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Evolutionary status of W Vir pulsating variables: Stellar evolution calculations for population II stars with initial
composition $Y_0=0.25$, $Z_0=10^{-3}$ and the initial stellar mass $M_0 =
0.82M_\odot$ were carried out from the main sequence to the white dwarf stage.
Twelve AGB and post--AGB evolutionary sequences were computed with different
values of the parameter in the Blocker mass loss rate formula
($0.01\le\eta_B\le 0.12$). Selected models of evolutionary sequences with
masses $M=0.536M_\odot$, $0.530M_\odot$ and $0.526M_\odot$ that experience the
loop in the Hertzsprung--Russel diagram due to the final helium flash were used
as initial conditions for solution of the equations of hydrodynamics describing
radial stellar oscillations. The region of instability to radial fundamental
mode pulsations is shown to extend from the asymptotic giant branch to
effective temperatures as high as $T_\mathrm{eff}\approx 6\times 10^3$ K.
Pulsation periods of hydrodynamic models are in the range from 15 to 50 day and
agree with periods of W~Vir pulsating stars. The models of intermediate
spectral type fundamental mode pulsators with periods $\Pi > 50$ day locate in
the upper part of the Hertzsprung--Russel diagram in the region of semiregular
pulsating variables. We conclude that W~Vir pulsating variables are the
low--mass post--AGB stars that experience the final helium flash.
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astro-ph
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COSMIC RAYS FROM ACCRETING ISOLATED NEUTRON STARS: Interstellar matter that is accreted onto isolated magnetic neutron stars in
the Galaxy ($\sim 10^9$ by number) is accelerated and reflected back by MHD
shocks, which envelope the stars. The integrated power in the Galaxy
$L_{cr,ns}$ is $ \sgreat 10^{40} {\rm erg \ s^{-1} }$, the energy distribution
is a power law of spectral index $> 2$, and the particle energy can be raised
to $10^6$ GeV, consistent with the power and spectrum of primary cosmic rays in
the Galaxy. The major contribution for $L_{cr,ns}$ comes from a minority of
$\sim 10^7$ isolated neutron stars which are located within dense clouds.
Sources in these clouds, that are generally spread within the Galactic disk,
can explain the concentration of high-energy cosmic rays in the Galactic plane,
as deduced from pion decay spectra in gamma-ray observations. The soft X-ray
luminosity from these neutron stars is consistent with the Galactic X-ray
background. The accretion may be associated with ion-neutral bias, that is
further enhanced by ion confinement in frozen-in magnetic fields, which can
raise the relative abundance of first ionization potential (FIP) elements in
the cosmic rays.
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astro-ph
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Systematic differences in simple stellar population model results:
Application to the M31 globular-like cluster system: Simple stellar population (SSP) synthesis models are useful tools for
studying the nature of unresolved star clusters in external galaxies. However,
the plethora of currently available SSP models gives rise to significant and
poorly documented systematic differences. Here we consider the outputs of the
commonly used Bruzual & Charlot and GALEV models, as well as a recently updated
SSP model suite which attempts to include the contributions of binary merger
products in the form of blue straggler stars (BS-SSP). We rederive the ages,
metallicities, extinction values and masses of 445 previously observed
globular-like clusters in M31 based on chi-square minimisation of their
spectral energy distributions with respect to these three different SSP models
and adopting a Chabrier-like stellar initial mass function. A comparison
between our new results and previous estimates of the same parameters shows
that the Bruzual & Charlot models yield the youngest ages and lowest masses,
while adoption of the BS-SSP models results in the oldest ages and highest mass
estimates. Similarly, the GALEV SSP models produce the lowest metallicities,
with the highest values resulting from the BS-SSP model suite. These trends are
caused by intrinsic differences associated with the models, and are not
significantly affected by the well-known age-metallicity degeneracy. Finally,
we note that the mass function of the massive M31 star clusters is similar to
that of the Milky Way's globular clusters, which implies that the two star
cluster systems likely formed under similar environmental conditions.
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astro-ph
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Dark Matter Halos in Galaxies and Globular Cluster Populations: We combine a new, comprehensive database for globular cluster populations in
all types of galaxies with a new calibration of galaxy halo masses based
entirely on weak lensing. Correlating these two sets of data, we find that the
mass ratio $\eta \equiv M_{GCS}/M_{h}$ (total mass in globular clusters,
divided by halo mass) is essentially constant at $\langle \eta \rangle \sim 4
\times 10^{-5}$, strongly confirming earlier suggestions in the literature.
Globular clusters are the only known stellar population that formed in
essentially direct proportion to host galaxy halo mass. The intrinsic scatter
in $\eta$ appears to be at most 0.2 dex; we argue that some of this scatter is
due to differing degrees of tidal stripping of the globular cluster systems
between central and satellite galaxies. We suggest that this correlation can be
understood if most globular clusters form at very early stages in galaxy
evolution, largely avoiding the feedback processes that inhibited the bulk of
field-star formation in their host galaxies. The actual mean value of $\eta$
also suggests that about $1/4$ of the \emph{initial} gas mass present in
protogalaxies collected into GMCs large enough to form massive, dense star
clusters. Finally, our calibration of $\langle \eta \rangle$ indicates that the
halo masses of the Milky Way and M31 are $(1.2\pm0.5)\times 10^{12} M_{\odot}$
and $(3.9\pm1.8)\times 10^{12} M_{\odot}$ respectively.
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astro-ph
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Ultraviolet C II and Si III Transit Spectroscopy and Modeling of the
Evaporating Atmosphere of GJ436b: Hydrogen gas evaporating from the atmosphere of the hot-Neptune GJ436b
absorbs over 50% of the stellar Ly$\alpha$ emission during transit. Given the
planet's atmospheric composition and energy-limited escape rate, this hydrogen
outflow is expected to entrain heavier atoms such as C and O. We searched for C
and Si in the escaping atmosphere of GJ436b using far-ultraviolet HST COS G130M
observations made during the planet's extended H I transit. These observations
show no transit absorption in the C II 1334,1335 \AA\ and Si III 1206 \AA\
lines integrated over [-100, 100] km s$^{-1}$, imposing 95% (2$\sigma$) upper
limits of 14% (C II) and 60% (Si III) depth on the transit of an opaque disk
and 22% (C II) and 49% (Si III) depth on an extended, highly asymmetric transit
similar to that of H I Ly$\alpha$. C$^+$ is likely present in the outflow
according to a simulation we carried out using a spherically-symmetric,
photochemical-hydrodynamical model. This simulation predicts a $\sim$2% transit
over the integrated bandpass, consistent with the data. At line center, we
predict the C II transit depth to be as high as 19%. Our model predicts a
neutral hydrogen escape rate of $1.6\times10^{9}$ g s$^{-1}$ ($3.1\times10^{9}$
g s$^{-1}$ for all species) for an upper atmosphere composed of hydrogen and
helium.
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astro-ph
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How Spatially Resolved Polarimetry Informs Black Hole Accretion Flow
Models: The Event Horizon Telescope (EHT) Collaboration has successfully produced
images of two supermassive black holes, enabling novel tests of black holes and
their accretion flows on horizon scales. The EHT has so far published total
intensity and linear polarization images, while upcoming images may include
circular polarization, rotation measure, and spectral index, each of which
reveals different aspects of the plasma and space-time. The next-generation EHT
(ngEHT) will greatly enhance these studies through wider recorded bandwidths
and additional stations, leading to greater signal-to-noise, orders of
magnitude improvement in dynamic range, multi-frequency observations, and
horizon-scale movies. In this paper, we review how each of these different
observables informs us about the underlying properties of the plasma and the
spacetime, and we discuss why polarimetric studies are well-suited to
measurements with sparse, long-baseline coverage.
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astro-ph
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Modeling the Mass Distribution in Spiral Galaxies: We use deep r-band photometry and Halpha rotation curves for a sample of 290
late-type spirals to model their mass distribution within the optical radius.
We examine luminosity profile decompositions into bulge and disk carefully and
confirm that bulge light is best modeled by a seeing-convolved exponential
profile. The optical rotation curves are well-reproduced with a combination of
bulge and "maximum" disk components only. No dark halo is needed. The disk
mass-to-light ratios (M/L's) correlate with the "size" of galaxies, as measured
by mass, luminosity, or disk scale length. Correcting for this scale effect
yields a narrow distribution of intrinsic M/L's for this galaxy population. By
combining these models with HI data for other samples, we confirm that the
luminous mass fraction increases with galaxy "size".
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astro-ph
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Yet another UFO in the X-ray spectrum of a high-z lensed QSO: Ultra-fast outflows (UFO) appear to be common in local active galactic nuclei
(AGN) and may be powerful enough ($\dot{E}_{kin}$$\geq$1\% of L$_{bol}$) to
effectively quench the star formation in their host galaxies. To test feedback
models based on AGN outflows, it is mandatory to investigate UFOs near the peak
of AGN activity, that is, at high-z where only a few studies are available to
date.
UFOs produce Fe resonant absorption lines measured above $\approx$7 keV. The
most critical problem in detecting such features in distant objects is the
difficulty in obtaining X-ray data with sufficient signal-to-noise. We
therefore selected a distant QSO that gravitational lensing made bright enough
for these purposes, the z=2.64 QSO MG J0414+0534, and observed it with
XMM-Newton for $\approx$78 ks.}
The X-ray spectrum of MG J0414+0534 is complex and shows signatures of cold
absorption (N$_{H}\approx$4$\times$10$^{22}$ cm$^{-2}$) and of the presence of
an iron emission line (E$\approx$6.4 keV, EW$=$95$\pm$53 eV) consistent with it
originating in the cold absorber. Our main result, however, is the robust
detection (more than 5$\sigma$) of an absorption line at E$_{int}\approx$9.2
keV (E$_{obs}\approx$2.5 keV observer frame). If interpreted as due to FeXXVI,
it implies gas outflowing at $v_{out}\approx$0.3c. To our knowledge, this is
the first detection of an UFO in a radio-loud quasar at z$\geq$1.5. We
estimated that the UFO mechanical output is
$\dot{E}_{kin}$$\approx$2.5$L_{bol}$ with
$\dot{p}_{out}/\dot{p}_{rad}\approx$17 indicating that it is capable of
installing significant feedback between the super-massive black hole (SMBH) and
the bulge of the host galaxy. We argue that this also suggests a magnetic
driving origin of the UFO.
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astro-ph
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MUCHFUSS - Massive Unseen Companions to Hot Faint Underluminous Stars
from SDSS: The project Massive Unseen Companions to Hot Faint Underluminous Stars from
SDSS (MUCHFUSS) aims at finding hot subdwarf stars with massive compact
companions (white dwarfs with masses $M>1.0 {\rm M_{\odot}}$, neutron stars or
black holes). The existence of such systems is predicted by binary evolution
calculations and some candidate systems have been found. We identified
$\simeq1100$ hot subdwarf stars from the Sloan Digital Sky Survey (SDSS). Stars
with high velocities have been reobserved and individual SDSS spectra have been
analysed. About 70 radial velocity variable subdwarfs have been selected as
good candidates for follow-up time resolved spectroscopy to derive orbital
parameters and photometric follow-up to search for features like eclipses in
the light curves. Up to now we found nine close binary sdBs with short orbital
periods ranging from $\simeq0.07 {\rm d}$ to $1.5 {\rm d}$. Two of them are
eclipsing binaries with companions that are most likely of substellar nature.
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astro-ph
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Electron Acceleration at Rippled Low-Mach-number Shocks in High-beta
Collisionless Cosmic Plasmas: Using large-scale fully-kinetic two-dimensional particle-in-cell simulations,
we investigate the effects of shock rippling on electron acceleration at
low-Mach-number shocks propagating in high-$\beta$ plasmas, in application to
merger shocks in galaxy clusters. We find that the electron acceleration rate
increases considerably when the rippling modes appear. The main acceleration
mechanism is stochastic shock-drift acceleration, in which electrons are
confined at the shock by pitch-angle scattering off turbulence and gain energy
from the motional electric field. The presence of multi-scale magnetic
turbulence at the shock transition and the region immediately behind the main
shock overshoot is essential for electron energization. Wide-energy non-thermal
electron distributions are formed both upstream and downstream of the shock.
The maximum energy of the electrons is sufficient for their injection into
diffusive shock acceleration. We show for the first time that the downstream
electron spectrum has a~power-law form with index $p\approx 2.5$, in agreement
with observations.
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astro-ph
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Analysis of the Data from Compton X-ray Polarimeters which Measure the
Azimuthal and Polar Scattering Angles: X-ray polarimetry has the potential to make key-contributions to our
understanding of galactic compact objects like binary black hole systems and
neutron stars, and extragalactic objects like active galactic nuclei, blazars,
and Gamma Ray Bursts. Furthermore, several particle astrophysics topics can be
addressed including uniquely sensitive tests of Lorentz invariance. In the
energy range from 10 keV to several MeV, Compton polarimeters achieve the best
performance. In this paper we evaluate the benefit that comes from using the
azimuthal and polar angles of the Compton scattered photons in the analysis,
rather than using the azimuthal scattering angles alone. We study the case of
an ideal Compton polarimeter and show that a Maximum Likelihood analysis which
uses the two scattering angles lowers the Minimum Detectable Polarization (MDP)
by ~20% compared to a standard analysis based on the azimuthal scattering
angles alone. The accuracies with which the polarization fraction and the
polarization direction can be measured improve by a similar amount. We conclude
by discussing potential applications of Maximum Likelihood analysis methods for
various polarimeter experiments.
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astro-ph
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Periodograms for Multiband Astronomical Time Series: This paper introduces the multiband periodogram, a general extension of the
well-known Lomb-Scargle approach for detecting periodic signals in time-domain
data. In addition to advantages of the Lomb-Scargle method such as treatment of
non-uniform sampling and heteroscedastic errors, the multiband periodogram
significantly improves period finding for randomly sampled multiband light
curves (e.g., Pan-STARRS, DES and LSST). The light curves in each band are
modeled as arbitrary truncated Fourier series, with the period and phase shared
across all bands. The key aspect is the use of Tikhonov regularization which
drives most of the variability into the so-called base model common to all
bands, while fits for individual bands describe residuals relative to the base
model and typically require lower-order Fourier series. This decrease in the
effective model complexity is the main reason for improved performance. We use
simulated light curves and randomly subsampled SDSS Stripe 82 data to
demonstrate the superiority of this method compared to other methods from the
literature, and find that this method will be able to efficiently determine the
correct period in the majority of LSST's bright RR Lyrae stars with as little
as six months of LSST data. A Python implementation of this method, along with
code to fully reproduce the results reported here, is available on GitHub.
