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Self-consistent Mean Field theory in weakly ionized media: We present a self-consistent mean field theory of the dynamo in 3D and
turbulent diffusion in 2D in weakly ionized gas. We find that in 3D, the
backreaction does not alter the beta effect while it suppresses the alpha
effect when the strength of a mean magnetic field exceeds a critical value.
These results suggest that a mean field dynamo operates much more efficiently
in weakly ionized gas compared to the fully ionized gas. Furthermore, we show
that in 2D, the turbulent diffusion is suppressed by back reaction when a mean
magnetic field reaches the same critical strength, with the upper bound on
turbulent diffusion given by its kinematic value. Astrophysical implications
are discussed. | astro-ph |
Negative Energy Cosmology and the Cosmological Constant: It is well known that string theories naturally compactify on anti-de Sitter
spaces, and yet cosmological observations show no evidence of a negative
cosmological constant in the early Universe's evolution. In this letter we
present two simple nonlocal modifications of the standard Friedmann cosmology
that can lead to observationally viable cosmologies with an initial (negative)
cosmological constant. The nonlocal operators we include are toy models for the
quantum cosmological backreaction. In Model I an initial quasiperiodic
oscillatory epoch is followed by inflation and a late time matter era,
representing a dark matter candidate. The backreaction in Model II quickly
compensates the negative cosmological term such that the Ricci curvature scalar
rapidly approaches zero, and the Universe ends up in a late time radiation era. | astro-ph |
The Most Massive White Dwarfs in the Solar Neighborhood: We present an analysis of the most massive white dwarf candidates in the
Montreal White Dwarf Database 100 pc sample. We identify 25 objects that would
be more massive than $1.3~M_{\odot}$ if they had pure H atmospheres and CO
cores, including two outliers with unusually high photometric mass estimates
near the Chandrasekhar limit. We provide follow-up spectroscopy of these two
white dwarfs and show that they are indeed significantly below this limit. We
expand our model calculations for CO core white dwarfs up to $M=1.334\
M_\odot$, which corresponds to the high-density limit of our equation-of-state
tables, $\rho = 10^9$ g cm$^{-3}$. We find many objects close to this maximum
mass of our CO core models. A significant fraction of ultramassive white dwarfs
are predicted to form through binary mergers. Merger populations can reveal
themselves through their kinematics, magnetism, or rapid rotation rates. We
identify four outliers in transverse velocity, four likely magnetic white
dwarfs (one of which is also an outlier in transverse velocity), and one with
rapid rotation, indicating that at least 8 of the 25 ultramassive white dwarfs
in our sample are likely merger products. | astro-ph |
Optical identification of the transient supersoft X-ray source RX
J0527.8-6954, in the LMC: Context. Close binary supersoft X-ray sources (CBSS) are binary systems that
contain a white dwarf with stable nuclear burning on its surface. These
sources, first discovered in the Magellanic Clouds, have high accretion rates
and near-Eddington luminosities (10^37 - 10^38 erg/s) with high temperatures (T
= 2 - 7 x 10^5 K). Aims. The total number of known objects in the MC is still
small and, in our galaxy, even smaller. We observed the field of the
unidentified transient supersoft X-ray source RX J0527.8-6954 in order to
identify its optical counterpart. Methods. The observation was made with the
IFU-GMOS on the Gemini South telescope with the purpose of identifying stars
with possible He II or Balmer emission or else of observing nebular extended
jets or ionization cones, features that may be expected in CBSS. Results. The
X-ray source is identified with a B5e V star that is associated with
subarcsecond extended Halpha emission, possibly bipolar. Conclusions. If the
primary star is a white dwarf, as suggested by the supersoft X-ray spectrum,
the expected orbital period exceeds 21 h; therefore, we believe that the 9.4 h
period found so far is not associated to this system. | astro-ph |
Horizontally Polarized Kink Oscillations Supported by Solar Coronal
Loops in an Asymmetric Environment: Kink oscillations are ubiquitously observed in solar coronal loops, their
understanding being crucial in the contexts of coronal seismology and
atmospheric heating. We study kink modes supported by a straight coronal loop
embeded in an asymmetric environment using three-dimensional
magnetohydrodynamic (MHD) simulations. We implement the asymmetric effect by
setting different exterior densities below and above the loop interior, and
initiate the simulation using a kink-like velocity perturbation perpendicular
to the loop plane, mimicking the frequently measured horizontally polarized
kink modes. We find that the external velocity fields show fan blade structures
propagating in the azimuthal direction as a result of the successive excitation
of higher azimuthal Fourier modes. Resonant absorption and phase mixing can
still occur despite an asymmetric environment, leading to the development of
small scales at loop boundaries. These small scales nonetheless develop
asymmetrically at the upper and lower boundaries due to the different gradients
of the Alfven speed. These findings enrich our understanding of kink modes in
coronal loops embedded within an asymmetric environment, providing insights
helpful for future high-resolution observations. | astro-ph |
Preliminary Spectral Analysis of SN 1994I: We present optical spectra of the Type Ic supernova 1994I in M51 and
preliminary non-LTE analysis of the spectra. Our models are not inconsistent
with the explosions of C+O cores of massive stars. While we find no direct
evidence for helium in the optical spectra, our models cannot rule out small
amounts of helium. More than 0.1~\msol\ of helium seems unlikely. | astro-ph |
Mechanisms and sites of ultra high energy cosmic ray origin: We shortly discuss several astrophysical scenarios leading to cosmic ray
acceleration up to extremely high energies reaching the scale of 10^{20} eV.
The processes suggested in the literature include acceleration at relativistic
jet terminal shocks and shear boundary layers, shocks in large scale accretion
flows onto supergalactic cosmic structures, particle reflections from
ultra-relativistic shocks postulated to exist in sources of gamma ray bursts,
the processes involving the neutron star rotating magnetospheres and dormant
quasars. Some of these objects can explain cosmic rays with highest energies if
one tunes the model parameters to limits enabling the highest acceleration
efficiency. We also note that some of the considered processes allow for
acceleration efficiency in the Hillas diagram, beta, to be much larger than
unity. The present paper is based on a review talk presented during the
European Cosmic Ray Symposium in Lodz (2000). | astro-ph |
Tensor to scalar ratio and large scale power suppression from pre-slow
roll initial conditions: We study corrections to the power spectra of curvature and tensor
perturbations and the tensor-to-scalar ratio in single field slow roll
inflation due to initial conditions imprinted by a fast-roll stage prior to
slow roll. For a wide range of initial inflaton kinetic energy, this stage
lasts only a few e-folds and merges smoothly with slow-roll leading to
non-Bunch-Davies initial conditions for modes that exit the Hubble radius
during slow roll. We describe a program that yields the dynamics in the
fast-roll stage while matching to the slow roll stage independent of the
inflationary potentials. Corrections to the power spectra are encoded in a
transfer function $\mathcal{T}_\alpha(k)$. We obtain $\mathcal{T}_\alpha(k)$ to
leading order in a Born approximation valid for modes of observational
relevance today. A fit yields $\mathcal{T}_\alpha(k) =1+
A_{\alpha}k^{-p}\cos[2\pi \omega k/H_{sr}+\varphi_\alpha]$, with $1.5 \lesssim
p \lesssim 2$, $\omega \simeq 1$ and $H_{sr}$ the Hubble scale during slow roll
inflation, where curvature and tensor perturbations feature the same $p,\omega$
for a wide range of initial conditions. These corrections lead to both a
suppression of the quadrupole and oscillatory features in both $P_R(k)$ and
$r(k_0)$ with a period of the order of the Hubble scale during slow roll
inflation. The results are independent of the specific inflationary potentials,
depending solely on the ratio of kinetic to potential energy $\kappa$ and the
slow roll parameters to leading order. For a wide range of $\kappa$ and the
values of $\epsilon_V;\eta_V$ corresponding to the upper bounds from Planck, we
find that the low quadrupole is consistent with the results from Planck, and
the oscillations in $r(k_0)$ could be observable if the modes corresponding to
the quadrupole and the pivot scale crossed the Hubble radius a few e-folds
after the onset of slow roll. | astro-ph |
SPITZER Observations of z~3 Lyman Break Galaxies: stellar masses and
mid-infrared properties: We describe the spectral energy distributions (SEDs) of Lyman Break Galaxies
(LBGs) at z~3 using deep mid-infrared and optical observations of the Extended
Groth Strip, obtained with IRAC and MIPS on board Spitzer and from the ground,
respectively. We focus on LBGs with detections at all four IRAC bands, in
particular the 26 galaxies with IRAC 8 micron band (rest--frame K-band)
detections. We use stellar population synthesis models and probe the stellar
content of these galaxies. Based on best--fit continuous star-formation models
we derive estimates of the stellar mass for these LBGs. As in previous studies,
we find that a fraction of LBGs have very red colors and large estimated
stellar masses (M_stellar > 5 x 10 ^{10} M_solar): the present Spitzer data
allow us, for the first time, to study these massive LBGs in detail. We discuss
the link between these LBGs and submm-luminous galaxies.We find that the number
density of these massive LBGs at high redshift is higher than predicted by
current semi-analytic models of galaxy evolution. | astro-ph |
Halo Mass Function and the Free Streaming Scale: The nature of structure formation around the particle free streaming scale is
still far from understood. Many attempts to simulate hot, warm, and cold dark
matter cosmologies with a free streaming cutoff have been performed with
cosmological particle-based simulations, but they all suffer from spurious
structure formation at scales below their respective free streaming scales --
i.e. where the physics of halo formation is most affected by free streaming. We
perform a series of high resolution numerical simulations of different WDM
models, and develop an approximate method to subtract artificial structures in
the measured halo mass function. The corrected measurements are then used to
construct and calibrate an extended Press-Schechter (EPS) model with sharp-$k$
window function and adequate mass assignment. The EPS model gives accurate
predictions for the low redshift halo mass function of CDM and WDM models, but
it significantly under-predicts the halo abundance at high redshifts. By taking
into account the ellipticity of the initial patches and connecting the
characteristic filter scale to the smallest ellipsoidal axis, we are able to
eliminate this inconsistency and obtain an accurate mass function over all
redshifts and all dark matter particle masses covered by the simulations. As an
additional application we use our model to predict the microhalo abundance of
the standard neutralino-CDM scenario and we give the first quantitative
prediction of the mass function over the full range of scales of CDM structure
formation. | astro-ph |
Mining the gap: evolution of the magnitude gap in X-ray galaxy groups
from the 3 square degree XMM coverage of CFHTLS: We present a catalog of 129 X-ray galaxy groups, covering a redshift range
0.04<z<1.23, selected in the ~3 square degree part of the CFHTLS W1 field
overlapping XMM observations performed under the XMM-LSS project. We carry out
a statistical study of the redshift evolution out to redshift one of the
magnitude gap between the first and the second brightest cluster galaxies of a
well defined mass-selected group sample. We find that the slope of the relation
between the fraction of groups and the magnitude gap steepens with redshift,
indicating a larger fraction of fossil groups at lower redshifts. We find that
22.2$\pm$6% of our groups at z$\leq$0.6 are fossil groups. We compare our
results with the predictions of three semi-analytic models based on the
Millennium simulation. The intercept of the relation between the magnitude of
the brightest galaxy and the value of magnitude gap becomes brighter with
increasing redshift. This trend is steeper than the model predictions which we
attribute to the younger stellar age of the observed brightest cluster
galaxies. This trend argues in favor of stronger evolution of the feedback from
active galactic nuclei at z<1 compared to the models. The slope of the relation
between the magnitude of the brightest cluster galaxy and the value of the gap
does not evolve with redshift and is well reproduced by the models, indicating
that the tidal galaxy stripping, put forward as an explanation of the
occurrence of the magnitude gap, is both a dominant mechanism and is
sufficiently well modeled. | astro-ph |
Cannonballs in the context of Gamma Ray Bursts: Formation sites ?: We investigate possible formation sites of the cannonballs (in the gamma ray
bursts context) by calculating their physical parameters, such as density,
magnetic field and temperature close to the origin. Our results favor scenarios
where the cannonballs form as instabilities (knots) within magnetized jets from
hyperaccreting disks. These instabilities would most likely set in beyond the
light cylinder where flow velocity with Lorentz factors as high as 2000 can be
achieved. Our findings challenge the cannonball model of gamma ray bursts if
these indeed form inside core-collapse supernovae (SNe) as suggested in the
literature; unless hyperaccreting disks and the corresponding jets are common
occurrences in core-collapse SNe. | astro-ph |
Proper motion of gamma-rays from microhalo sources: I discuss the prospects of detecting the smallest dark matter bound
structures present in the Milky Way by searching for the proper motion of
$\gamma$-ray sources in the upcoming GLAST all sky map. I show that for dark
matter particle candidates that couple to photons the detection of at least one
$\gamma$-ray microhalo source with proper motion places a constraint on the
couplings and mass of the dark matter particle. For SUSY dark matter, proper
motion detection implies that the mass of the particle is less than 500 GeV and
the kinetic decoupling temperature is in the range of [4-100] MeV. | astro-ph |
The interactions of winds from massive young stellar objects: X-ray
emission, dynamics, and cavity evolution: 2D axis-symmetric hydrodynamical simulations are presented which explore the
interaction of stellar and disk winds with surrounding infalling cloud
material. The star, and its accompanying disk, blow winds inside a cavity
cleared out by an earlier jet. The collision of the winds with their
surroundings generates shock heated plasma which reaches temperatures up to
~10^8 K. Attenuated X-ray spectra are calculated from solving the equation of
radiative transfer along lines-of-sight. This process is repeated at various
epochs throughout the simulations to examine the evolution of the intrinsic and
attenuated flux. We find that the dynamic nature of the wind-cavity interaction
fuels intrinsic variability in the observed emission on timescales of several
hundred years. This is principally due to variations in the position of the
reverse shock which is influenced by changes in the shape of the cavity wall.