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astro-ph
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Galaxy Formation with BECDM: I. Turbulence and relaxation of idealised
haloes: We present a theoretical analysis of some unexplored aspects of relaxed
Bose-Einstein condensate dark matter (BECDM) haloes. This type of ultralight
bosonic scalar field dark matter is a viable alternative to the standard cold
dark matter (CDM) paradigm, as it makes the same large-scale predictions as CDM
and potentially overcomes CDM's small-scale problems via a galaxy-scale de
Broglie wavelength. We simulate BECDM halo formation through mergers, evolved
under the Schr\"odinger-Poisson equations. The formed haloes consist of a
soliton core supported against gravitational collapse by the quantum pressure
tensor and an asymptotic $r^{-3}$ NFW-like profile. We find a fundamental
relation of the core=to-halo mass with the dimensionless invariant $\Xi \equiv
\lvert E \rvert/M^3/(Gm/\hbar)^2$ or $M_{\rm c}/M \simeq 2.6 \Xi^{1/3}$,
linking the soliton to global halo properties. For $r \geq 3.5 \,r_{\rm c}$
core radii, we find equipartition between potential, classical kinetic, and
quantum gradient energies. The haloes also exhibit a conspicuous turbulent
behavior driven by the continuous reconnection of vortex lines due to wave
interference. We analyse the turbulence 1D velocity power spectrum and find a
$k^{-1.1}$ power-law. This suggests the vorticity in BECDM haloes is
homogeneous, similar to thermally-driven counterflow BEC systems from condensed
matter physics, in contrast to a $k^{-5/3}$ Kolmogorov power-law seen in
mechanically-driven quantum systems. The mode where the power spectrum peaks is
approximately the soliton width, implying the soliton-sized granules carry most
of the turbulent energy in BECDM haloes.
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astro-ph
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Neutrino spectra evolution during proto-neutron star deleptonization: The neutrino-driven wind, which occurs after the onset of a core-collapse
supernova explosion, has long been considered as the possible site for the
synthesis of heavy r-process elements in the Universe. Only recently, it has
been possible to simulate supernova explosions up to ~10 seconds, based on
three-flavor Boltzmann neutrino transport. These simulations show that the
neutrino luminosities and spectra of all flavors are very similar and their
difference even decreases during the deleptonization of the proto-neutron star.
As a consequence, the ejecta are always proton rich which rules out the
possible production of heavy r-process elements (Z>56). We perform a detailed
analysis of the different weak processes that determine the neutrino spectra.
Non-electron flavor (anti)neutrinos are produced and interact only via
neutral-current processes, while electron (anti)neutrinos have additional
contributions from charge-current processes. The latter are dominated by ve
absorption on neutrons and anti-ve absorption on protons. At early times,
charge-current processes are responsible for spectral differences between.
However, as the region of neutrino decoupling moves to higher densities during
deleptonization, charge-current reactions are suppressed by final state
Pauli-blocking. anti-ve absorption on protons is suppressed due to the
continuously increasing chemical potential of the neutrons. ve absorption on
neutrons is blocked by the increasing degeneracy of the electrons. These
effects result in negligible contributions from charge-current reactions on
timescales on the order of tens of seconds, depending on the progenitor star.
Hence, the neutrino spectra are mainly determined from neutral-current
processes which do not distinguish between the different flavors and results in
the convergence of the spectra. These findings are independent of the
charge-current reaction rates used...
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astro-ph
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Identifying hot subdwarf stars from photometric data using Gaussian
mixture model and graph neural network: Hot subdwarf stars are very important for understanding stellar evolution,
stellar astrophysics, and binary star systems. Identifying more such stars can
help us better understand their statistical distribution, properties, and
evolution. In this paper, we present a new method to search for hot subdwarf
stars in photometric data (b, y, g, r, i, z) using a machine learning
algorithm, graph neural network, and Gaussian mixture model. We use a Gaussian
mixture model and Markov distance to build the graph structure, and on the
graph structure, we use a graph neural network to identify hot subdwarf stars
from 86 084 stars, when the recall, precision, and f1 score are maximized on
the original, weight and synthetic minority oversampling technique datasets.
Finally, from 21 885 candidates, we selected approximately 6 000 stars that
were the most similar to the hot subdwarf star.
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astro-ph
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VLA Detection of RRLs from the radio nucleus of NGC 253 : Ionization by
a weak AGN, an obscured SSC or a compact SNR ?: We have imaged the H92alpha and H75alpha radio recombination line (RRL)
emissions from the starburst galaxy NGC 253 with a resolution of ~4 pc. The
peak of the RRL emission at both frequencies coincides with the unresolved
radio nucleus. Both lines observed towards the nucleus are extremely wide, with
FWHM of ~200 km /s. Modeling the RRL and radio continuum data for the radio
nucleus shows that the lines arise in gas whose density is ~10^4 \cc and mass
is few thousand Msun, which requires an ionizing flux of (6-20)x10^{51} photons
/s. We consider a SNR expanding in a dense medium, a star cluster and also an
AGN as potential ionizing sources. Based on dynamical arguments, we rule out an
SNR as a viable ionizing source. A star cluster model was considered and the
dynamics of the ionized gas in a stellar-wind driven structure was
investigated. Such a model is consistent with the properties of the ionized gas
only for a cluster younger than ~10^5 years. The existence of such a young
cluster at the nucleus seems improbable. The third model assumes the ionizing
source to be an AGN at the nucleus. In this model, it was shown that the
observed X-ray flux is too weak to account for the required ionizing photon
flux. However, the ionization requirement can be explained if the accretion
disk is assumed to have a Big Blue Bump in its spectrum. Hence, we favor an AGN
at the nucleus as the source responsible for ionizing the observed RRLs. A
hybrid model consisting of a inner ADAF disk and an outer thin disk is
suggested, which could explain the radio, UV and the X-ray luminosities of the
nucleus.
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astro-ph
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The Hawaii Infrared Parallax Program. I. Ultracool Binaries and the L/T
Transition: We present the first results from our high-precision infrared (IR) astrometry
program at the Canada-France-Hawaii Telescope. We measure parallaxes for 83
ultracool dwarfs (spectral types M6--T9) in 49 systems, with a median
uncertainty of 1.1 mas (2.3%) and as good as 0.7 mas (0.8%). We provide the
first parallaxes for 48 objects in 29 systems, and for another 27 objects in 17
systems, we significantly improve upon published results, with a median (best)
improvement of 1.7x (5x). Three systems show astrometric perturbations
indicative of orbital motion; two are known binaries (2MASSJ0518-2828AB and
2MASSJ1404-3159AB) and one is spectrally peculiar (SDSSJ0805+4812). In
addition, we present here a large set of Keck adaptive optics imaging that more
than triples the number of binaries with L6--T5 components that have both
multi-band photometry and distances. Our data enable an unprecedented look at
the photometric properties of brown dwarfs as they cool through the L/T
transition. Going from \approxL8 to \approxT4.5, flux in the Y and J bands
increases by \approx0.7 mag and \approx0.5 mag, respectively (the Y- and J-band
"bumps"), while flux in the H, K, and L' bands declines monotonically. This
wavelength dependence is consistent with cloud clearing over a narrow range of
temperature, since condensate opacity is expected to dominate at 1.0--1.3
micron. Interestingly, despite more than doubling the near-IR census of L/T
transition objects, we find a conspicuous paucity of objects on the
color--magnitude diagram just blueward of the late-L/early-T sequence. This
"L/T gap" occurs at MKO(J-H) = 0.1--0.3 mag, MKO(J-K) = 0.0--0.4 mag, and
implies that the last phases of cloud evolution occur rapidly. Finally, we
provide a comprehensive update to the absolute magnitudes of ultracool dwarfs
as a function of spectral type using a combined sample of 314 objects.
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astro-ph
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Quantum vacuum and accelerated expansion: A new approach to extraction of quantum vacuum energy, in the context of the
accelerated expansion, is proposed, and it is shown that experimentally
realistic orders of values can be derived. The idea has been implemented in the
framework of the Friedmann-Lemaitre-Robertson-Walker geometry in the language
of the effective action in the relativistic formalism of Schwinger's proper
time and Seeley-DeWitt's heat kernel expansion.
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astro-ph
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Almost All of Kepler's Multiple Planet Candidates are Planets: We present a statistical analysis that demonstrates that the overwhelming
majority of Kepler candidate multiple transiting systems (multis) indeed
represent true, physically-associated transiting planets. Binary stars provide
the primary source of false positives among Kepler planet candidates, implying
that false positives should be nearly randomly-distributed among Kepler
targets. In contrast, true transiting planets would appear clustered around a
smaller number of Kepler targets if detectable planets tend to come in systems
and/or if the orbital planes of planets encircling the same star are
correlated. There are more than one hundred times as many Kepler planet
candidates in multi-candidate systems as would be predicted from a random
distribution of candidates, implying that the vast majority are true planets.
Most of these multis are multiple planet systems orbiting the Kepler target
star, but there are likely cases where (a) the planetary system orbits a
fainter star, and the planets are thus significantly larger than has been
estimated, or (b) the planets orbit different stars within a binary/multiple
star system. We use the low overall false positive rate among Kepler multis,
together with analysis of Kepler spacecraft and ground-based data, to validate
the closely-packed Kepler-33 planetary system, which orbits a star that has
evolved somewhat off of the main sequence. Kepler-33 hosts five transiting
planets with periods ranging from 5.67 to 41 days.
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astro-ph
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ROSAT PSPC and Hri Observations of the Composite Starburst/Seyfert 2
Galaxy NGC 1672: The nearby barred spiral galaxy NGC 1672 is thought to have a weak Seyfert
nucleus in addition to its strong starburst activity. Observations with the
PSPC and HRI instruments on board the ROSAT X-ray satellite show that three
X-ray sources with luminosities (1--2)\times 10^{40} erg/s are clearly
identified with NGC 1672. The strongest X-ray source lies at the nucleus, and
the other two lie near the ends of the prominent bar, locations that are also
bright in H-alpha and near-infrared images. The nuclear source is resolved by
the HRI on about the scale of the recently identified nuclear ring, and one of
the sources at the ends of the bar is also probably resolved. The X-ray
spectrum of the nuclear source is quite soft, having a Raymond--Smith plasma
temperature of about 0.7 keV and little evidence for intrinsic absorption. The
ROSAT band X-ray flux of the nuclear source appears to be dominated not by
X-ray binary emission but rather by diffuse gas emission. While the properties
of the nuclear source are generally supportive of a superbubble interpretation,
its large density and emission measure stretch the limits that can be
comfortably accommodated by such models. We do not detect direct emission from
the putative Seyfert nucleus, although an alternative model for the nuclear
source is thermal emission from gas that is photoionized by a hidden Seyfert
nucleus. The spectra of the other two X-ray sources are harder than that of the
nuclear source, and superbubble models for them have the same strengths and
weaknesses.
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astro-ph
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The Host Galaxy of GRB 060505: Host ISM Properties: We investigate the ISM environment of GRB 060505. Using optical emission-line
diagnostic ratios, we compare the ISM properties of the GRB 060505 host region
with the hosts of unambiguous long- and short-duration GRBs. We show that the
metallicity, ionization state, and star formation rate of the GRB 060505
environment are more consistent with short-duration GRBs than with
long-duration GRBs. We compare the metallicity and star formation rates of the
GRB 060505 region with four other star-forming regions within the GRB 060505
host galaxy. We find no significant change in metallicity or star formation
rate between the GRB 060505 region and the other four host regions. Our results
are consistent with a compact-object-merger progenitor for GRB 060505.
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astro-ph
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Properties of Ellipticity Correlation with Atmospheric Structure from
Gemini South: Cosmic shear holds great promise for a precision independent measurement of
$\Omega\rm_m$, the mass density of the universe relative to the critical
density. The signal is expected to be weak, so a thorough understanding of
systematic effects is crucial. An important systematic effect is the
atmosphere: shear power introduced by the atmosphere is larger than the
expected signal. Algorithms exist to extract the cosmic shear from the
atmospheric component, though a measure of their success applied to a range of
seeing conditions is lacking.
To gain insight into atmospheric shear, Gemini South imaging in conjunction
with ground condition and satellite wind data were obtained. We find that under
good seeing conditions Point-Spread-Function (PSF) correlations persist well
beyond the separation typical of high-latitude stars. Under these conditions,
ellipticity residuals based on a simple PSF interpolation can be reduced to
within a factor of a few of the shot-noise induced ellipticity floor. We also
find that the ellipticity residuals are highly correlated with wind direction.
Finally, we correct stellar shapes using a more sophisticated procedure and
generate shear statistics from stars. Under all seeing conditions in our data
set the residual correlations lie everywhere below the target signal level. For
good seeing we find that the systematic error attributable to atmospheric
turbulence is comparable in magnitude to the statistical error (shape noise)
over angular scales relevant to present lensing surveys.
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astro-ph
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Radiative Transfer Effect on Ultraviolet Pumping of the 21cm Line in the
High Redshift Universe: During the epoch of reionization the 21cm signal is sensitive to the
scattering rate of the ultraviolet photons, redshifting across the Lyman_alpha
resonance. Here we calculate the photon scattering rate profile for a single
ultraviolet source. After taking into account previously neglected natural
broadening of the resonance line, we find that photons approach the resonance
frequency and experience most scatterings at a significantly smaller distance
from the source than naively expected r=(dnu/nu_0)(c/H), where dnu=nu-nu_0 is
the initial frequency offset, and the discrepancy increases as the initial
frequency offset decreases. As a consequence, the scattering rate P(r) drops
much faster with increasing distance than the previously assumed 1/r^2 profile.
Near the source (r<1Mpc comoving), the scattering rate of photons that redshift
into the Ly_alpha resonance converges to P(r) \propto r^{-7/3}. The scattering
rate of Ly_alpha photons produced by splitting of photons that redshift into a
higher resonance (Ly_gamma, Ly_delta, etc.) is only weakly affected by the
radiative transfer, while the sum of scattering rates of Ly_alpha photons
produced from all higher resonances also converges to P(r) \propto r^{-7/3}
near the source. At 15<z<35, on scales of ~0.01-20Mpc/h (comoving), the total
scattering rate of Ly_alpha photons from all Lyman resonances is found to be
higher by a factor of ~1+0.3[(1+z)/20]^{2/3} than obtained without full
radiative transfer. Consequently, during the early stage of reionization, the
differential brightness of 21cm signal against the cosmic microwave background
is also boosted by a similar factor.
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astro-ph
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On the Efficiency of Thermal Conduction in Galaxy Clusters: Galaxy clusters host a large reservoir of diffuse plasma with
radially-varying temperature profiles. The efficiency of thermal conduction in
the intracluster medium (ICM) is complicated by the existence of turbulence and
magnetic fields, and has received a lot of attention in the literature.