The collision of the winds with the cavity wall can cause clumps of cloud
material to be stripped away. Mixing of these clumps into the winds mass-loads
the flow and enhances the X-ray emission measure. The position and shape of the
reverse shock plays a key role in determining the strength and hardness of the
X-ray emission. In some models the reverse shock is oblique to much of the
stellar and disk outflows, whereas in others it is closely normal over a wide
range of polar angles. For reasonable stellar and disk wind parameters the
integrated count rate and spatial extent of the intensity peak for X-ray
emission agree with \textit{Chandra} observations of the deeply embedded MYSOs
S106 IRS4, Mon R2 IRS3 A, and AFGL 2591.(abridged) | astro-ph |
Transition from galactic to extragalactic cosmic rays: The study of the transition between galactic and extragalactic cosmic rays
can shed more light on the end of the Galactic cosmic rays spectrum and the
beginning of the extragalactic one. Three models of transition are discussed:
ankle, dip and mixed composition models. All these models describe the
transition as an intersection of a steep galactic component with a flat
extragalactic one. Severe bounds on these models are provided by the Standard
Model of Galactic Cosmic Rays according to which the maximum acceleration
energy for Iron nuclei is of the order of $E_{\rm Fe}^{\rm max} \approx 1\times
10^{17}$ eV. In the ankle model the transition is assumed at the ankle, a flat
feature in the all particle spectrum which observationally starts at energy
$E_a \sim (3 - 4)\times 10^{18}$ eV. This model needs a new high energy
galactic component with maximum energy about two orders of magnitude above that
of the Standard Model. The origin of such component is discussed. As
observations are concerned there are two signatures of the transition: change
of energy spectra and mass composition. In all models a heavy galactic
component is changed at the transition to a lighter or proton component. | astro-ph |
The Fermi Bubbles as a Scaled-up Version of Supernova Remnants: In this study, we treat the Fermi bubbles as a scaled-up version of supernova
remnants (SNRs). The bubbles are created through activities of the
super-massive black hole (SMBH) or starbursts at the Galactic center (GC).
Cosmic-rays (CRs) are accelerated at the forward shocks of the bubbles like
SNRs, which means that we cannot decide whether the bubbles were created by the
SMBH or starbursts from the radiation from the CRs. We follow the evolution of
CR distribution by solving a diffusion-advection equation, considering the
reduction of the diffusion coefficient by CR streaming. In this model,
gamma-rays are created through hadronic interaction between CR protons and the
gas in the Galactic halo. In the GeV band, we can well reproduce the observed
flat distribution of gamma-ray surface brightness, because some amount of gas
is left behind the shock. The edge of the bubbles is fairly sharp owing to the
high gas density behind the shock and the reduction of the diffusion
coefficient there. The latter also contributes the hard gamma-ray spectrum of
the bubbles. We find that the CR acceleration at the shock has started when the
bubbles were small, and the time-scale of the energy injection at the GC was
much smaller than the age of the bubbles. We predict that if CRs are
accelerated to the TeV regime, the apparent bubble size should be larger in the
TeV band, which could be used to discriminate our hadronic model from other
leptonic models. We also present neutrino fluxes. | astro-ph |
Spectral Energy Distributions of Companion Galaxies to z$\sim$6 Quasars: Massive, quiescent galaxies are already observed at redshift $z\sim4$, i.e.
$\sim$1.5 Gyr after the Big Bang. Current models predict them to be formed via
massive, gas-rich mergers at $z>6$. Recent ALMA observations of the cool gas
and dust in $z\gtrsim$6 quasars have discovered [CII]- and far infrared-bright
galaxies adjacent to several quasars. In this work, we present sensitive
imaging and spectroscopic follow-up observations, with HST/WFC3, Spitzer/IRAC,
VLT/MUSE, Magellan/FIRE and LBT/LUCI-MODS, of ALMA-detected, dust-rich
companion galaxies of four quasars at $z\gtrsim 6$, specifically acquired to
probe their stellar content and unobscured star formation rate. Three companion
galaxies do not show significant emission in the observed optical/IR wavelength
range. The photometric limits suggest that these galaxies are highly
dust-enshrouded, with unobscured star formation rates SFR$_{\rm UV}<$few
M$_{\odot}$ yr$^{-1}$, and a stellar content of $M_{*}<$10$^{10}$ M$_{\odot}$
yr$^{-1}$. However, the companion to PJ167-13 shows bright rest-frame UV
emission (F140W AB = 25.48). Its SED resembles that of a star-forming galaxy
with a total SFR$\sim$50 M$_{\odot}$ yr$^{-1}$ and $M_{*}\sim 9 \times 10^{9}$
M$_{\odot}$. All the companion sources are consistent with residing on the
galaxy main sequence at $z\sim$6. Additional, deeper data from future
facilities, such as JWST, are needed in order to characterize these gas-rich
sources in the first Gyr of cosmic history. | astro-ph |
The MOSDEF Survey: Neon as a Probe of ISM Physical Conditions at High
Redshift: We present results on the properties of neon emission in $z\sim2$
star-forming galaxies drawn from the MOSFIRE Deep Evolution Field (MOSDEF)
survey. Doubly-ionized neon ([NeIII]3869) is detected at $\geq3\sigma$ in 61
galaxies, representing $\sim$25% of the MOSDEF sample with H$\alpha$, H$\beta$,
and [OIII]$5007$ detections at similar redshifts. We consider the neon
emission-line properties of both individual galaxies with [NeIII]3869
detections and composite $z\sim2$ spectra binned by stellar mass. With no
requirement of [NeIII]3869 detection, the latter provide a more representative
picture of neon emission-line properties in the MOSDEF sample. The
[NeIII]3869/[OII]3727 ratio (Ne3O2) is anti-correlated with stellar mass in
$z\sim2$ galaxies, as expected based on the mass-metallicity relation. It is
also positively correlated with the [OIII]$5007$/[OII]$3727$ ratio (O32), but
$z\sim2$ line ratios are offset towards higher Ne3O2 at fixed O32, compared
with both local star-forming galaxies and individual H~II regions. Despite the
offset towards higher Ne3O2 at fixed O32 at $z\sim2$, biases in inferred
Ne3O2-based metallicity are small. Accordingly, Ne3O2 may serve as an important
metallicity indicator deep into the reionization epoch. Analyzing additional
rest-optical line ratios including [NeIII]$3869$/[OIII]$5007$ (Ne3O3) and
[OIII]$5007$/H$\beta$ (O3H$\beta$), we conclude that the nebular emission-line
ratios of $z\sim2$ star-forming galaxies suggest a harder ionizing spectrum
(lower stellar metallicity, i.e., Fe/H) at fixed gas-phase oxygen abundance,
compared to systems at $z\sim0$. These new results based on neon lend support
to the physical picture painted by oxygen, nitrogen, hydrogen, and sulfur
emission, of an ionized ISM in high-redshift star-forming galaxies irradiated
by chemically young, $\alpha$-enhanced massive stars. | astro-ph |
X-ray diagnostics of Cassiopeia A's "Green Monster": evidence for dense
shocked circumstellar plasma: The recent survey of the core-collapse supernova remnant Cassiopeia A (CasA)
with the MIRI instrument on board the James Webb Space Telescope (JWST)
revealed a large structure in the interior region, referred to as the "Green
Monster". Although its location suggests that it is an ejecta structure, the