Previous studies suggest that the magnetothermal instability developed in outer
regions of galaxy clusters would drive magnetic field lines preferentially
radial, resulting in efficient conduction along the radial direction. Using a
series of spherically-symmetric simulations, here we investigate the impact of
thermal conduction on the observed temperature distributions in outer regions
of three massive clusters, and find that thermal conduction substantially
modifies the ICM temperature profile. Within 3 Gyr, the gas temperature at a
representative radius of $0.3r_{500}$ typically decreases by ~10 - 20% and the
average temperature slope between $0.3r_{500}$ and $r_{500}$ drops by ~ 30 -
40%, indicating that the observed ICM would not stay in a long-term equilibrium
state in the presence of thermal conduction. However, X-ray observations show
that the outer regions of massive clusters have remarkably similar
radially-declining temperature profiles, suggesting that they should be quite
stable. Our study thus suggests that the effective conductivity along the
radial direction must be suppressed below the Spitzer value by a factor of 10
or more, unless additional heating sources offset conductive cooling and
maintain the observed temperature distributions. Our study provides a
smoking-gun evidence for the suppression of parallel conduction along magnetic
field lines in low-collisionality plasmas by kinetic mirror or whistler
instabilities.
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astro-ph
|
Mapping the Clumpy Structures within Submillimeter Galaxies using
Laser-Guide Star Adaptive Optics Spectroscopy: We present the first integral-field spectroscopic observations of
high-redshift submillimeter-selected galaxies (SMGs) using Laser Guide Star
Adaptive Optics (LGS-AO). We target H-alpha emission of three SMGs at redshifts
z~1.4-2.4 with the OH-Suppressing Infrared Imaging Spectrograph (OSIRIS) on
Keck. The spatially-resolved spectroscopy of these galaxies reveals unresolved
broad H-alpha line regions (FWHM>1000 km/s) likely associated with an AGN and
regions of diffuse star formation traced by narrow-line H-alpha emission
(FWHM<500 km/s) dominated by multiple Halpha-bright stellar clumps, each
contributing 1-30% of the total clump-integrated H-alpha emission. We find that
these SMGs host high star-formation rate surface densities, similar to local
extreme sources, such as circumnuclear starbursts and luminous infrared
galaxies. However, in contrast to these local environments, SMGs appear to be
undergoing such intense activity on significantly larger spatial scales as
revealed by extended H-alpha emission over 4-16 kpc. H-alpha kinematics show no
evidence of ordered global motion as would be found in a disk, but rather large
velocity offsets (~few x 100 km/s) between the distinct stellar clumps.
Together with the asymmetric distribution of the stellar clumps around the AGN
in these objects, it is unlikely that we are unveiling a clumpy disk structure
as has been suggested in other high-redshift populations of star-forming
galaxies. The SMG clumps in this sample may correspond to remnants of
originally independent gas-rich systems that are in the process of merging,
hence triggering the ultraluminous SMG phase.
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astro-ph
|
Searching for an Intermediate Mass Black Hole in the Blue Compact Dwarf
galaxy MRK 996: The possibility is explored that accretion on an intermediate mass black hole
contributes to the ionisation of the interstellar medium of the Compact Blue
Dwarf galaxy MRK996. Chandra observations set tight upper limits (99.7 per cent
confidence level) in both the X-ray luminosity of the posited AGN,
Lx(2-10keV)<3e40erg/s, and the black hole mass, <1e4/\lambda Msolar, where
\lambda, is the Eddington ratio. The X-ray luminosity upper limit is
insufficient to explain the high ionisation line [OIV]25.89\mu m, which is
observed in the mid-infrared spectrum of the MRK996 and is proposed as evidence
for AGN activity. This indicates that shocks associated with supernovae
explosions and winds of young stars must be responsible for this line. It is
also found that the properties of the diffuse X-ray emission of MRK996 are
consistent with this scenario, thereby providing direct evidence for shocks
that heat the galaxy's interstellar medium and contribute to its ionisation.
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astro-ph
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Stability of planetary, single M dwarf, and binary star companions to
Kepler detached eclipsing binaries and a possible five body system: In this study we identify 11 Kepler systems (KIC 5255552, 5653126, 5731312,
7670617, 7821010, 8023317, 10268809, 10296163, 11519226, 11558882 and 12356914)
with a "flip-flop" effect in the eclipse timing variations O-C diagrams of the
systems, report on what these systems have in common and whether these systems
are dynamically stable. These systems have previously reported high eccentric
binary stars with highly eccentric third bodies/outer companions. We find that
all of the additional bodies in the system are dynamically stable for the
configurations previously reported and are therefore likely to exist as
described. We also provide additional evidence of KIC5255552 being a quadruple
star system comprised of an eclipsing binary pair and non-eclipsing binary pair
with the possibility of a fifth body in the system. With the advent of the NASA
TESS exoplanet survey, its precision photometric monitoring offers an
opportunity to help confirm more local eclipsing binary star companions,
including planets.
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astro-ph
|
Dependence of fragmentation in self-gravitating accretion discs on small
scale structure: We propose a framework for understanding the fragmentation criterion for
self-gravitating discs which, in contrast to studies that emphasise the
`gravoturbulent' nature of such discs, instead focuses on the properties of
their quasi-regular spiral structures. Within this framework there are two
evolutionary paths to fragmentation: i) collapse on the free-fall time, which
requires that the ratio of cooling time to dynamical time ($\beta$) $< 3$ and
ii) quasistatic collapse on the cooling time at a rate that is sufficiently
fast that fragments are compact enough to withstand disruption when they
encounter spiral features in the disc.
We perform 2D grid simulations which demonstrate numerically converged
fragmentation at $\beta < 3$ (in good agreement with Paardekooper et al. (2011)
and others) and argue that this is a consequence of the fact that such
simulations smooth the gravitational force on the scale $H$, the scale height
of the disc. Such simulations thus only allow fragmentation via route i) above
since they suppress the quasistatic contraction of fragments on scales $< H$;
the inability of fragments to contract to significantly smaller scales then
renders them susceptible to disruption at the next spiral arm encounter.
On the other hand, 3D simulations indeed show fragmentation at higher $\beta$
via route ii). We derive an analytic prediction of fragmentation by route ii)
when $\beta \lesssim 12$, based on the requirement that fragments must contract
sufficiently to withstand disruption by spiral arms. We also discuss the
necessary numerical requirements on both grid based and SPH codes if they are
to model fragmentation via route ii).
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astro-ph
|
Improved Weak Lensing Photometric Redshift Calibration via StratLearn
and Hierarchical Modeling: Discrepancies between cosmological parameter estimates from cosmic shear
surveys and from recent Planck cosmic microwave background measurements
challenge the ability of the highly successful $\Lambda$CDM model to describe
the nature of the Universe. To rule out systematic biases in cosmic shear
survey analyses, accurate redshift calibration within tomographic bins is key.
In this paper, we improve photo-$z$ calibration via Bayesian hierarchical
modeling of full galaxy photo-$z$ conditional densities, by employing
$\textit{StratLearn}$, a recently developed statistical methodology, which
accounts for systematic differences in the distribution of the spectroscopic
training/source set and the photometric target set. Using realistic simulations
that were designed to resemble the KiDS+VIKING-450 dataset, we show that
$\textit{StratLearn}$-estimated conditional densities improve the galaxy
tomographic bin assignment, and that our $\textit{StratLearn}$-Bayesian
framework leads to nearly unbiased estimates of the target population means.
This leads to a factor of $\sim 2$ improvement upon the previously best
photo-$z$ calibration method. Our approach delivers a maximum bias per
tomographic bin of $\Delta \langle z \rangle = 0.0095 \pm 0.0089$, with an
average absolute bias of $0.0052 \pm 0.0067$ across the five tomographic bins.
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astro-ph
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Topology of the Galaxy Distribution in the Hubble Deep Fields: We have studied topology of the distribution of the high redshift galaxies
identified in the Hubble Deep Field (HDF) North and South. The two-dimensional
genus is measured from the projected distributions of the HDF galaxies at
angular scales from $3.8''$ to $ 6.1''$. We have also divided the samples into
three redshift slices with roughly equal number of galaxies using photometric
redshifts to see possible evolutionary effects on the topology.
The genus curve of the HDF North clearly indicates clustering of galaxies
over the Poisson distribution while the clustering is somewhat weaker in the
HDF South. This clustering is mainly due to the nearer galaxies in the samples.
We have also found that the genus curve of galaxies in the HDF is consistent
with the Gaussian random phase distribution with no significant redshift
dependence.
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astro-ph
|
Numerical study of Cosmic Ray Diffusion in MHD turbulence: We study diffusion of Cosmic Rays (CRs) in turbulent magnetic fields using
test particle simulations. Electromagnetic fields are produced in direct
numerical MHD simulations of turbulence and used as an input for particle
tracing, particle feedback on turbulence being ignored. Statistical transport
coefficients from the test particle runs are compared with earlier analytical
predictions. We find qualitative correspondence between them in various aspects
of CR diffusion. In the incompressible case, that we consider in this paper,
the dominant scattering mechanism occurs to be the non-resonant mirror
interactions with the slow-mode perturbations. Perpendicular transport roughly
agrees with being produced by magnetic field wandering.
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astro-ph
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The Galactic Plane Observed by XMM-Newton: In A0-1 we proposed an ambitious long-term survey of selected regions of our
Galaxy (the XGPS survey) using the EPIC CCD cameras on XMM-Newton. The first
phase of the programme, which aims to survey a strip of the Galactic Plane in
the Scutum region, is currently underway. Here we report on the preliminary
results from the first 15 survey pointings. We show that the XGPS survey
strategy of fairly shallow (5-10 ks) exposures but wide-angle coverage is well
tuned to the goal of providing a large catalogue of predominantly Galactic
sources at relatively faint X-ray fluxes in the hard 2-6 keV band.
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astro-ph
|
Possible Quasi-Periodic Oscillations from Unstable Accretion: 3D MHD
Simulations: We investigate the photometric variability of magnetized stars, particularly
neutron stars, accreting through a magnetic Rayleigh-Taylor-type instability at
the disk-magnetosphere interface, and compare it with the variability during
stable accretion, with the goal of looking for possible quasi-periodic
oscillations. The lightcurves during stable accretion show periodicity at the
star's frequency and sometimes twice that, due to the presence of two funnel
streams that produce antipodal hotspots near the magnetic poles. On the other
hand, lightcurves during unstable accretion through tongues penetrating the
magnetosphere are more chaotic due to the stochastic behaviour of the tongues,
and produce noisier power spectra. However, the power spectra do show some
signs of quasi-periodic variability. Most importantly, the rotation frequency
of the tongues and the resulting hotspots is close to the inner-disk orbital
frequency, except in the most strongly unstable cases. There is therefore a
high probability of observing QPOs at that frequency in longer simulations. In
addition, the lightcurves in the unstable regime show periodicity at the star's
rotation frequency in many of the cases investigated here, again except in the
most strongly unstable cases which lack funnel flows and the resulting
antipodal hotspots. The noisier power spectra result in the fractional rms
amplitudes of the Fourier peaks being smaller.
We also study in detail the effect of the misalignment angle between the
rotation and magnetic axes of the star on the variability, and find that at
misalignment angles $\gtrsim 25^\circ$, the star's period always appears in the
lightcurves.
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astro-ph
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High time resolution observations of the January 2000 glitch in the Vela
pulsar: Pulsars are rotating neutron stars, sweeping the emission regions from the
magnetic poles across our line of sight. Isolated neutron stars lose angular
momentum through dipole radiation and (possibly) particle winds, hence they
slow down extremely steadily, making them amongst the most reliable timing
sources available. However, it is well known that younger pulsars can suffer
glitches, when they suddenly deviate from their stable rotation period. On 2000
January 16 (MJD 51559) the rate of pulsation from the Vela pulsar (B0833-45)
showed such a fractional period change of {\rm $3.1\times 10^{-6}$}, the
largest recorded for this pulsar. The glitch was detected and reported by the
Hobart radio telescope. The speedy announcement allowed the X-ray telescope,
Chandra, and others, to make Target of Opportunity observations. The data
placed an upper limit of 40 seconds for the transition time from the original
to the new period. Four relaxation timescales are found, which are believed to
be due to the variable coupling between the crust and the interior fluid. One
is very short, about 60 seconds; the others have been previously reported and
are 0.56, 3.33 and 19.1 days in length.
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astro-ph
|
Jupiter's equatorial quasi-quadrennial oscillation forced by internal
thermal forcing: Observations have shown that there exists downward propagation of alternating
westward/eastward jets in Jupiter's equatorial stratosphere, with a
quasi-period between four and six years. This phenomenon is generally called
the quasi-quadrennial oscillation (QQO). Here, we simulate the QQO by injecting
isotropic small-scale thermal disturbances into a three-dimensional general
circulation model of Jupiter. It is found that the internal thermal disturbance
is able to excite a wealth of waves that generate the equatorial QQO and
multiple jet streams at middle and high latitudes of both hemispheres. The
dominant wave mode in generating the QQO-like oscillation is that with a zonal
wavenumber of 10. Inhomogeneous evolution of potential vorticity favors the
emergence of the off-equatorial zonal jets. The off-equatorial jets migrate to
the equator, strengthen the deep equatorial jets, and result in the prolonging
of the QQO-like oscillations.
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astro-ph
|
First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations:
Preliminary Maps and Basic Results: We present full sky microwave maps in five bands (23 to 94 GHz) from the WMAP
first year sky survey. Calibration errors are <0.5% and the low systematic
error level is well specified. The 2<l<900 anisotropy power spectrum is cosmic
variance limited for l<354 with a signal-to-noise ratio >1 per mode to l=658.
The temperature-polarization cross-power spectrum reveals both acoustic
features and a large angle correlation from reionization. The optical depth of
reionization is 0.17 +/- 0.04, which implies a reionization epoch of 180+220-80
Myr (95% CL) after the Big Bang at a redshift of 20+10-9 (95% CL) for a range
of ionization scenarios. This early reionization is incompatible with the
presence of a significant warm dark matter density. The age of the best-fit
universe is 13.7 +/- 0.2 Gyr old. Decoupling was 379+8-7 kyr after the Big Bang
at a redshift of 1089 +/- 1. The thickness of the decoupling surface was dz=195
+/- 2. The matter density is Omega_m h^2 = 0.135 +0.008 -0.009, the baryon
density is Omega_b h^2 = 0.0224 +/- 0.0009, and the total mass-energy of the
universe is Omega_tot = 1.02 +/- 0.02. The spectral index of scalar
fluctuations is fit as n_s = 0.93 +/- 0.03 at wavenumber k_0 = 0.05 Mpc^-1,
with a running index slope of dn_s/d ln k = -0.031 +0.016 -0.018 in the
best-fit model. This flat universe model is composed of 4.4% baryons, 22% dark
matter and 73% dark energy. The dark energy equation of state is limited to
w<-0.78 (95% CL). Inflation theory is supported with n_s~1, Omega_tot~1,
Gaussian random phases of the CMB anisotropy, and superhorizon fluctuations. An
admixture of isocurvature modes does not improve the fit. The tensor-to-scalar
ratio is r(k_0=0.002 Mpc^-1)<0.90 (95% CL).
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astro-ph
|
The SONYC survey: Towards a complete census of brown dwarfs in star
forming regions: SONYC, short for "Substellar Objects in Nearby Young Clusters", is a survey
program to provide a census of the substellar population in nearby star forming
regions. We have conducted deep optical and near-infrared photometry in five
young regions (NGC1333, rho Ophiuchi, Chamaeleon-I, Upper Sco, and Lupus-3),
combined with proper motions, and followed by extensive spectroscopic campaigns
with Subaru and VLT, in which we have obtained more than 700 spectra of
candidate low-mass objects. We have identified and characterized more than 60
new substellar objects, among them a handful of objects with masses close to,
or below the Deuterium burning limit. Through SONYC and surveys by other
groups, the substellar IMF is now well characterized down to ~ 5 - 10 MJup, and
we find that the ratio of the number of stars with respect to brown dwarfs lies
between 2 and 6. A comprehensive survey of NGC 1333 reveals that, down to
~5MJup, free-floating objects with planetary masses are 20-50 times less
numerous than stars, i.e. their total contribution to the mass budget of the
clusters can be neglected.
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astro-ph
|
Evolution of the L1 halo family in the radial solar sail CRTBP: We present a detailed investigation of the dramatic changes that occur in the
$\mathcal{L}_1$ halo family when radiation pressure is introduced into the
Sun-Earth circular restricted three-body problem (CRTBP). This
photo-gravitational CRTBP can be used to model the motion of a solar sail
orientated perpendicular to the Sun-line. The problem is then parameterized by
the sail lightness number, the ratio of solar radiation pressure acceleration
to solar gravitational acceleration. Using boundary-value problem numerical
continuation methods and the AUTO software package (Doedel et al. 1991) the
families can be fully mapped out as the parameter $\beta$ is increased.
Interestingly, the emergence of a branch point in the retrograde satellite
family around the Earth at $\beta\approx0.0387$ acts to split the halo family
into two new families. As radiation pressure is further increased one of these
new families subsequently merges with another non-planar family at
$\beta\approx0.289$, resulting in a third new family. The linear stability of
the families changes rapidly at low values of $\beta$, with several small
regions of neutral stability appearing and disappearing. By using existing
methods within AUTO to continue branch points and period-doubling bifurcations,
and deriving a new boundary-value problem formulation to continue the folds and
Krein collisions, we track bifurcations and changes in the linear stability of
the families in the parameter $\beta$ and provide a comprehensive overview of
the halo family in the presence of radiation pressure. The results demonstrate
that even at small values of $\beta$ there is significant difference to the
classical CRTBP, providing opportunity for novel solar sail trajectories.
Further, we also find that the branch points between families in the solar sail
CRTBP provide a simple means of generating certain families in the classical
case.
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astro-ph
|
Neutrinos as a diagnostic of cosmic ray Galactic/extra-galactic
transition: Motivated by a recent change in viewing the onset of the extra-galactic
component in the cosmic ray spectrum, we have fitted the observed data down to
$10^{8.6}$ GeV and have obtained the corresponding power emissivity. This
transition energy is well below the threshold for resonant $p\gamma$ absorption
on the cosmic microwave background, and thus source evolution is an essential
ingredient in the fitting procedure. Two-parameter fits in the spectral and
redshift evolution indices show that a standard Fermi $E_i^{-2}$ source
spectrum is excluded at larger than 95% confidence level (CL). Armed with the
primordial emissivity, we follow Waxman and Bahcall to derive the associated
neutrino flux on the basis of optically thin sources. For $pp$ interactions as
the generating mechanism, the neutrino flux exceeds the AMANDA-B10 90%CL upper
limits. In the case of $p\gamma$ dominance, the flux is marginally consistent
with AMANDA-B10 data. In the new scenario the source neutrino flux dominates
over the cosmogenic flux at all energies. Thus, should data from AMANDA-II
prove consistent with the model, we show that IceCube can measure the
characteristic power law of the neutrino spectrum, and thus provide a window on
the source dynamics.
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astro-ph
|
Unusual Burst Emission from the New Soft Gamma Repeater SGR1627-41: In June-July,1998 the Konus-Wind burst spectrometer observed a series of
bursts from the new soft gamma repeater SGR1627-41. Time histories and energy
spectra of the bursts have been studied, revealing fluences and peak fluxes in
the ranges of 3x10^{-7} - 7.5x10^{-6} erg cm^{-2} and 10^{-5} - 10^{-4}erg
cm^{-2}/s respectively. One event, 18 June 6153.5sUT stands out dramatically
from this series. Its fluence is ~7x10^{-4} erg cm^{-2} and peak flux
~2x10^{-2} erg cm^{-2}/s. These values from a source at a distance of 5.8 kpc
yield an energy output of ~3x10^{42}erg and maximum luminosity of ~8x10^{43}
erg/s, similar to the values for the famous March 5, 1979 and August27,1998
events. In terms of energy, this event is another giant outburst seen in a
third SGR! However, this very energetic burst differs significantly from the
other giant outbursts. It exhibits no separate initial pulse with a fast rise
time, no extended tail, and no pulsations. It is rather similar to ordinary
repeated bursts but is a few hundred times stronger in intensity. According to
the magnetar model by Thompson and Duncan (1995) such a burst may be initiated
by a strong starquake when a crust fracture propagates over the whole surface
of a neutron star.
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astro-ph
|
On the Particle Acceleration Mechanisms in a Double Radio Relic Galaxy
Cluster, Abell 1240: We present a 368 ks deep Chandra observation of Abell~1240, a binary merging
galaxy cluster at a redshift of 0.195 with two Brightest Cluster Galaxies
(BCGs) may have passed each other 0.3 Gyr ago. Building upon previous
investigations involving GMRT, VLA, and LOFAR data, our study focuses on two
prominent extended radio relics at the north-west (NW) and south-east (SE) of
the cluster core. By leveraging the high-resolution Chandra imaging, we have
identified two distinct surface brightness edges at $\sim$ 1 Mpc and 1.2 Mpc NW
and SE of the cluster center, respectively, coinciding with the outer edges of
both relics. Our temperature measurements hint the edges to be shock front
edges. The Mach numbers, derived from the gas density jumps, yield
$\cal{M}_{\rm SE}$ = 1.49$^{+0.22}_{-0.24}$ for the South Eastern shock and
$\cal{M}_{\rm NW}$ = 1.41$^{+0.17}_{-0.19}$ for the North Western shock. Our
estimated Mach numbers are remarkably smaller compared to those derived from
radio observations ($\cal{M}_{\rm SE}$ = 2.3 and $\cal{M}_{\rm NW}$ = 2.4),
highlighting the prevalence of a re-acceleration scenario over direct
acceleration of electrons from the thermal pool. Furthermore, we compare the
observed temperature profiles across both shocks with that of predictions from
collisional vs. collisionless models. Both shocks favor the Coulomb collisional
model, but we could not rule out a purely collisionless model due to pre-shock
temperature uncertainties.
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astro-ph
|
Thermal structure and cooling of neutron stars with magnetized envelopes: The thermal structure of neutron stars with magnetized envelopes is studied
using modern physics input. The relation between the internal (T_i) and local
surface temperatures is calculated and fitted by analytic expressions for
magnetic field strengths B from 0 to 10^{16} G and arbitrary inclination of the
field lines to the surface. The luminosity of a neutron star with dipole
magnetic field is calculated and fitted as a function of B, T_i, stellar mass
and radius. In addition, we simulate cooling of neutron stars with magnetized
envelopes. In particular, we analyse ultramagnetized envelopes of magnetars and
also the effects of the magnetic field of the Vela pulsar on the determination
of critical temperatures of neutron and proton superfluids in its core.
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astro-ph
|
The ALHAMBRA survey: an empirical estimation of the cosmic variance for
merger fraction studies based on close pairs: Our goal is to estimate empirically, for the first time, the cosmic variance
that affects merger fraction studies based on close pairs. We compute the
merger fraction from photometric redshift close pairs with 10h^-1 kpc <= rp <=
50h^-1 kpc and Dv <= 500 km/s, and measure it in the 48 sub-fields of the
ALHAMBRA survey. We study the distribution of the measured merger fractions,
that follow a log-normal function, and estimate the cosmic variance sigma_v as
the intrinsic dispersion of the observed distribution. We develop a maximum
likelihood estimator to measure a reliable sigma_v and avoid the dispersion due
to the observational errors (including the Poisson shot noise term). The cosmic
variance of the merger fraction depends mainly on (i) the number density of the
populations under study, both for the principal (n_1) and the companion (n_2)
galaxy in the close pair, and (ii) the probed cosmic volume V_c. We find a
significant dependence on neither the search radius used to define close
companions, the redshift, nor the physical selection (luminosity or stellar
mass) of the samples. We provide a parametrisation of the cosmic variance with
n_1, n_2, and V_c, sigma_v = 0.48 n_1^{-0.54} V_c^{-0.48} (n_2/n_1)^{-0.37}.
Thanks to this prescription, future merger fraction studies based on close
pairs could account properly for the cosmic variance on their results.
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astro-ph
|
Signatures of a Maxwellian Component in Shock-Accelerated Electrons in
GRBs: Recent particle-in-cell simulations suggest that a large fraction of the
energy dissipated in a relativistic shock is deposited into a Maxwellian
distribution of electrons that is connected to the high-energy power-law tail.
Here, we explore the observational implications of such a mixed
thermal-nonthermal particle distribution for the afterglow and prompt emission
of gamma-ray bursts. When the Maxwellian component dominates the energy budget,
the afterglow lightcurves show a very steep decline phase followed by a more
shallow decay when the characteristic synchrotron frequency crosses the
observed band. The steep decay appears in the X-rays at ~100 sec after the
burst and is accompanied by a characteristic hard-soft-hard spectral evolution
that has been observed in a large number of early afterglows. If internal
shocks produce a similar mixed electron distribution, a bump is expected at the
synchrotron peak of the nu*f_nu spectrum.
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astro-ph
|
Nonlinear cascades in two-dimensional turbulent magnetoconvection: The dynamics of spectral transport in two-dimensional turbulent convection of
electrically conducting fluids is studied by means of direct numerical
simulations (DNS) in the frame of the magnetohydrodynamic (MHD) Boussinesq
approximation. The system performs quasi-oscillations between two different
regimes of small-scale turbulence: one dominated by nonlinear MHD interactions,
the other governed by buoyancy forces. The self-excited change of turbulent
states is reported here for the first time. The process is controlled by the
ideal invariant cross-helicity, $H^\mathrm{C}=\int_S \mathrm{d}S
\mathbf{v}\cdot\mathbf{b}$. The observations are explained by the interplay of
convective driving with the nonlinear spectral transfer of total MHD energy and
cross-helicity.
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astro-ph
|
The Greater Taurus-Auriga Ecosystem I: There Is A Distributed Older
Population: The Taurus-Auriga association and its associated molecular cloud are a
benchmark population for studies of star and planet formation. The census of
Taurus-Auriga has been assembled over seven decades and has inherited the
biases, incompleteness, and systematic uncertainties of the input studies. The
notably unusual shape of the inferred IMF and the existence of several isolated
disk-bearing stars suggest that additional (likely disk-free) members might
remain to be discovered. We therefore have begun a global reassessment of the
membership of Taurus-Auriga that exploits new data and better definitions of
youth and kinematic membership. As a first step, we reconsider the membership
of all disk-free candidate members from the literature with spectral type
$\ge$F0, $3^h50^m<\alpha<5^h40^m$, and $14^{\circ}<\delta<34^{\circ}$. We
combine data from the literature with Keck/HIRES and UH88/SNIFS spectra to test
the membership of these candidates using HR diagram positions, proper motions,
RVs, H$\alpha$, lithium, and surface gravity. We find 218 confirmed or likely
Taurus members, 160 confirmed or likely interlopers, and only 18 that still
lack sufficient evidence to draw firm conclusions. A significant fraction of
these stars (81/218=37%) are not included in the most recent canonical member
lists. Intriguingly, there are few additional members in the immediate vicinity
of the molecular clouds, preserving the IMFs that have been deemed anomalous in
past work. Many of the likely Taurus members are distributed broadly across the
search area. When combined with known disk hosts, our updated census reveals
two regimes: a high-density population with a high disk fraction (indicative of
youth) that broadly traces the molecular clouds, and a low-density population
with low disk fraction (hence likely older) that most likely represents
previous generations of star formation.
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astro-ph
|
A Swift Fix II: Physical Parameters of Type I Superluminous Supernovae: In November 2020, the Swift team announced a major update to the calibration
of the UltraViolet and Optical Telescope (UVOT) data to correct for the gradual
loss of sensitivity over time. Beginning in roughly 2015, the correction
affected observations in the three near ultraviolet (UV) filters, reaching
levels of up to 0.3 mag immediately prior to the correction. Over the same time
period, an increasing number of Type I superluminous supernovae (SLSNe-I) were
discovered and studied. Many SLSNe-I are hot (T$_\textrm{eff}$ $\approx 10,000$
K) near peak, and therefore accurate UV data are imperative towards properly
understanding their physical properties and energetics. We re-compute Swift
UVOT photometry for SLSNe-I discovered between 2014 and 2021 with at least 5
Swift observations in 2015 or later. We calculate host-subtracted magnitudes
for each SLSN and fit their spectral energy distributions with modified
blackbodies to obtain the radius and temperature evolution. We also fit
multi-band photometry using the Modular Open Source Fitter for Transients
(MOSFiT) to obtain key parameters such as the spin period (P), magnetic field
strength (B), ejecta mass (M$_\textrm{ej}$), and kinetic energy
(E$_\textrm{kin}$). From our MOSFiT modeling, we also estimate the peak
UV/optical luminosity (L$_\textrm{peak}$) and total radiative energy
(E$_\textrm{rad}$). Under the assumption of magnetar-powered SLSNe we find
several strong trends, including anti-correlations between P and both
L$_\textrm{peak}$ and E$_\textrm{rad}$, a correlation between E$_\textrm{kin}$
and E$_\textrm{rad}$, and an anti-correlation between B and E$_\textrm{rad}$.
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astro-ph
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The Evolution and Star Formation History of M33: We construct a parameterized model to explore the main properties of the star
formation history of M33. We assume that the disk originates and grows by the
primordial gas infall and adopt the simple form of gas accretion rate with one
free parameter, the infall time-scale. We also include the contribution of gas
outflow process. A major update of the model is that we adopt a molecular
hydrogen correlated star formation law and calculate the evolution of the
atomic and molecular gas separately. Comparisons between the model predictions
and the observational data show that the model predictions are very sensitive
to the adopted infall time-scale, while the gas outflow process mainly
influences the metallicity profile. The model adopting a moderate outflow rate
and an inside-out formation scenario can be in good agreement with most of
observed constraints of M33 disk. We also compare the model predictions based
on the molecular hydrogen correlated star formation law and that based on the
Kennicutt star formation law. Our results imply that the molecular hydrogen
correlated star formation law should be preferred to describe the evolution of
the M33 disk, especially the radial distributions of both the cold gas and the
stellar population.