infrared properties of the "Green Monster" hint at a circumstellar medium (CSM)
origin. In this companion paper to the JWST Cas A paper, we investigate the
filamentary X-ray structures associated with the "Green Monster" using Chandra
X-ray Observatory data. We extracted spectra along the "Green Monster" as well
as from shocked CSM regions. Both the extracted spectra and a principal
component analysis show that the "Green Monster" emission properties are
similar to those of the shocked CSM. The spectra are well-fit by a model
consisting of a combination of a non-equilibrium-ionization model and a
power-law component, modified by Galactic absorption. All the "Green Monster"
spectra show a blueshift corresponding to a radial velocity of around -2300
km/s, suggesting that the structure is on the near side of Cas A. The
ionization age is around $n_{e}t = 1.5 \times 10^{11}$ cm$^{-3}s. This
translates into a pre-shock density of ~12 cm$^{-3}$, higher than previous
estimates of the unshocked CSM. The relatively high net and relatively low
radial velocity suggest that this structure has a relatively high density
compared to other shocked CSM plasma. This analysis provides yet another piece
of evidence that the CSM around Cas A's progenitor was not that of a smooth
steady wind profile. | astro-ph |
Dust temperature and the submillimeter-radio flux density ratio as a
redshift indicator: It is difficult to identify the distant galaxies selected in existing
submillimeter (submm)-wave surveys, because their positional accuracy is only
several arcseconds. Currently, centimeter-wave VLA observations are required in
order to determine sub-arcsec positions, and so to make reliable optical
identifications. Carilli & Yun (1999) pointed out that the ratio of the radio
and submm-wave flux densities provides a redshift indicator for dusty
star-forming galaxies, when compared with the tight correlation observed
between the far-infrared and radio flux densities for low-redshift galaxies.
This method provides a useful, albeit imprecise, indication of the distance to
a submm-selected galaxy. However, because the degeneracy between the effects of
increasing the redshift of a galaxy and decreasing its dust temperature is not
broken, it does not provide an unequivocal redshift estimate. | astro-ph |
Mixing Bispectrum Multipoles Under Geometric Distortions: We derive general expressions for how the Alcock-Paczynski distortions affect
the power spectrum and the bispectrum of cosmological fields. We compute
explicit formulas for the mixing coefficients of bispectrum multipoles in the
linear approximation. The leading-order effect for the bispectrum is the
uniform dilation of all three wavevectors. The mixing coefficients depend on
the shape of the bispectrum triplet. Our results for the bispectrum multipoles
are framed in terms of the "natural" basis of the lengths of three wavevectors
but can be easily generalized for other bases and reduction schemes. Our
validation tests confirm that the linear approximation is extremely accurate
for all power spectrum multipoles. The linear approximation is accurate for the
bispectrum monopole but results in sub-percent level inaccuracies for the
bispectrum quadrupole and fails for the bispectrum hexadecapole. Our results
can be used to simplify the analysis of the bispectrum from galaxy surveys,
especially the measurement of the Baryon Acoustic Oscillation peak position.
They can be used to replace numeric schemes with exact analytic formulae. | astro-ph |
Machine Learning of Interstellar Chemical Inventories: The characterization of interstellar chemical inventories provides valuable
insight into the chemical and physical processes in astrophysical sources. The
discovery of new interstellar molecules becomes increasingly difficult as the
number of viable species grows combinatorially, even when considering only the
most thermodynamically stable. In this work, we present a novel approach for
understanding and modeling interstellar chemical inventories by combining
methodologies from cheminformatics and machine learning. Using multidimensional
vector representations of molecules obtained through unsupervised machine
learning, we show that identification of candidates for astrochemical study can
be achieved through quantitative measures of chemical similarity in this vector
space, highlighting molecules that are most similar to those already known in
the interstellar medium. Furthermore, we show that simple, supervised learning
regressors are capable of reproducing the abundances of entire chemical
inventories, and predict the abundance of not yet seen molecules. As a
proof-of-concept, we have developed and applied this discovery pipeline to the
chemical inventory of a well-known dark molecular cloud, the Taurus Molecular
Cloud 1 (TMC-1); one of the most chemically rich regions of space known to
date. In this paper, we discuss the implications and new insights machine
learning explorations of chemical space can provide in astrochemistry. | astro-ph |
Formation Scenario of Magnetars: The Puzzle of Isolation: Magnetars (SGRs and AXPs) are one of the most evolutionary paths of a neutron
star. These objects have an ultra-strong magnetic field $B \sim 10^{15}$ G at
their surface and show persistent X-ray pulsations and transient bursts. Till
date there are 14 magnetars known: 5 SGRs (4 confirmed, 1 candidate) and 9 AXPs
(7 confirmed, 2 candidates). It is an open puzzle that all these objects are
isolated and none have been found in binaries. We discuss the formation
scenario which can lead to such a situation. | astro-ph |
Titan's Dynamic Love Number Implies Stably-Stratified Ocean: The dynamic quadrupole Love number of Titan measured by \Cassini is
$k_\mathrm{2,obs}=0.616\pm 0.067$, strongly indicating a global subsurface
ocean. However, the theoretical Love number due to equilibrium tides is at most
$k_\mathrm{2,eq}^\mathrm{max}\approx 0.48$ in the absence of an ice shell on
top of the ocean. In reality, there is an outer ice shell of thickness $
100\,\mathrm{km}$, reducing the equilibrium-tide Love number to
$k_\mathrm{2,eq}\approx 0.42$. Therefore, other types of tidal response, like
dynamic tides, may be also present in Titan. We propose that the ocean is
stably stratified. As a result, there exist standing ocean waves (gravity
modes) with eigen-frequencies close to the tidal frequency. Such a gravity mode
(g-mode) is resonantly excited. It bends the outer ice shell radially and thus
enhances the dynamic Love number by $k_\mathrm{2,g}$. In order for
$k_\mathrm{2,g}$ to account for the discrepancy between $k_\mathrm{2,eq}$ and
$k_\mathrm{2,obs}$, the Brunt-Vaisala frequency in the ocean is required to be
$3.3\times 10^{-4}\,\mathrm{rad\, s^{-1}}$. It is compatible with the
volatile-rich model for Titan that was proposed to explain the methane-rich
atmosphere. The three components of the tidal potential with azimuthal degrees,
$m=-2,0,2$, correspond to the three components of the quadrupole Love number,
$k_\mathrm{2,-2}$, $k_\mathrm{2,0}$ and $k_\mathrm{2,2}$. They can excite
retrograde, axisymmetric and prograde g-modes equally in the absence of
rotation. However, Coriolis force induced by Titan's rotation breaks the
symmetry among these modes. Most likely, only one of the Love-number components
is significantly enhanced by a g-mode, while the other two are still attributed
to equilibrium tides. This prediction is testable by observation. If confirmed,
the smaller components of the Love number can be used to constrain the