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astro-ph
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HAT-P-47b AND HAT-P-48b: Two Low Density Sub-Saturn-Mass Transiting
Planets on the Edge of the Period--Mass Desert: We report the discovery of two new transiting extrasolar planets orbiting
moderately bright (V = 10.7 and 12.2 mag) F stars (masses of 1.39 Msun and 1.10
Msun, respectively). The planets have periods of P = 4.7322 d and 4.4087 d, and
masses of 0.21 MJ and 0.17 MJ which are almost half-way between those of
Neptune and Saturn. With radii of 1.31 RJ and 1.13 RJ, these very low density
planets are the two lowest mass planets with radii in excess that of Jupiter.
Comparing with other recent planet discoveries, we find that sub-Saturns
(0.18MJ < Mp < 0.3MJ) and super-Neptunes (0.05MJ < Mp < 0.18MJ) exhibit a wide
range of radii, and their radii exhibit a weaker correlation with irradiation
than higher mass planets. The two planets are both suitable for measuring the
Rossiter-McLaughlin effect and for atmospheric characterization. Measuring the
former effect would allow an interesting test of the theory that star-planet
tidal interactions are responsible for the tendency of close-in giant planets
around convective envelope stars to be on low obliquity orbits. Both planets
fall on the edge of the short period Neptunian desert in the semi-major
axis-mass plane.
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astro-ph
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X-ray Reverberation Observational Modelling in Active Galactic Nuclei: X-ray reverberation in Active Galactic Nuclei, believed to be the result of
the reprocessing of coronal photons by the underlying accretion disc, has
allowed us to probe the properties of the inner-most regions of the accretion
flow and the central black hole. Our current model (KYNREFREV) computes the
time-dependent reflection spectra of the disc as a response to a flash of
primary power-law radiation from a point source corona located on the axis of
the black hole accretion disc (lamp-post geometry). Full relativistic effects
are taken into account. The ionization of the disc is set for each radius
according to the amount of the incident primary flux and the density of the
accretion disc. We detect wavy residuals around the best-fit reverberation
model time lags at high frequencies. This result suggests that the simple
lamp-post geometry does not fully explain the X-ray source/disc configuration
in Active Galactic Nuclei. There has been a noticeable progress into the
development of codes for extended coronae (Wilkins+16, Chainakun & Young 2017,
Taylor & Reynolds 2018a,b). Indeed, the model from Chainakun & Young (2017),
consisting of two axial point sources illuminating an accretion disc that
produce the reverberation lags is able to reproduce the observed time-lag
versus frequency spectra. The goal of this paper is to observationally justify
the need for an extended corona in order to provide (in the near future) with a
mathematical formulation of a model for an extended corona in its simplest
form.
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astro-ph
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A Viscous Heating Mechanism for the Hot Plasma in the Galactic Center
Region: In addition to lines originating in a soft phase at ~0.8 keV and to cold
molecular clouds, the X-ray spectra from the Galactic center region also
exhibit properties similar to those of a diffuse, thin, very hot plasma at 8
keV on a scale of hundreds of parsecs. This phase is surprising for more than
one reason. First, such a hot plasma should not be bound to the Galactic plane
and the power needed to sustain the escaping matter would be higher then any
known source. Second, there is no known mechanism able to heat the plasma to
more than a few keV. Recently we have suggested that, hydrogen having escaped,
the hot plasma could be a helium plasma, heavy enough to be gravitationally
confined. In this case, the required power is much more reasonable. We present
here a possible heating mechanism which taps the gravitational energy of the
molecular clouds. We note that the 8 keV plasma is highly viscous and we show
how viscous friction of molecular clouds flowing within the hot phase can
dissipate energy in the gas and heat it. We detail the MHD wake of a spherical
cloud by considering the different MHD waves the cloud can excite. We find that
most of the energy is dissipated by the damping of Alfvenic perturbations in
two possible manners, namely by non-linear effects and by a large scale
curvature of the field lines. Depending on the field strength, this heating can
balance the radiative cooling. We note that the plasma parameters may be
optimal to make the dissipation most efficient, suggesting a self-regulation
mechanism. The loss of kinetic and gravitational energy also causes accretion
of the clouds and may have significant action on the gas dynamics in this
region between the large scale, bar dominated flow and the central accretion to
the massive black hole.
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astro-ph
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Cosmological parameters from the comparison of peculiar velocities with
predictions from the 2M++ density field: Peculiar velocity measurements are the only tool available in the
low-redshift Universe for mapping the large-scale distribution of matter and
can thus be used to constrain cosmology. Using redshifts from the 2M++ redshift
compilation, we reconstruct the density of galaxies within 200 Mpc/h, allowing
for the first time good sampling of important superclusters such as the Shapley
Concentration. We compare the predicted peculiar velocities from 2M++ to
Tully-Fisher and SNe peculiar velocities. We find a value of $\beta^* \equiv
\Omega_{\rm{m}}^{0.55}/b^* = 0.431 \pm 0.021$, suggesting
$\Omega_{\rm{m}}^{0.55}\sigma_{\rm{8,lin}} = 0.401 \pm 0.024$, in good
agreement with other probes. The predicted peculiar velocity of the Local Group
arising from the 2M++ volume alone is $540 \pm 40$ km/s, towards $l = 268 \pm
4, b= 38 \pm 6$, only $10^\circ$ out of alignment with the Cosmic Microwave
Background dipole. To account for velocity contributions arising from sources
outside the 2M++ volume, we fit simultaneously for $\beta^*$ and an external
bulk flow in our analysis. We find that an external bulk flow is preferred at
the 5.1$\sigma$ level, and the best fit has a velocity of $159\pm23$ km/s
towards $l=304 \pm 11, b = 6 \pm 13$. Finally, the predicted bulk flow of a 50
Mpc/h Gaussian-weighted volume centred on the Local Group is $230 \pm 30$ km/s,
in the direction $l=293\pm 8, b = 14 \pm 10$, in agreement with predictions
from $\Lambda$CDM.
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astro-ph
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Multi-band optical variability of the Blazar S5 0716+714 in outburst
state during 2014-2015: We analyzed the multi-band optical behaviour of the BL Lacertae object, S5
0716+714, during its outburst state from 2014 November - 2015 March. We took
data on 23 nights at three observatories, one in India and two in Bulgaria,
making quasi-simultaneous observations in B, V, R, and I bands. We measured
multi-band optical fluxes, colour and spectral variations for this blazar on
intraday and short timescales. The source was in a flaring state during the
period analyzed and displayed intense variability in all wavelengths. R band
magnitude of 11.6 was attained by the target on 18 Jan 2015, which is the
brightest value ever recorded for S5 0716+714. The discrete correlation
function method yielded good correlation between the bands with no measurable
time lags, implying that radiation in these bands originate from the same
region and by the same mechanism. We also used the structure function technique
to look for characteristic timescales in the light curves. During the times of
rapid variability, no evidence for the source to display spectral changes with
magnitude was found on either of the timescales. The amplitude of variations
tends to increase with increasing frequency with a maximum of $\sim$ 22% seen
during flaring states in B band. A mild trend of larger variability amplitude
as the source brightens was also found. We found the duty cycle of our source
during the analyzed period to be $\sim$ 90%. We also investigated the optical
spectral energy distribution of S5 0716+714 using B, V, R, and I data points
for 21 nights. We briefly discuss physical mechanisms most likely responsible
for its flux and spectral variations.
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astro-ph
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Red-Sequence Galaxies at High Redshift by the COMBO-17+4 Survey: We investigate the evolution of the galaxy population since redshift 2 with a
focus on the colour bimodality and mass density of the red sequence. We obtain
precise and reliable photometric redshifts up to z=2 by supplementing the
optical survey COMBO-17 with observations in four near-infrared bands on 0.2
square degrees of the COMBO-17 A901-field. Our results are based on an
H-band-selected catalogue of 10692 galaxies complete to H=21.7. We measure the
rest-frame colour (U_280-V) of each galaxy, which across the redshift range of
our interest requires no extrapolation and is robust against moderate redshift
errors by staying clear of the 4000A-break. We measure the colour-magnitude
relation of the red sequence as a function of lookback time from the peak in a
colour error-weighted histogram, and thus trace the galaxy bimodality out to
z~1.65. The (U_280-V) of the red sequence is found to evolve almost linearly
with lookback time. At high redshift, we find massive galaxies in both the red
and the blue population. Red-sequence galaxies with log M_*/M_sun>11 increase
in mass density by a factor of ~4 from z~2 to 1 and remain nearly constant at
z<1. However, some galaxies as massive as log M_*/M_sun=11.5 are already in
place at z~2.
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astro-ph
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The DRAO Synthesis Telescope: The DRAO Synthesis Telescope (ST) is a forefront telescope for imaging
large-scale neutral hydrogen and polarized radio continuum emission at
arcminute resolution. Equipped for observations at 1420 and 408 MHz, the ST
completed the Canadian Galactic Plane Survey, providing pioneering measurements
of arcminute-scale structure in HI emission and self-absorption and of the
diffuse polarized emission, using a fine grid of Rotation Measures to chart the
large-scale Galactic magnetic field, and advancing the knowledge of the
Galactic rotation curve. In this paper we describe a plan for renewal of the
Synthesis Telescope that will create a forefront scientific instrument, a
testbed for new radio astronomy technologies, and a training ground for the
next generation of Canadian radio astronomers and radio telescope engineers.
The renewed telescope will operate across the entire range 400 to 1800 MHz.
Collaborations between DRAO and university partners have already demonstrated a
novel feed antenna to cover this range, low-noise amplifiers, and a new
GPU-based correlator of bandwidth 400 MHz. The renewed ST will provide
excellent sensitivity to extended HI, covering the Galactic disk and halo,
spectro-polarimetry with unprecedented resolution in angle and in Faraday
depth, the ability to search for OH masers in all four 18-cm lines
simultaneously, and sensitive recombination-line observations stacked over as
many as forty transitions. As a testbed the renewed ST will evaluate low-cost
digital clocking and sampling techniques of wide significance for the ngVLA,
SKA, and other future telescopes, and a prototype of the digital correlator
developed at DRAO for SKA-mid.
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astro-ph
|
Solar Activity and Space Weather: After providing an overview of solar activity as measured by the sunspot
number (SSN) and space weather events during solar cycles (SCs) 21-24, we focus
on the weak solar activity in SC 24. The weak solar activity reduces the number
of energetic eruptions from the Sun and hence the number of space weather
events. The speeds of coronal mass ejections (CMEs), interplanetary (IP)
shocks, and the background solar wind all declined in SC 24. One of the main
heliospheric consequences of weak solar activity is the reduced total (magnetic
+ gas) pressure, magnetic field strength, and Alfv\'en speed. There are three
groups of phenomena that decline to different degrees in SC 24 relative to the
corresponding ones in SC 23: (i) those that decline more than SSN does, (ii)
those that decline like SSN, and (iii) those that decline less than SSN does.
The decrease in the number of severe space weather events such as high-energy
solar energetic particle (SEP) events and intense geomagnetic storms is deeper
than the decline in SSN. CMEs expand anomalously and hence their magnetic
content is diluted resulting in weaker geomagnetic storms. The reduction in the
number of intense geomagnetic storms caused by corotating interaction regions
is also drastic. The diminished heliospheric magnetic field in SC 24 reduces
the efficiency of particle acceleration, resulting in fewer high-energy SEP
events. The numbers of IP type II radio bursts, IP socks, and high-intensity
energetic storm particle events closely follow the number of fast and wide CMEs
(and approximately SSN). The number of halo CMEs in SC 24 declines less than
SSN does, mainly due to the weak heliospheric state. Phenomena such as IP CMEs
and magnetic clouds related to frontside halos also do not decline
significantly. The mild space weather is likely to continue in SC 25, whose
strength has been predicted to be not too different from that of SC 24.
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astro-ph
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Strong stellar-driven outflows shape the evolution of galaxies at cosmic
dawn: We study galaxy mass assembly and cosmic star formation rate (SFR) at
high-redshift (z$\gt$4), by comparing data from multiwavelength surveys with
predictions from the GAlaxy Evolution and Assembly (GAEA) model. GAEA
implements a stellar feedback scheme partially based on cosmological
hydrodynamical simulations, that features strong stellar driven outflows and
mass-dependent timescale for the re-accretion of ejected gas. In previous work,
we have shown that this scheme is able to correctly reproduce the evolution of
the galaxy stellar mass function (GSMF) up to $z\sim3$. We contrast model
predictions with both rest-frame Ultra-Violet (UV) and optical luminosity
functions (LF), which are mostly sensible to the SFR and stellar mass,
respectively. We show that GAEA is able to reproduce the shape and redshift
evolution of both sets of LFs. We study the impact of dust on the predicted LFs
and we find that the required level of dust attenuation is in qualitative
agreement with recent estimates based on the UV continuum slope. The
consistency between data and model predictions holds for the redshift evolution
of the physical quantities well beyond the redshift range considered for the
calibration of the original model. In particular, we show that GAEA is able to
recover the evolution of the GSMF up to z$\sim$7 and the cosmic SFR density up
to z$\sim$10.
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astro-ph
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Calibrating Galaxy Redshifts Using Absorption by the Surrounding
Intergalactic Medium: Rest-frame UV spectral lines of star-forming galaxies are systematically
offset from the galaxies' systemic redshifts, probably because of large-scale
outflows. We calibrate galaxy redshifts measured from rest-frame UV lines by
utilizing the fact that the mean HI Ly-alpha absorption profiles around the
galaxies, as seen in spectra of background objects, must be symmetric with
respect to the true galaxy redshifts if the galaxies are oriented randomly with
respect to the lines of sight to the background objects. We use 15 QSOs at
z~2.5-3 and more than 600 foreground galaxies with spectroscopic redshifts at
z~1.9-2.5. All galaxies are within 2 Mpc proper from the lines of sight to the
background QSOs. We find that LyA emission and ISM absorption redshifts require
systematic shifts of v_LyA=-295(+35)(-35) km/s and v_ISM=145(+70)(-35) km/s.
Assuming a Gaussian distribution, we put 1-sigma upper limits on possible
random redshift offsets of <220 km/s for LyA and <420 km/s for ISM redshifts.
For the small subset (<10%) of galaxies for which near-IR spectra have been
obtained, we can compare our results to direct measurements based on nebular
emission lines which we confirm to mark the systemic redshifts. While our v_ISM
agrees with the direct measurements, our v_LyA is significantly smaller.
However, when we apply our method to the near-IR subsample which is
characterized by slightly different selection effects, the best-fit velocity
offset comes into agreement with the direct measurement. This confirms the
validity of our approach, and implies that no single number appropriately
describes the whole population of galaxies, in line with the observation that
the line offset depends on galaxy spectral morphology. This method provides
accurate redshift calibrations and will enable studies of circumgalactic matter
around galaxies for which rest-frame optical observations are not available.
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astro-ph
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Stimulated Raman scattering of water maser lines in astrophysical
plasmas: Radiative transfer equations are derived and solved for the stimulated Raman
scattering of water maser lines in the astrophysical plasmas with electron
density of about 10^6 - 10^7 cm-3. In stimulated Raman scattering, the energy
of water maser line is transferred to the side band modes: Stokes mode and
anti-Stokes mode. The Stokes mode is easily produced by backward Raman
scattering while the anti-Stokes mode is created by the interacting
intersecting masers in the plasma. The intensity of the Stokes mode is higher
than that of the anti-Stokes mode. These side band modes are proposed as
explanation for the extreme velocity features observed in the galaxy NGC 4258.
The threshold value of the brightness temperature for the Raman scattering is
about 10^16 - 10^19 K, and it is satisfied in the case of NGC 4258.
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astro-ph
|
Tomography of the environment of the COSMOS/AzTEC-3 submillimeter galaxy
at z=5.3 revealed by Lyalpha and MUSE observations: We study the members of the protocluster around AzTEC3 submillimeter galaxy
at z=5.3. We analyzed the data from the MUSE instrument in an area of 1.4x1.4
arcmin^2 around AzTEC3 and derived information on the Lya line in emission. We
compared the Lya profile of various regions of the environment with the zELDA
radiative transfer model, revealing the neutral gas distribution and
kinematics. We identified 10 Lya emitting sources, including 2 regions with
extended emission: one embedding AzTEC3 and LBG3, a star-forming galaxy located
12 kpc north of the SMG and another toward LBG-1, a star-forming galaxy located
90 kpc to the southeast. The sources appear distributed in an elongated
configuration of about 70'' in extent. The number of sources confirms the
overdensity around AzTEC3. For the AzTEC3+LBG3 system, the Lya emission appears
redshifted and more spatially extended than the [CII] line emission. Similarly,
the Lya line spectrum is broader in velocity than [CII] for LBG1. In the former
spectrum, the Lya emission is elongated to the north of LBG3 and to the south
of AzTEC3, where a faint Lya emitting galaxy is also located. The elongated
structures could resemble tidal features due to the interaction of the two
galaxies with AzTEC3. Also, we find a bridge of gas, revealed by the Lya
emission between AzTEC3 and LBG3. The Lya emission toward LBG1 embeds its three
components. The HI kinematics support the idea of a merger of the three
components. Given the availability of CO and [CII] observations from previous
campaigns, and our Lya information, we find evidence of starburst-driven
phenomena and interactions around AzTEC-3. The stellar mass of the galaxies of
the overdensity and the Lya luminosity of the HI nebula associated with AzTEC-3
imply a dark matter halo of 10^12 Msun at z=5.3 that could evolve into a
cluster of 2x10^14 Msun at z=0.
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astro-ph
|
Accretion by the Secondary in Eta Carinae During the Spectroscopic
Event: I. Flow Parameters: We examine the influence of the gravity of the companion (the secondary) to
the massive primary star Eta Carinae on the winds blown by the primary and the
secondary. The two winds collide with each other after passing through two
respective shock waves, and escape the system while strongly emitting in the
X-ray band. While during most of the 5.5 years orbital period, the companion's
gravity has a negligible effect on the winds, we find that near periastron, the
companion's gravity may significantly influence the flow and the companion
might accrete from the primary's wind under certain circumstances. Near
periastron passage, the collision region of the two winds may collapse onto the
secondary star, a process that could substantially reduce the X-ray luminosity.
We suggest that such an accretion process produces the long, almost flat, X-ray
minimum in Eta Carinae.
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astro-ph
|
Wetter Stratospheres on High Obliquity Planets: We investigate how obliquity affects stratospheric humidity using a 3D
general circulation model and find the stratosphere under high obliquity could
be over 3 orders of magnitude moister than under the low obliquity equivalent,
even with the same global annual mean surface temperature. Three complexities
that only exist under high obliquity are found to be causally relevant. 1)
Seasonal variation under high obliquity causes extremely high surface
temperatures to occur during polar days, moistening the polar air that may
eventually enter the stratosphere. 2) Unlike the low obliquity scenario where
the cold trap efficiently freezes out water vapor, the high obliquity
stratosphere gets most of its moisture input from high latitudes, and thus
largely bypasses the cold trap. 3) A high obliquity climate tends to be warmer
than its low obliquity equivalent, thus moistening the atmosphere as a whole.
We found each of the above factors could significantly increase stratospheric
humidity. These results indicate that, for an earth-like exoplanet, it is more
likely to detect water from surface evaporation if the planet is under high
obliquity. The water escape could cause a high obliquity planet to loss
habitability before the runaway greenhouse takes place.
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astro-ph
|
Diagnostic of the temperature and differential emission measure (DEM)
based on Hinode/XRT data: We discuss here various methodologies and an optimal strategy of the
temperature and emission measure diagnostics based on Hinode X-Ray Telescope
data. As an example of our results we present determination of the temperature
distribution of the X-rays emitting plasma using filters ratio method and three
various methods of the calculation of the differential emission measure (DEM).
We have found that all these methods give results similar to the two filters
ratio method. Additionally, all methods of the DEM calculation gave similar
solutions. We can state that the majority of the pairs of the Hinode filters
allows one to derive the temperature and emission measure in the isothermal
plasma approximation using standard diagnostic based on two filters ratio
method. In cases of strong flares one can also expect well conformity of the
results obtained using a Withbroe - Sylwester, genetic algorithm and
least-square methods of the DEM evaluation.
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astro-ph
|
Special point on the mass radius diagram of hybrid stars: An analytical study that explains the existence of a very small region on the
mass radius diagram of hybrid stars where all of the lines representing the
sequences of models with different constant values of the bag constant B
intersect is presented. This circumstance is shown to be a consequence of the
linear dependence of pressure on energy density in the quark cores of hybrid
stars.
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astro-ph
|
Common envelope events with low-mass giants: understanding the energy
budget: Common envelope events are important interactions between two binary stars
that lead to the formation of close binary systems. We present here a
systematic three-dimensional study in which we model common envelope events
with low-mass giant donors. The results allow us to revise the energy formalism
that is usually used to determine common envelope event outcomes. We show that
the energy budget for this type of system should include the recombination
energy, and that it also must take into account that a significant fraction of
the released orbital energy is taken away by the ejecta. We provide three ways
in which our results can be used by binary population synthesis studies: a
relation that links the observed post-common envelope binary with the initial
binary parameters, a fitting formula for the $\alpha_{\rm ce}\lambda$ parameter
of the standard energy formalism, and a revised energy formalism that takes
into account both the recombination energy and the energy that is taken away by
the ejecta.
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astro-ph
|
Detecting and interpreting statistical lensing by absorbers: We propose a method for detecting gravitational magnification of distant
sources, like quasars, due to absorber systems detected in their spectra. We
first motivate the use of metal absorption lines rather than Lyman-alpha lines,
then we show how to relate the observed moments of the source magnitude
distribution to the mass distribution of absorbers. In order to illustrate the
feasibility of the method, we use a simple model to estimate the amplitude of
the effect expected for MgII absorption lines, and show that their lensing
signal might already be detectable in large surveys like the SDSS. Our model
suggests that quasars behind strong MgII absorbers are in average brightened by
-0.05 to -0.2 magnitude due to magnification. One must therefore revisit the
claim that, in magnitude limited surveys, quasars with strong absorbers tend to
be missed due to extinction effects. In addition to constraining the mass of
absorber systems, applying our method will allow for the quantification of this
bias.
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astro-ph
|
Uncertainties in the Galactic dark matter distribution: an update: We present here a quantitative, accurate estimate of the impact of
uncertainties of astrophysical nature on the determination of the dark matter
distribution within our Galaxy, the Milky Way. Based on an update of a previous
analysis, this work is motivated by recent new determinations of astrophysical
quantities of relevance -- such as the Galactic parameters (R0,V0) -- from the
GRAVITY collaboration and the GAIA satellite, respectively. We find that even
with these state-of-the-art determination and a range of uncertainties -- both
statistical and systematic -- much narrowed with respect to previous work, the
uncertainties on the dark matter distribution and their impact on searches of
physics beyond the standard model stays sizable.
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astro-ph
|
A Time-Efficient, Data Driven Modelling Approach For Predicting The
Geomagnetic Impact of Coronal Mass Ejections: To understand the global-scale physical processes behind coronal mass
ejection (CME)-driven geomagnetic storms and predict their intensity as a space
weather forecasting measure, we develop an interplanetary CME flux
rope-magnetosphere interaction module using 3D magnetohydrodynamics. The
simulations adequately describe ICME-forced dynamics of the magnetosphere
including the imposed magnetotail torsion. These interactions also result in
induced currents which is used to calculate the geomagnetic perturbation.
Through a suitable calibration, we estimate a proxy of geoeffectiveness -- the
Storm Intensity index (STORMI) -- that compares well with the Dst/SYM-H Index.
Simulated impacts of two contrasting coronal mass ejections quantified by the
STORMI index exhibit a high linear correlation with the corresponding Dst and
SYM-H indices. Our approach is relatively simple, has fewer parameters to be
fine-tuned, is time-efficient compared to complex fluid-kinetic methods.
Furthermore, we demonstrate that flux rope erosion does not significantly
affect our results. Thus our method has the potential to significantly extend
the time window for predictability -- an outstanding challenge in geospace
environment forecasting -- if early predictions of near-Earth CME flux rope
structures based on near-Sun observations are available as inputs. This study
paves the way for early warnings based on operational predictions of CME-driven
geomagnetic storms.
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astro-ph
|
Generation of large-scale magnetic fields due to fluctuating $α$ in
shearing systems: We explore the growth of large-scale magnetic fields in a shear flow, due to
helicity fluctuations with a finite correlation time, through a study of the
Kraichnan-Moffatt model of zero-mean stochastic fluctuations of the $\alpha$
parameter of dynamo theory. We derive a linear integro-differential equation
for the evolution of large-scale magnetic field, using the first-order
smoothing approximation and the Galilean invariance of the $\alpha$-statistics.
This enables construction of a model that is non-perturbative in the shearing
rate $S$ and the $\alpha$-correlation time $\tau_\alpha$. After a brief review
of the salient features of the exactly solvable white-noise limit, we consider
the case of small but non-zero $\tau_\alpha$. When the large-scale magnetic
field varies slowly, the evolution is governed by a partial differential
equation. We present modal solutions and conditions for the exponential growth
rate of the large-scale magnetic field, whose drivers are the Kraichnan
diffusivity, Moffatt drift, Shear and a non-zero correlation time. Of
particular interest is dynamo action when the $\alpha$-fluctuations are weak;
i.e. when the Kraichnan diffusivity is positive. We show that in the absence of
Moffatt drift, shear does not give rise to growing solutions. But shear and
Moffatt drift acting together can drive large scale dynamo action with growth
rate $\gamma \propto |S|$.
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astro-ph
|
A Comparison of the UV and HI Properties of the Extended UV (XUV) Disk
Galaxies NGC 2541, NGC 5832 and ESO406-042: We present a UV study of 3 extended UV (XUV) galaxies that we have observed
with the UVIT and the GMRT. XUV galaxies show filamentary or diffuse star
formation well beyond their optical disks, in regions where the disk surface
density lies below the threshold for star formation. GALEX observations found
that surprisingly 30% of all the nearby spiral galaxies have XUV disks. The XUV
galaxies can be broadly classified as type 1 and type 2 XUV disks. The type 1
XUV disks have star formation that is linked to that in their main disk, and
the UV emission appears as extended, filamentary spiral arms. The UV luminosity
is associated with compact star forming regions along the extended spiral arms.
The star formation is probably driven by slow gas accretion from nearby
galaxies or the intergalactic medium (IGM). But the type 2 XUV disks have star
formation associated with an outer low luminosity stellar disk that is often
truncated near the optical radius of the galaxy. The nature of the stellar
disks in type 2 XUV disks are similar to that of the diffuse stellar disks of
low surface brightness galaxies. The star formation in type 2 XUV disks is
thought to be due to rapid gas accretion or gas infall from nearby high
velocity clouds (HVCs), interacting galaxies or the IGM. In this paper we
investigate the star formation properties of the XUV regions of two type 2
galaxies and one mixed XUV type galaxy and compare them with the neutral
hydrogen (HI) emission in their disks. We present preliminary results of our
UVIT (FUV and NUV) observations of NGC 2541, NGC 5832 and ESO406-042, as well
as GMRT observations of their HI emission. We describe the UV emission
morphology, estimate the star formation rates and compare it with the HI
distribution in these type 2 and mixed XUV galaxies.
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astro-ph
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PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar: (Abridged) We investigate the impact of radiative feedback from massive stars
on their natal cloud and focus on the transition from the HII region to the
atomic PDR (crossing the ionisation front (IF)), and the subsequent transition
to the molecular PDR (crossing the dissociation front (DF)). We use
high-resolution near-IR integral field spectroscopic data from NIRSpec on JWST
to observe the Orion Bar PDR as part of the PDRs4All JWST Early Release Science
Program. The NIRSpec data reveal a forest of lines including, but not limited
to, HeI, HI, and CI recombination lines, ionic lines, OI and NI fluorescence
lines, Aromatic Infrared Bands (AIBs including aromatic CH, aliphatic CH, and
their CD counterparts), CO2 ice, pure rotational and ro-vibrational lines from
H2, and ro-vibrational lines HD, CO, and CH+, most of them detected for the
first time towards a PDR. Their spatial distribution resolves the H and He
ionisation structure in the Huygens region, gives insight into the geometry of
the Bar, and confirms the large-scale stratification of PDRs. We observe
numerous smaller scale structures whose typical size decreases with distance
from Ori C and IR lines from CI, if solely arising from radiative recombination
and cascade, reveal very high gas temperatures consistent with the hot
irradiated surface of small-scale dense clumps deep inside the PDR. The H2
lines reveal multiple, prominent filaments which exhibit different
characteristics. This leaves the impression of a "terraced" transition from the
predominantly atomic surface region to the CO-rich molecular zone deeper in.
This study showcases the discovery space created by JWST to further our
understanding of the impact radiation from young stars has on their natal
molecular cloud and proto-planetary disk, which touches on star- and planet
formation as well as galaxy evolution.
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astro-ph
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Spherical Harmonic Analysis of the Angular Distribution of Gamma-Ray
Bursts: We compute the angular power spectrum C_l of the BATSE 3B catalog, and find
no evidence for clustering on any scale. These constraints bridge the entire
range from small scales, probing source clustering and repetition, to large
scales constraining possible Galactic anisotropies, or those from nearby
cosmological large scale structures.