thickness of the outer ice shell. | astro-ph |
The Stability of Tidal Equilibrium for Hierarchical Star-Planet-Moon
Systems: Motivated by the current search for exomoons, this paper considers the
stability of tidal equilibrium for hierarchical three-body systems containing a
star, a planet, and a moon. In this treatment, the energy and angular momentum
budgets include contributions from the planetary orbit, lunar orbit, stellar
spin, planetary spin, and lunar spin. The goal is to determine the optimized
energy state of the system subject to the constraint of constant angular
momentum. Due to the lack of a closed form solution for the full three-body
problem, however, we must use use an approximate description of the orbits. We
first consider the Keplerian limit and find that the critical energy states are
saddle points, rather than minima, so that these hierarchical systems have no
stable tidal equilibrium states. We then generalize the calculation so that the
lunar orbit is described by a time-averaged version of the circular restricted
three-body problem. In this latter case, the critical energy state is a shallow
minimum, so that a tidal equilibrium state exists. In both cases, however, the
lunar orbit for the critical point lies outside the boundary (roughly half the
Hill radius) where (previous) numerical simulations indicate dynamical
instability. These results suggest that star-planet-moon systems have no viable
long-term stable states analogous to those found for two-body systems. | astro-ph |
Fermi LAT detection of two high Galactic latitude gamma-ray sources,
Fermi J1049.7+0435 and J1103.2+1145: The Second Fermi LAT source catalog (Nolan et al. 2012) includes as many as
1,873 sources, but initial attempts to identify counterparts at other
wavelengths resulted in 575 sources remaining unidentified. The 2FGL catalog is
based on the first 24 months of LAT observation since its launch in 2008, but
the LAT has now accumulated more than 5 years of high-energy gamma-ray data
almost flawlessly, presenting the possibility of finding new sources which were
too faint to be detected in the first two years of data or showed flaring
activity after the catalog was created. In this paper we report on two new
gamma-ray sources serendipitously discovered in the constellation Leo and
discuss possible counterparts based on radio observations. | astro-ph |
Too Many, Too Few, or Just Right? The Predicted Number and Distribution
of Milky Way Dwarf Galaxies: We predict the spatial distribution and number of Milky Way dwarf galaxies to
be discovered in the DES and LSST surveys, by completeness correcting the
observed SDSS dwarf population. We apply most massive in the past, earliest
forming, and earliest infall toy models to a set of dark matter-only simulated
Milky Way/M31 halo pairs from Exploring the Local Volume In Simulations
(ELVIS). The observed spatial distribution of Milky Way dwarfs in the LSST-era
will discriminate between the earliest infall and other simplified models for
how dwarf galaxies populate dark matter subhalos. Inclusive of all toy models
and simulations, at 90% confidence we predict a total of 37-114 L $\gtrsim
10^3$L$_{\odot}$ dwarfs and 131-782 L $\lesssim 10^3$L$_{\odot}$ dwarfs within
300 kpc. These numbers of L $\gtrsim 10^3$L$_{\odot}$ dwarfs are dramatically
lower than previous predictions, owing primarily to our use of updated
detection limits and the decreasing number of SDSS dwarfs discovered per sky
area. For an effective $r_{\rm limit}$ of 25.8 mag, we predict: 3-13 L $\gtrsim
10^3$L$_{\odot}$ and 9-99 L $\lesssim 10^3$L$_{\odot}$ dwarfs for DES, and
18-53 L $\gtrsim 10^3$L$_{\odot}$ and 53-307 L $\lesssim 10^3$L$_{\odot}$
dwarfs for LSST. These enormous predicted ranges ensure a coming decade of
near-field excitement with these next generation surveys. | astro-ph |
The Anisotropy of the Microwave Background to l = 3500: Mosaic
Observations with the Cosmic Background Imager: Using the Cosmic Background Imager, a 13-element interferometer array
operating in the 26-36 GHz frequency band, we have observed 40 sq deg of sky in
three pairs of fields, each ~ 145 x 165 arcmin, using overlapping pointings
(mosaicing). We present images and power spectra of the cosmic microwave
background radiation in these mosaic fields. We remove ground radiation and
other low-level contaminating signals by differencing matched observations of
the fields in each pair. The primary foreground contamination is due to point
sources (radio galaxies and quasars). We have subtracted the strongest sources
from the data using higher-resolution measurements, and we have projected out
the response to other sources of known position in the power-spectrum analysis.
The images show features on scales ~ 6 - 15 arcmin, corresponding to masses ~
(5 - 80)*10^{14} Msun at the surface of last scattering, which are likely to be
the seeds of clusters of galaxies. The power spectrum estimates have a
resolution Delta-l = 200 and are consistent with earlier results in the
multipole range l <~ 1000. The power spectrum is detected with high
signal-to-noise ratio in the range 300 <~ l <~ 1700. For 1700 <~ l <~ 3000 the
observations are consistent with the results from more sensitive CBI deep-field
observations. The results agree with the extrapolation of cosmological models
fitted to observations at lower l, and show the predicted drop at high l (the
"damping tail"). | astro-ph |
Radio-quiet neutron star 1E 1207.4-5209: a possible strong
Gravitational-wave source: There are four puzzles on 1E 1207.4-5209: (1) the characteristic age of the
pulsar is much higher than the estimated age of the supernova remnant; (2) the
magnetic field inferred from spin-down is significantly different from the
value obtained from the cyclotron absorption lines; (3) the spinning down of
the pulsar is non-monotonic; (4) the magnitude of the frequency's first
derivative varies significantly and its sign is also variable. The third puzzle
can be explained by a wide binary system, with orbital period from 0.2 to 6 yr.
This letter proposes that all four puzzles can be explained naturally by an
ultra-compact binary with orbital period of between 0.5 and 3.3 min. With the
shortest orbital period and a close distance of 2 kpc, the characteristic
amplitude of gravitational waves is $h\sim3\times10^{-21}$. It would be an
excellent source of gravitational-wave detectors such as the Laser
Interferometer Space Antenna. | astro-ph |
Giant eruptions of very massive stars: Giant eruptions or supernova-impostor events are far more mysterious than
true supernovae. An extreme example can release as much radiative energy as a
SN, ejecting several M_sun of material. These events involve continuous
radiation-driven outflows rather than blast waves. They constitute one of the
main unsolved problems in stellar astrophysics, but have received surprisingly
little theoretical effort. Here I note some aspects that are not yet familiar
to most astronomers. | astro-ph |
The SKA and the Unknown Unknowns: As new scientists and engineers join the SKA project and as the pressures
come on to maintain costs within a chosen envelope it is worth restating and
updating the rationale for the 'Exploration of the Unknown' (EoU). Maintaining
an EoU philosophy will prove a vital ingredient for realizing the SKA's
discovery potential. Since people make the discoveries enabled by technology a
further axis in capability parameter space, the'human bandwidth' is emphasised.