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astro-ph
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High Energy Astroparticle Physics: We give a brief (and highly incomplete) overview of the current experimental
and theoretical status of high energy cosmic rays and their secondary
gamma-rays and neutrinos. We focus on the role of large scale magnetic fields
and on multi-messenger aspects linking these three channels. We also recall
that the flavor composition of neutrino fluxes from astrophysical sources
contains information on both the source conditions and neutrino physics.
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astro-ph
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Stellar streams from black hole-rich star clusters: Nearly a hundred progenitor-less, thin stellar streams have been discovered
in the Milky Way, thanks to Gaia and related surveys. Most streams are believed
to have formed from star clusters and it was recently proposed that extended
star clusters -- rich in stellar-mass black holes (BHs) -- are efficient in
creating streams. To understand the nature of stream progenitors better, we
quantify the differences between streams originating from star clusters with
and without BHs using direct $N$-body models and a new model for the density
profiles of streams based on time-dependent escape rates from clusters. The QSG
(Quantifying Stream Growth) model facilitates the rapid exploration of
parameter space and provides an analytic framework to understand the impact of
different star cluster properties and escape conditions on the structure of
streams. Using these models it is found that, compared to streams from BH-free
clusters on the same orbit, streams of BH-rich clusters: (1) are approximately
five times more massive; (2) have a peak density three times closer to the
cluster 1 Gyr post-dissolution (for orbits of Galactocentric radius > 10 kpc),
and (3) have narrower peaks and more extended wings in their density profile.
We discuss other observable stream properties that are affected by the presence
of BHs in their progenitor cluster, namely the width of the stream, its radial
offset from the orbit, and the properties of the gap at the progenitor's
location. Our results provide a step towards using stellar streams to constrain
the BH content of dissolved (globular) star clusters.
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astro-ph
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Engineering physics of superconducting hot-electron bolometer mixers: Superconducting hot-electron bolometers are presently the best performing
mixing devices for the frequency range beyond 1.2 THz, where good quality
superconductor-insulator-superconductor (SIS) devices do not exist. Their
physical appearance is very simple: an antenna consisting of a normal metal,
sometimes a normal metal-superconductor bilayer, connected to a thin film of a
narrow, short superconductor with a high resistivity in the normal state. The
device is brought into an optimal operating regime by applying a dc current and
a certain amount of local- oscillator power. Despite this technological
simplicity its operation has been found to be controlled by many different
aspects of superconductivity, all occurring simultaneously. A core ingredient
is the understanding that there are two sources of resistance in a
superconductor: a charge conversion resistance occurring at an
normal-metal-superconductor interface and a resistance due to time- dependent
changes of the superconducting phase. The latter is responsible for the actual
mixing process in a non-uniform superconducting environment set up by the
bias-conditions and the geometry. The present understanding indicates that
further improvement needs to be found in the use of other materials with a
faster energy-relaxation rate. Meanwhile several empirical parameters have
become physically meaningful indicators of the devices, which will facilitate
the technological developments.
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astro-ph
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Choked accretion: from radial infall to bipolar outflows by breaking
spherical symmetry: Steady state, spherically symmetric accretion flows are well understood in
terms of the Bondi solution. Spherical symmetry however, is necessarily an
idealized approximation to reality. Here we explore the consequences of
deviations away from spherical symmetry, first through a simple analytic model
to motivate the physical processes involved, and then through hydrodynamical,
numerical simulations of an ideal fluid accreting onto a Newtonian gravitating
object. Specifically, we consider axisymmetric, large-scale, small amplitude
deviations in the density field such that the equatorial plane is over dense as
compared to the polar regions. We find that the resulting polar density
gradient dramatically alters the Bondi result and gives rise to steady state
solutions presenting bipolar outflows. As the density contrast increases, more
and more material is ejected from the system, attaining speeds larger than the
local escape velocities for even modest density contrasts. Interestingly,
interior to the outflow region, the flow tends locally towards the Bondi
solution, with a resulting total mass accretion rate through the inner boundary
$choking$ at a value very close to the corresponding Bondi one. Thus, the
numerical experiments performed suggest the appearance of a maximum achievable
accretion rate, with any extra material being ejected, even for very small
departures from spherical symmetry.
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astro-ph
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Rational Approximation Formula for Chandrasekhar's H-function for
Isotropic Scattering: We first establish a simple procedure to obtain with 11-figure accuracy the
values of Chandrasekhar's H-function for isotropic scattering using a
closed-form integral representation and the Gauss-Legendre quadrature. Based on
the numerical values of the function produced by this method for various values
of the single scattering albedo and the cosine of the azimuth angle of the
direction of radiation emergent from or incident upon a semi-infinite
scattering-absorbing medium, we propose a rational approximation formula, which
allows us to reproduce the correct values of the H-function within a relative
error of 2.1/100000 without recourse to any iterative procedure or root-finding
process.
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astro-ph
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The Voyage of Metals in the Universe from Cosmological to Planetary
Scales: the need for a Very High-Resolution, High Throughput Soft X-ray
Spectrometer: Metals form an essential part of the Universe at all scales. Without metals
we would not exist, and the Cosmos would look completely different. Metals are
primarily born through nuclear processes in stars. They leave their cradles
through winds or explosions, and then start their journey through space. This
can lead them in and out of astronomical objects on all scales, ranging from
comets, planets, stars, entire galaxies, groups and clusters of galaxies to the
largest structures of the Universe. Their wanderings are fundamental in
determining how these objects, and the entire universe, evolve. In addition,
their bare presence can be used to trace what these structures look like. The
scope of this paper is to highlight the most important open astrophysical
problems that will be central in the next decades and for which a deep
understanding of the Universe-wandering metals, their physical and kinematical
states and their chemical composition represents the only viable solution. The
majority of these studies can only be efficiently performed through High
Resolution Spectroscopy in the soft X-ray band.
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astro-ph
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Origin of the Galactic Halo: accretion vs. in situ formation: We test the hypothesis that the classical and ultra-faint dwarf spheroidal
satellites of the our Galaxy have been the building blocks of the Galactic halo
by comparing their [O/Fe] and [Ba/Fe] vs. [Fe/H] patterns with the ones
observed in Galactic halo stars. The [O/Fe] ratio deviates substantially from
the observed abundance ratios in the Galactic halo stars for [Fe/H] > -2 dex,
while they overlap for lower metallicities. On the other hand, for the neutron
capture elements, the discrepancy is extended at all the metallicities,
suggesting that the majority of stars in the halo are likely to have been
formed in situ. We present the results for a model considering the effects of
an enriched gas stripped from dwarf satellites on the chemical evolution of the
Galactic halo. We find that the resulting chemical abundances of the halo stars
depend on the adopted infall time-scale, and the presence of a threshold in the
gas for star formation.
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astro-ph
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The Cluster of Galaxies Abell 970: We present a dynamical analysis of the galaxy cluster Abell 970 based on a
new set of radial velocities measured at ESO, Pic du Midi and Haute-Provence
observatories. Our analysis indicates that this cluster has a substructure and
is out of dynamical equilibrium. This conclusion is also supported by
differences in the positions of the peaks of the surface density distribution
and X-ray emission, as well as by the evidence of a large scale velocity
gradient in the cluster. We also found a discrepancy between the masses
inferred with the virial theorem and with the X-ray emission, what is expected
if the galaxies and the gas inside the cluster are not in hydrostatic
equilibrium. Abell 970 has a modest cooling flow, as is expected if it is out
of equilibrium as suggested by Allen (1998). We propose that cooling flows may
have an intermittent behavior, with phases of massive cooling flows being
followed by phases without significant cooling flows after the acretion of a
galaxy group massive enough to disrupt the dynamical equilibrium in the center
of the clusters. A massive cooling flow will be established again, after a new
equilibrium is achieved.
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astro-ph
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Continuum and line modelling of discs around young stars. I. 300000 disc
models for Herschel/GASPS: We have combined the thermo-chemical disc code ProDiMo with the Monte Carlo
radiative transfer code MCFOST to calculate a grid of ~300000 circumstellar
disc models, systematically varying 11 stellar, disc and dust parameters
including the total disc mass, several disc shape parameters and the
dust-to-gas ratio. For each model, dust continuum and line radiative transfer
calculations are carried out for 29 far IR, sub-mm and mm lines of [OI], [CII],
12CO and o/p-H2O under 5 inclinations. The grid allows to study the influence
of the input parameters on the observables, to make statistical predictions for
different types of circumstellar discs, and to find systematic trends and
correlations between the parameters, the continuum fluxes, and the line fluxes.
The model grid, comprising the calculated disc temperatures and chemical
structures, the computed SEDs, line fluxes and profiles, will be used in
particular for the data interpretation of the Herschel open time key programme
GASPS. The calculated line fluxes show a strong dependence on the assumed UV
excess of the central star, and on the disc flaring. The fraction of models
predicting [OI] and [CII] fine-structure lines fluxes above Herschel/PACS and
Spica/SAFARI detection limits are calculated as function of disc mass. The
possibility of deriving the disc gas mass from line observations is discussed.
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astro-ph
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The near-infrared size-luminosity relations for Herbig Ae/Be disks: We report the results of a sensitive K-band survey of Herbig Ae/Be disk sizes
using the 85-m baseline Keck Interferometer. Targets were chosen to span the
maximum range of stellar properties to probe the disk size dependence on
luminosity and effective temperature. For most targets, the measured
near-infrared sizes (ranging from 0.2 to 4 AU) support a simple disk model
possessing a central optically-thin (dust-free) cavity, ringed by hot dust
emitting at the expected sublimation temperatures (T_sub~1000-1500K).
Furthermore, we find a tight correlation of disk size with source luminosity R
propto L^(1/2) for Ae and late Be systems (valid over more than 2 decades in
luminosity), confirming earlier suggestions based on lower-quality data.
Interestingly, the inferred dust-free inner cavities of the highest luminosity
sources (Herbig B0-B3 stars) are under-sized compared to predictions of the
optically-thin cavity model, likely due to optically-thick gas within the inner
AU.
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astro-ph
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Time resolved spectroscopy of the multiperiodic pulsating subdwarf B
star PG1605+072: We present results for the 2m spectroscopic part of the MultiSite
Spectroscopic Telescope campaign, which took place in May/June 2002. In order
to perform an asteroseismological analysis on the multiperiodic pulsating
subdwarf B star PG 1605+072 we used over 150 hours of time resolved
spectroscopy in order to search for and analyse line profile variations by
using phase binning. We succeeded in finding variations in effective
temperature and gravity for four modes. A pilot analysis using the
\textit{BRUCE} and \textit{KYLIE} programs and assuming strong rotation and low
inclination favours models with $l=1$ or $l=2$ with $m\leq0$.
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astro-ph
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Unveiling the evolutionary state of three B supergiant stars: PU Gem,
$ε$ CMa and $η$ CMa: We aim to combine asteroseismology, spectroscopy, and evolutionary models to
establish a comprehensive picture of the evolution of Galactic blue supergiant
stars (BSG). To start such an investigation, we selected three BSG candidates
for our analysis: HD 42087 (PU Gem), HD 52089 ($\epsilon$ CMa) and HD 58350
($\eta$ CMa). These stars show pulsations and were suspected to be in an
evolutionary stage either preceding or succeding the red supergiant (RSG)
stage.
For our analysis, we utilized the 2-min cadence TESS data to study the
photometric variability and obtained new spectroscopic observations at the
CASLEO observatory. We calculated CMFGEN non-LTE radiative transfer models and
derived stellar and wind parameters using the iterative spectral analysis
pipeline XTGRID. The spectral modeling was limited to changing only the
effective temperature, surface gravity, CNO abundances, and mass-loss rates.
Finally, we compared the derived metal abundances with predictions from Geneva
stellar evolution models. The frequency spectra of all three stars show either
stochastic oscillations, nonradial strange modes, or a rotational splitting.
We conclude that the rather short sectoral observing windows of TESS prevent
establishing a reliable mode identification of low frequencies connected to
mass-loss variabilities. The spectral analysis confirmed gradual changes in the
mass-loss rates and the derived CNO abundances comply with the values reported
in the literature. We were able to achieve a quantitative match with stellar
evolution models for the stellar masses and luminosities. However, the
spectroscopic surface abundances turned out to be inconsistent with theoretical
predictions. The stars show N enrichment, typical for CNO cycle processed
material, but the abundance ratios do not reflect the associated levels of C
and O depletion.
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astro-ph
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The Oxygen Bottleneck for Technospheres: As oxygen is essential for respiration and metabolism for multicellular
organisms on Earth, its presence may be crucial for the development of a
complex biosphere on other planets. And because life itself, through
photosynthesis, contributed to creating our oxygen-rich atmosphere, oxygen has
long been considered as a possible biosignature. Here we consider the
relationship between atmospheric oxygen and the development of technology. We
argue that only planets with substantial oxygen partial pressure ($p_{\rm
O_2}$) will be capable of developing advanced technospheres and hence
technosignatures that we can detect. But open-air combustion (needed, for
example, for metallurgy), is possible only in Earth-like atmospheres when
$p_{\rm O_2}\ge 18\%$. This limit is higher than the one needed to sustain a
complex biosphere and multicellular organisms. We further review other possible
planetary atmospheric compositions and conclude that oxygen is the most likely
candidate for the evolution of technological species. Thus, the presence of
$p_{\rm O_2}\ge 18\%$ in exoplanet atmospheres may represent a contextual prior
required for the planning and interpretation of technosignature searches.
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astro-ph
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Coupling dark-baryonic matter density profile for vacuum decay scenarios: The cosmological consequences of an interacting model in which vacuum decay
law is deducted from the effect that vacuum decay has on the dark matter
evolution are investigated. Here, the baryonic matter is also considered as a
fluid gravitationally coupled with dark matter. It is made a careful analysis
to constrain this model with the observational data of growth rate of cosmic
structures. The theoretical growth rate is followed since the primordial
recombination and the main physical processes on the baryonic component are
considered. As a complementary constraint, this model is compared with the
observed CMB-BAO ratio as well with the gas mass fraction of cluster of
galaxies. We found the best fit values for dark matter $\Omega_{d0} = 0.269
^{+0.023}_{-0.023}$ and for the decay parameter $\epsilon = 0.02
^{+0.04}_{-0.05}$.
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astro-ph
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Epicyclic frequencies for rotating strange quark stars: the importance
of stellar oblateness: Kilohertz QPOs can be used as a probe of the inner regions of accretion disks
in compact stars and hence also of the properties of the central object. Most
models of kHz QPOs involve epicyclic frequencies to explain their origin. We
compute the epicyclic frequencies of nearly circular orbits around rotating
strange quark stars. The MIT bag model is used to model the equation of state
of quark matter and the uniformly rotating stellar configurations are computed
in full general relativity. The vertical epicyclic frequency and the related
nodal precession rate of inclined orbits are very sensitive to the oblateness
of the rotating star. For slowly rotating stellar models of moderate and high
mass strange stars, the sense of the nodal precession changes at a certain
rotation rate. At lower stellar rotation rates the orbital nodal precession is
prograde, as it is in the Kerr metric, while at higher rotation rates the
precession is retrograde, as it is for Maclaurin spheroids. Thus,
qualitatively, the orbits around rapidly rotating strange quark stars are
affected more strongly by the effects of stellar oblateness than by the effects
of general relativity. We show that epicyclic and orbital frequencies
calculated numerically for small mass strange stars are in very good agreement
with analytical formulae for Maclaurin spheroids.
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astro-ph
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Spatial correlation function and pairwise velocity dispersion of
galaxies: CDM models versus the Las Campanas Survey: We show, with the help of large N-body simulations, that the real-space
two-point correlation function and pairwise velocity dispersion of galaxies can
both be measured reliably from the Las Campanas Redshift Survey. The real-space
correlation function is well fitted by the power law $\xi(r)=(r_0/r)^\gamma$
with $r_0=(5.06\pm0.12)\mpc$ and $\gamma=1.862\pm 0.034$, and the pairwise
velocity dispersion at $1\mpc$ is $(570\pm 80)\kms$. A detailed comparison
between these observational results and the predictions of current CDM
cosmogonies is carried out. We construct 60 mock samples for each theoretical
model from a large set of high resolution N-body simulations, which allows us
to include various observational selection effects in the analyses and to use
exactly the same methods for both real and theoretical samples. We demonstrate
that such a procedure is essential in the comparison between models and
observations. The observed two-point correlation function is significantly
flatter than the mass correlation function in current CDM models on scales $\la
1\mpc$. The observed pairwise velocity dispersion is also lower than that of
dark matter particles in these models. We propose a simple antibias model to
explain these discrepancies. This model assumes that the number of galaxies per
unit dark matter mass, $N/M$, decreases with the mass of dark haloes. The
predictions of CDM models with $\sigma_8\Omega_0^{0.6}\sim 0.4$-0.5 and
$\Omega_0 h\sim 0.2$ are in agreement with the observational results, if the
trend of $N/M$ with $M$ is at the level already observed for rich clusters of
galaxies. Thus CDM models with cluster-abundance normalization are consistent
with the observed correlation function and pairwise velocity dispersion of
galaxies. A high level of velocity bias is not required in these models.
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astro-ph
|
Balmer jump variations of HD 215441 (Babcock's star): Balmer jump discontinuity of the peculiar star HD 215441 was measured on 14
continuous energy distributions obtained by S. J. Adelman at Kitt Peak National
and Palomar Observatories. The results show that the Balmer jump vary by about
0.05 dex over the cycle of the star. A comparison with new Kurucz model
atmospheres shown that the Balmer jump for the phase 0.5 corresponds to a 14750
K, log(g)=3.0 and for the phase 1.0 corresponds to a 15750 K, log(g)=3.0.
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astro-ph
|
The dynamics of eccentric accretion discs in superhump systems: We have applied an eccentric accretion disc theory in simplified form to the
case of an accretion disc in a binary system, where the disc contains the 3:1
Lindblad resonance. This is relevant to the case of superhumps in SU Ursae
Majoris cataclysmic variables and other systems, where it is thought that this
resonance leads to growth of eccentricity and a modulation in the light curve
due to the interaction of a precessing eccentric disc with tidal stresses. A
single differential equation is formulated which describes the propagation,
resonant excitation and viscous damping of eccentricity. The theory is first
worked out in the simple case of a narrow ring and leads to the conclusion that
the eccentricity distribution is locally suppressed by the presence of the
resonance, creating a dip in the eccentricity at the resonant radius.
Application of this theory to the superhump case confirms this conclusion and
produces a more accurate expression for the precession rate of the disc than
has been previously accomplished with simple dynamical estimates.
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astro-ph
|
The habitability of a stagnant-lid Earth: Plate tectonics is a fundamental component for the habitability of the Earth.
Yet whether it is a recurrent feature of terrestrial bodies orbiting other
stars or unique to the Earth is unknown. The stagnant lid may rather be the
most common tectonic expression on such bodies. To understand whether a
stagnant-lid planet can be habitable, i.e. host liquid water at its surface, we
model the thermal evolution of the mantle, volcanic outgassing of H$_2$O and
CO$_2$, and resulting climate of an Earth-like planet lacking plate tectonics.
We used a 1D model of parameterized convection to simulate the evolution of
melt generation and the build-up of an atmosphere of H$_2$O and CO$_2$ over 4.5
Gyr. We then employed a 1D radiative-convective atmosphere model to calculate
the global mean atmospheric temperature and the boundaries of the habitable
zone (HZ). The evolution of the interior is characterized by the initial
production of a large amount of partial melt accompanied by a rapid outgassing
of H$_2$O and CO$_2$. At 1 au, the obtained temperatures generally allow for
liquid water on the surface nearly over the entire evolution. While the outer
edge of the HZ is mostly influenced by the amount of outgassed CO$_2$, the
inner edge presents a more complex behaviour that is dependent on the partial
pressures of both gases. At 1 au, the stagnant-lid planet considered would be
regarded as habitable. The width of the HZ at the end of the evolution, albeit
influenced by the amount of outgassed CO$_2$, can vary in a non-monotonic way
depending on the extent of the outgassed H$_2$O reservoir. Our results suggest
that stagnant-lid planets can be habitable over geological timescales and that
joint modelling of interior evolution, volcanic outgassing, and accompanying
climate is necessary to robustly characterize planetary habitability.
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astro-ph
|
The IceCube Neutrino Observatory Part V: Neutrino Oscillations and
Supernova Searches: Papers on neutrino oscillation and supernova searches submitted to the 33nd
International Cosmic Ray Conference (Rio de Janeiro 2013) by the IceCube
Collaboration.
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astro-ph
|
An analysis of the FIR/RADIO Continuum Correlation in the Small
Magellanic Cloud: The local correlation between far-infrared (FIR) emission and radio-continuum
(RC) emission for the Small Magellanic Cloud (SMC) is investigated over scales
from 3 kpc to 0.01 kpc. Here, we report good FIR/RC correlation down to ~15 pc.
The reciprocal slope of the FIR/RC emission correlation (RC/FIR) in the SMC is
shown to be greatest in the most active star forming regions with a power law
slope of ~1.14 indicating that the RC emission increases faster than the FIR
emission. The slope of the other regions and the SMC are much flatter and in
the range of 0.63-0.85. The slopes tend to follow the thermal fractions of the
regions which range from 0.5 to 0.95. The thermal fraction of the RC emission
alone can provide the expected FIR/RC correlation. The results are consistent
with a common source for ultraviolet (UV) photons heating dust and Cosmic Ray
electrons (CRe-s) diffusing away from the star forming regions. Since the CRe-s
appear to escape the SMC so readily, the results here may not provide support
for coupling between the local gas density and the magnetic field intensity.
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astro-ph
|
Imprint of a 2 Myr old source on the cosmic ray anisotropy: We study numerically the anisotropy of the cosmic ray (CR) flux emitted by a
single source calculating the trajectories of individual CRs. We show that the
contribution of a single source to the observed anisotropy is instead
determined solely by the fraction the source contributes to the total CR
intensity, its age and its distance,and does not depend on the CR energy at
late times. Therefore the observation of a constant dipole anisotropy indicates
that a single source dominates the CR flux in the corresponding energy range. A
natural explanation for the plateau between 2--20 TeV observed in the CR
anisotropy is thus the presence of a single, nearby source. For the source age
of 2 Myr, as suggested by the explanation of the antiproton and positron data
from PAMELA and AMS-02 through a local source [arXiv:astro-ph/1504.06472], we
determine the source distance as $\sim 200$ pc. Combined with the contribution
of the global CR sea calculated in the escape model, we can explain
qualitatively the data for the dipole anisotropy. Our results suggest that the
assumption of a smooth CR source distribution should be abandoned between 200
GeV and 1 PeV.
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astro-ph
|
Hall drift and the braking indices of young pulsars: Braking index measurements of young radio pulsars are all smaller than the
value expected for spin down by magnetic dipole braking. We investigate
magnetic field evolution in the neutron star crust due to Hall drift as an
explanation for observed braking indices. Using numerical simulations and a
semi-analytic model, we show that a $\approx 10^{14}\ {\rm G}$ quadrupolar
toroidal field in the neutron star crust at birth leads to growth of the dipole
moment at a rate large enough to agree with measured braking indices. A key
factor is the density at which the crust yields to magnetic stresses that build
up during the evolution, which sets a characteristic minimum Hall timescale.
The observed braking indices of pulsars with inferred dipole fields of
$\lesssim 10^{13}\ {\rm G}$ can be explained in this picture, although with a
significant octupole component needed in some cases. For the stronger field
pulsars, those with $B_d\gtrsim 10^{13}\ {\rm G}$, we find that the magnetic
stresses in the crust exceed the maximum shear stress before the pulsar reaches
its current age, likely quenching the Hall effect. This may have implications
for the magnetar activity seen in the high magnetic field radio pulsar
PSR~J1846-0258. Observations of braking indices may therefore be a new piece of
evidence that neutron stars contain subsurface toroidal fields that are
significantly stronger than the dipole field, and may indicate that the Hall
effect is important in a wider range of neutron stars than previously thought.
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astro-ph
|
Elastic and breaking properties of epitaxial face-centered crystals in
neutron star crusts and white dwarf cores: Crystallization of dense matter in neutron star crusts and white dwarf cores
may be similar to epitaxial crystal growth in terrestrial laboratories. However
in stellar crystals, the spacing between horizontal planes has to gradually
increase with the outward movement of the crystallization front, tracing
decrease of the electron density. This process produces Coulomb crystals with
stretched rather than cubic elementary cells. We extend the analysis of the
elastic and breaking properties of such crystals to the face-centered (fc)
lattice. Shear deformations orthogonal to the stretch direction have been
studied for 22 crystallographic shear planes. A common property for all these
planes is a reduction and eventual nulling of the breaking shear strain with
deviation from the unstretched configuration. The effective shear moduli for
deformations orthogonal to the stretch direction have been calculated. It is
possible that the epitaxial crystallization in compact stars results in a
formation of large-scale crystallites or, at least, in growth of the whole
crystallization front perpendicular to particular crystallographic planes. For
fc structure growth orthogonal to the $\{111\}$ planes, we expect that, at any
density, $\sim 5\%$ ($\sim 0.5\%$) of crystallite height is occupied by layers
one (two) orders of magnitude weaker than the bulk of the crystallite. This may
be important for realistic modeling of crustquakes on neutron stars.
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astro-ph
|
Faint End of 1.3 mm Number Counts Revealed by ALMA: We present the faint end of number counts at 1.3 mm (238 GHz) obtained with
the Atacama Large Millimeter/submillimeter Array (ALMA). Band 6 observations
were carried out targeting 20 star-forming galaxies at z ~ 1.4 in the
Subaru/XMM-Newton Deep Survey field. In the observations, we serendipitously
detect 15 sources (>=3.8 sigma, S(1.3 mm) = 0.15-0.61 mJy) other than the
targeted sources. We create number counts by using these `sub-mJy sources',
which probe the faintest flux range among surveys at millimeter wavelengths.
The number counts are consistent with (flux-scaled) number counts at 850 um and
870 um obtained with gravitational lensing clusters. The ALMA number counts
agree well with model predictions, which suggest that these sub-mJy populations
are more like `normal' star-forming galaxies than `classical' SMGs with intense
star-forming activity. In this flux range, ~80% of the extragalactic background
light at 1.3 mm is resolved into individual sources.
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astro-ph
|
Lyot-based Ultra-Fine Pointing Control System for Phase Mask
Coronagraphs: High performance coronagraphic imaging at small inner working angle requires
efficient control of low order aberrations. The absence of accurate pointing
control at small separation not only degrades coronagraph starlight rejection
but also increases the risk of confusing planet's photons with starlight
leaking next to the coronagraph focal plane mask center. Addressing this issue
is essential for preventing coronagraphic leaks, and we have thus developed a
new concept, the Lyot-based pointing control system (LPCS), to control pointing
errors and other low order aberrations within a coronagraph. The LPCS uses
residual starlight reflected by the Lyot stop at the pupil plane. Our
simulation has demonstrated pointing errors measurement accuracy between 2-12
nm for tip-tilt at 1.6 micron with a four quadrant phase mask coronagraph.
|
astro-ph
|
On the accuracy of N-body simulations at very large scales: We examine the deviation of Cold Dark Matter particle trajectories from the
Newtonian result as the size of the region under study becomes comparable to or
exceeds the particle horizon. To first order in the gravitational potential,
the general relativistic result coincides with the Zel'dovich approximation and
hence the Newtonian prediction on all scales. At second order, General
Relativity predicts corrections which overtake the corresponding second order
Newtonian terms above a certain scale of the order of the Hubble radius.
However, since second order corrections are very much suppressed on such
scales, we conclude that simulations which exceed the particle horizon but use
Newtonian equations to evolve the particles, reproduce the correct trajectories
very well. The dominant relativistic corrections to the power spectrum on
scales close to the horizon are at most of the order of $\sim 10^{-5}$ at
$z=49$ and $\sim 10^{-3}$ at $z=0$. The differences in the positions of real
space features are affected at a level below $10^{-6}$ at both redshifts. Our
analysis also clarifies the relation of N-body results to relativistic
considerations.
|
astro-ph
|
The immediate environment of the Class 0 protostar VLA1623, on scales of
~50-100 AU, observed at millimetre and centimetre wavelengths: We present high angular resolution observations, taken with the Very Large
Array (VLA) and Multiple Element Radio Linked Interferometer Network (MERLIN)
radio telescopes, at 7mm and 4.4cm respectively, of the prototype Class 0
protostar VLA1623. At 7mm we detect two sources (VLA1623A & B) coincident with
the two previously detected components at the centre of this system. The
separation between the two is 1.2arcsec, or ~170AU at an assumed distance of
139pc. The upper limit to the size of the source coincident with each component
of VLA1623 is ~0.7arcsec, in agreement with previous findings. This corresponds
to a diameter of ~100AU at an assumed distance of 139pc. Both components show
the same general trend in their broadband continuum spectra, of a steeper dust
continuum spectrum shortward of 7mm and a flatter spectrum longward of this.
We estimate an upper limit to the VLA1623A disc mass of <0.13Msol and an
upper limit to its radius of ~50AU. The longer wavelength data have a spectral
index of \alpha~0.6+/-0.3. This is too steep to be explained by optically thin
free-free emission. It is most likely due to optically thick free-free
emission. Alternatively, we speculate that it might be due to the formation of
larger grains or planetesimals in the circumstellar disc. We estimate the mass
of VLA1623B to be <0.15M$sol. We can place a lower limit to its size of ~30x7
AU, and an upper limit to its diameter of ~100AU. The longer wavelength data of
VLA1623B also have a spectral index of \alpha~0.6+/-0.3. The nature of VLA1623B
remains a matter of debate. It could be a binary companion to the protostar, or
a knot in the radio jet from VLA1623A.
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astro-ph
|
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