Using the morphological approach pioneered by Zwicky, a currently unexploited
region of observational parameter space can be identified viz: time variable
spectral patterns on all spectral and angular scales, one interesting example
would be 'spectral transients'. We should be prepared to build up to 10 percent
less collecting area for a given overall budget in order to enhance the ways in
which SKA1 can be flexibly utilized. | astro-ph |
Magnetic field emergence in mesogranular-sized exploding granules
observed with SUNRISE/IMaX data: We report on magnetic field emergences covering significant areas of
exploding granules. The balloon-borne mission SUNRISE provided high spatial and
temporal resolution images of the solar photosphere. Continuum images,
longitudinal and transverse magnetic field maps and Dopplergrams obtained by
IMaX onboard SUNRISE are analyzed by Local Correlation Traking (LCT),
divergence calculation and time slices, Stokes inversions and numerical
simulations are also employed. We characterize two mesogranular-scale exploding
granules where $\sim$ 10$^{18}$ Mx of magnetic flux emerges. The emergence of
weak unipolar longitudinal fields ($\sim$100 G) start with a single visible
magnetic polarity, occupying their respective granules' top and following the
granular splitting. After a while, mixed polarities start appearing,
concentrated in downflow lanes. The events last around 20 min. LCT analyses
confirm mesogranular scale expansion, displaying a similar pattern for all the
physical properties, and divergence centers match between all of them. We found
a similar behaviour with the emergence events in a numerical MHD simulation.
Granule expansion velocities are around 1 \kms while magnetic patches expand at
0.65 \kms. One of the analyzed events evidences the emergence of a loop-like
structure. Advection of the emerging magnetic flux features is dominated by
convective motion resulting from the exploding granule due to the magnetic
field frozen in the granular plasma. Intensification of the magnetic field
occurs in the intergranular lanes, probably because of being directed by the
downflowing plasma. | astro-ph |
A transiting warm giant planet around the young active star TOI-201: We present the confirmation of the eccentric warm giant planet TOI-201 b,
first identified as a candidate in \textit{TESS} photometry (Sectors 1-8,
10-13, and 27-28) and confirmed using ground-based photometry from NGTS and
radial velocities from FEROS, HARPS, CORALIE, and \textsc{Minerva}-Australis.
TOI-201 b orbits a young ($\mathrm{0.87^{+0.46}_{-0.49} \, Gyr}$) and
bright(V=9.07 mag) F-type star with a $\mathrm{52.9781 \, d}$ period. The
planet has a mass of $\mathrm{0.42^{+0.05}_{-0.03}\, M_J}$, a radius of
$\mathrm{1.008^{+0.012}_{-0.015}\, R_J}$, and an orbital eccentricity of
$0.28^{+0.06}_{-0.09}$; it appears to still be undergoing fairly rapid cooling,
as expected given the youth of the host star. The star also shows long-term
variability in both the radial velocities and several activity indicators,
which we attribute to stellar activity. The discovery and characterization of
warm giant planets such as TOI-201 b is important for constraining formation
and evolution theories for giant planets. | astro-ph |
Supernovae and their host galaxies - IV. The distribution of supernovae
relative to spiral arms: Using a sample of 215 supernovae (SNe), we analyze their positions relative
to the spiral arms of their host galaxies, distinguishing grand-design (GD)
spirals from non-GD (NGD) galaxies. We find that: (1) in GD galaxies, an offset
exists between the positions of Ia and core-collapse (CC) SNe relative to the
peaks of arms, while in NGD galaxies the positions show no such shifts; (2) in
GD galaxies, the positions of CC SNe relative to the peaks of arms are
correlated with the radial distance from the galaxy nucleus. Inside (outside)
the corotation radius, CC SNe are found closer to the inner (outer) edge. No
such correlation is observed for SNe in NGD galaxies nor for SNe Ia in either
galaxy class; (3) in GD galaxies, SNe Ibc occur closer to the leading edges of
the arms than do SNe II, while in NGD galaxies they are more concentrated
towards the peaks of arms. In both samples of hosts, the distributions of SNe
Ia relative to the arms have broader wings. These observations suggest that
shocks in spiral arms of GD galaxies trigger star formation in the leading
edges of arms affecting the distributions of CC SNe (known to have short-lived
progenitors). The closer locations of SNe Ibc vs. SNe II relative to the
leading edges of the arms supports the belief that SNe Ibc have more massive
progenitors. SNe Ia having less massive and older progenitors, have more time
to drift away from the leading edge of the spiral arms. | astro-ph |
Transverse Sizes of CIV Absorption Systems Measured from Multiple QSO
Sightlines: We present tomography of the circum-galactic metal distribution at redshift
1.7 to 4.5 derived from echellete spectroscopy of binary quasars. We find CIV
systems at similar redshifts in paired sightlines more often than expected for
sightline-independent redshifts. As the separation of the sightlines increases
from 36 kpc to 907 kpc, the amplitude of this clustering decreases. At the
largest separations, the CIV systems cluster similar to Lyman-break galaxies
(Adelberger et al. 2005a). The CIV systems are significantly less correlated
than these galaxies, however, at separations less than R_1 ~ 0.42 +/- 0.15 h-1
comoving Mpc. Measured in real space, i.e., transverse to the sightlines, this
length scale is significantly smaller than the break scale estimated from the
line-of-sight correlation function in redshift space (Scannapieco et al.
2006a). Using a simple model, we interpret the new real-space measurement as an
indication of the typical physical size of enriched regions. We adopt this size
for enriched regions and fit the redshift-space distortion in the line-of-sight
correlation function. The fitted velocity kick is consistent with the peculiar
velocity of galaxies as determined by the underlying mass distribution and
places an upper limit on the outflow (or inflow) speed of metals. The implied
time scale for dispersing metals is larger than the typical stellar ages of
Lyman-break galaxies (Shapley et al. 2001), and we argue that enrichment by
galaxies at z > 4.3 played a greater role in dispersing metals. To further
constrain the growth of enriched regions, we discuss empirical constraints on
the evolution of the CIV correlation function with cosmic time. This study
demonstrates the potential of tomography for measuring the metal enrichment
history of the circum-galactic medium. | astro-ph |
Radial velocities and membership of stars in the old, distant open
cluster Berkeley 29: Multi slit spectroscopy at the Telescopio Nazionale Galileo was employed to
measure radial velocities for 20 stars in the direction of the old open cluster
Berkeley 29, the farthest known in our Galaxy. Membership information was
derived for stars along all the red giant branch, in particular near its tip,
and on the red clump. The sample of bona-fide cluster members was used to
revise the cluster distance to about 15 kpc, on the basis of an empirical
comparison with the red clump in open clusters with known distances. A
metallicity [Fe/H] = -0.74 +/- 0.18 was also estimated using the colours of
spectroscopically confirmed red giant stars. | astro-ph |
Radial Migration in Disk Galaxies I: Transient Spiral Structure and
Dynamics: We seek to understand the origin of radial migration in spiral galaxies by
analyzing in detail the structure and evolution of an idealized, isolated
galactic disk. To understand the redistribution of stars, we characterize the
time-evolution of properties of spirals that spontaneously form in the disk.
Our models unambiguously show that in such disks, single spirals are unlikely,
but that a number of transient patterns may coexist in the disk. However, we
also show that while spirals are transient in amplitude, at any given time the
disk favors patterns of certain pattern speeds. Using several runs with
different numerical parameters we show that the properties of spirals that
occur spontaneously in the disk do not sensitively depend on resolution. The
existence of multiple transient patterns has large implications for the orbits
of stars in the disk, and we therefore examine the resonant scattering
mechanisms that profoundly alter angular momenta of individual stars. We
confirm that the corotation scattering mechanism described by Sellwood & Binney
(2002) is responsible for the largest angular momentum changes in our
simulations. | astro-ph |
Some caveats about the evolution of the N/O abundance and the star
formation history: We carefully analyze how the abundance of Nitrogen over Oxygen evolves when
dependent on metallicity stellar yields with a primary component of N
proceeding from AGBs stars are used. We show the results obtained with a
chemical evolution models grid, calculated with variable star formation
efficiencies, which produce different star formation histories. Finally we see
how the N/O abundance is related on the evolutionary history. | astro-ph |
The TESS-Keck Survey I: A Warm Sub-Saturn-mass Planet and a Caution
about Stray Light in TESS Cameras: We report the detection of a Saturn-size exoplanet orbiting HD 332231 (TOI
1456) in light curves from the Transiting Exoplanet Survey Satellite (TESS). HD
332231, an F8 dwarf star with a V-band magnitude of 8.56, was observed by TESS
in Sectors 14 and 15. We detect a single-transit event in the Sector 15
presearch data conditioning (PDC) light curve. We obtain spectroscopic
follow-up observations of HD 332231 with the Automated Planet Finder, Keck I,
and SONG telescopes. The orbital period we infer from the radial velocity (RV)
observations leads to the discovery of another transit in Sector 14 that was
masked by PDC due to scattered light contamination. A joint analysis of the
transit and RV data confirms the planetary nature of HD 332231 b, a Saturn-size
($0.867^{+0.027}_{-0.025} \; R_{\rm J}$), sub-Saturn-mass ($0.244\pm0.021 \;
M_{\rm J}$) exoplanet on a 18.71 day circular orbit. The low surface gravity of
HD 332231 b and the relatively low stellar flux it receives make it a
compelling target for transmission spectroscopy. Also, the stellar obliquity is
likely measurable via the Rossiter-McLaughlin effect, an exciting prospect
given the 0.14 au orbital separation of HD 332231 b. The spectroscopic
observations do not provide substantial evidence for any additional planets in
the HD 332231 system, but continued RV monitoring is needed to further
characterize this system. We also predict that the frequency and duration of
masked data in the PDC light curves for TESS Sectors 14-16 could hide transits
of some exoplanets with orbital periods between 10.5 and 17.5 days. | astro-ph |
Magnetospheres and Disk Accretion in Herbig Ae/Be Stars: We present evidence of magnetically-mediated disk accretion in Herbig Ae/Be
stars. Magnetospheric accretion models of Balmer and sodium profiles calculated
with appropriate stellar and rotational parameters are in qualitative agreement
with the observed profiles of the Herbig Ae star UX Ori, and yield a mass
accretion rate of ~ 10^{-8} Msun/yr. If more recent indications of an extremely
large rotation rate for this object are correct, the magnetic field geometry
must deviate from that of a standard dipole in order to produce line emission
consistent with observed flux levels. Models of the associated accretion shock
qualitatively explain the observed distribution of excess fluxes in the Balmer
discontinuity for a large ensemble of Herbig Ae/Be stars, and imply typically
small mass accretion rates, < 10^{-7} Msun/yr. In order for accretion to
proceed onto the star, significant amounts of gas must exist inside the dust
destruction radius, which is potentially problematic for recently advocated
scenarios of "puffed" inner dust wall geometries. However, our models of the
inner gas disk show that for the typical accretion rates we have derived, the
gas should be generally optically thin, thus allowing direct stellar
irradiation of the inner dust edge of the disk. | astro-ph |
HST F160W Imaging of Very Massive Galaxies at $1.5<z<3.0$: Diversity of
Structures and the Effect of Close Pairs on Number Density Estimates: We present a targeted follow-up Hubble Space Telescope WFC3 F160W imaging
study of very massive galaxies $(\log(M_{\rm{star}}/M_{\odot})> 11.2)$ selected
from a combination of ground-based near-infrared galaxy surveys (UltraVISTA,
NMBS-II, UKIDSS UDS) at $1.5<z<3$. We find that these galaxies are diverse in
their structures, with $\sim1/3$ of the targets being composed of close pairs,
and span a wide range in sizes. At $1.5<z<2.5$, the sizes of both star-forming
and quiescent galaxies are consistent with the extrapolation of the stellar
mass-size relations determined at lower stellar masses. At $2.5<z<3.0$,
however, we find evidence that quiescent galaxies are systematically larger
than expected based on the extrapolation of the relation derived using lower
stellar mass galaxies. We used the observed light profiles of the blended
systems to decompose their stellar masses and investigate the effect of the
close pairs on the measured number densities of very massive galaxies in the
early universe. We estimate correction factors to account for close-pair blends
and apply them to the observed stellar mass functions measured using
ground-based surveys. Given the large uncertainties associated with this
extreme population of galaxies, there is currently little tension between the
(blending-corrected) number density estimates and predictions from theoretical
models. Although we currently lack the statistics to robustly correct for
close-pair blends, we show that this is a systematic effect which can reduce
the observed number density of very massive galaxies by up to a factor of
$\sim1.5$, and should be accounted for in future studies of stellar mass
functions. | astro-ph |
Kappa-mechanism excitation of retrograde mixed modes in rotating B-type
stars: I examine the stability of retrograde mixed modes in rotating B-type stars.
These modes can be regarded as a hybrid between the Rossby modes that arise
from conservation of vorticity, and the Poincare modes that are gravity waves
modified by the Coriolis force. Using a non-adiabatic pulsation code based
around the traditional approximation, I find that the modes are unstable in
mid- to late-B type stars, due to the same iron-bump opacity mechanism usually
associated with SPB and $\beta$ Cep stars. At one half of the critical rotation
rate, the instability for $m=1...4$ modes spans the spectral types B4 to A0.
Inertial-frame periods of the unstable modes range from 100 days down to a
fraction of a day, while normalized growth rates can reach in excess of
$10^{-5}$.
I discuss the relevance of these findings to SPB and pulsating Be stars, and
to the putative Maia class of variable star. I also outline some of the
questions raised by this discovery of a wholly-new class of pulsational
instability in early-type stars. | astro-ph |
Stability of non-homegeneous models and fine tuning of initial state: We apply phase space analysis to inhomogeneous cosmological model given by
Lema\^itre-Tolman model. We describe some general conditions required to
interpret the model stable enough and, in the present paper, apply them to two
special cases: dust filled homogeneous model with and without cosmological
constant. We find that such stability explaining all present astrophysical
observations can not be achieved due to instabilities in phase space. This
hints that non-homogeneous models are not likely to be physically viable,
although any conclusive analysis requires more realistic modeling of
non-homogeneous universe. | astro-ph |
Dealing with large gaps in asteroseismic time series: With long data sets available for asteroseismology from space missions, it is
sometimes necessary to deal with time series that have large gaps. This is
becoming particularly relevant for TESS, which is revisiting many fields on the
sky every two years. Because solar-like oscillators have finite mode lifetimes,
it has become tempting to close large gaps by shifting time stamps. Using
actual data from the Kepler Mission, we show that this results in artificial
structures in the power spectrum that compromise the measurements of mode
frequencies and linewidths. | astro-ph |
The continued optical to mid-IR evolution of V838 Monocerotis: The eruptive variable V838 Monocerotis gained notoriety in 2002 when it
brightened nine magnitudes in a series of three outbursts and then rapidly
evolved into an extremely cool supergiant. We present optical, near-IR, and
mid-IR spectroscopic and photometric observations of V838 Monocerotis obtained
between 2008 and 2012 at the Apache Point Observatory 3.5m, NASA IRTF 3m, and
Gemini South 8m telescopes. We contemporaneously analyze the optical & IR
spectroscopic properties of V838 Monocerotis to arrive at a revised spectral
type L3 supergiant and effective temperature Teff~2000--2200 K. Because there
are no existing optical observational data for L supergiants in the optical, we
speculate that V838 Monocerotis may represent the prototype for L supergiants
in this wavelength regime. We find a low level of Halpha emission present in
the system, consistent with interaction between V838 Monocerotis and its B3V
binary; however, we cannot rule out a stellar collision as the genesis event,
which could result in the observed Halpha activity. Based upon a two-component
blackbody fit to all wavelengths of our data, we conclude that, as of 2009, a
shell of ejecta surrounded V838 Monocerotis at a radius of R=263+/-10 AU with a
temperature of T=285+/-2 K. This result is consistent with IR interferometric
observations from the same era and predictions from the Lynch et al. model of
the expanding system, which provides a simple framework for understanding this
complicated system. | astro-ph |
Rocking the Lighthouse: Circumpulsar Asteroids and Radio Intermittency: We propose that neutral, circumpulsar debris entering the light cylinder can
account for many time-dependent pulsar phenomena that are otherwise difficult
to explain. Neutral material avoids propeller ejection and injects sufficient
charges -- after heating, evaporation, and ionization -- to alter current flows
and pair-production and thus trigger, detune, or extinguish coherent emission.
Relevant phenomena, with time scales from seconds to months, include nulls,
rotating radio transients (RRATs), rapid changes in pulse profile (``mode
changes''), variable subpulse drift rates, quasi-periodic bursts from B1931+24,
and torque variations. Over the 10 Myr lifetime of a canonical pulsar with
trillion-gauss surface magnetic field, less than a millionth of an Earth mass
of material is needed to modulate the Goldreich-Julian current by 100%.
Circumpulsar material originates from metal-rich, supernova fallback gas that
aggregates into asteroids. Debris disks can inject sufficient material on time
scales of interest, yet be too tenuous to form large planets detectable in
pulse timing data. Asteroid migration results from collisions and the
radiation-driven Yarkovsky and Poynting-Robertson effects. For B1931+24, an
asteroid in a $\sim 40$~day elliptical orbit pollutes the magnetosphere
stochastically through collisions with other debris. Injection is less likely
for hot, young and highly magnetized pulsars or millisecond pulsars that
pre-ionize any debris material well outside their small magnetospheres.
Injection effects will therefore be most prominent in long-period, cooler
pulsars, consistent with the distribution of relevant objects in perid and
period derivative. A pulsar's spin history and its radiation-beam orientation
may influence whether it displays nulling, RRATs and other effects. | astro-ph |
The Unidentified Galactic EGRET Sources: The nature of the unidentified gamma-ray point sources in the galactic plane
is a long standing puzzle of gamma-ray astronomy. Kaaret and Cottam (1996)
showed that 16 of the 25 unidentified gamma-ray point sources near the galactic
plane lie in or near associations of massive stars, likely sites of pulsar
formation, and that the intrinsic luminosity distribution of the sources is
consistent with that of known gamma-ray pulsars. Here, I show there are
approximately 5-7 times as many visible gamma-ray pulsars as supernovae
remnants. This strengthens the hypothesis that pulsars constitute the majority
of the unidentified gamma-ray point sources near the galactic plane. | astro-ph |
An opening criterion for dust gaps in protoplanetary discs: We aim to understand under which conditions a low mass planet can open a gap
in viscous dusty protoplanetary discs. For this purpose, we extend the theory
of dust radial drift to include the contribution from the tides of an embedded
planet and from the gas viscous forces. From this formalism, we derive i) a
grain size-dependent criterion for dust gap opening in discs, ii) an estimate
of the location of the outer edge of the dust gap and iii) an estimate of the
minimum Stokes number above which low-mass planets are able to carve gaps which
appear only in the dust disc. These analytical estimates are particularly
helpful to appraise the minimum mass of an hypothetical planet carving gaps in
discs observed at long wavelengths and high resolution. We validate the theory
against 3D SPH simulations of planet-disc interaction in a broad range of dusty
protoplanetary discs. We find a remarkable agreement between the theoretical
model and the numerical experiments. | astro-ph |
Estimation of lunar surface dielectric constant using MiniRF SAR data: A new model has been developed to estimate the dielectric constant of the
lunar surface using Synthetic Aperture Radar (SAR) data. Continuous
investigation on the dielectric constant of the lunar surface is a high
priority task due to future lunar mission's goals and possible exploration of
human outposts. For this purpose, derived anisotropy and backscattering
coefficients of SAR images are used. The SAR images are obtained from Miniature
Radio Frequency (MiniRF) radar onboard Lunar Reconnaissance Orbiter (LRO).
These images are available in the form of Stokes parameters, which are used to
derive the coherency matrix. The derived coherency matrix is further
represented in terms of particle anisotropy. This coherency matrix's elements
compared with Cloud's coherency matrix, which results in the new relationship
between particle anisotropy and coherency matrix elements (backscattering
coefficients). Following this, estimated anisotropy is used to determine the
dielectric constant. Our model estimates the dielectric constant of the lunar
surface without parallax error. The produce results are also comparable with
the earlier estimate. As an advantageous, our method estimates the dielectric
constant without any apriori information about the density or composition of
lunar surface materials. The proposed approach can also be useful for
determining the dielectric properties of Mars and other celestial bodies. | astro-ph |
Unveiling the Secrets of Gamma Ray Bursts: Gamma Ray Bursts (GRBs) are unpredictable and brief flashes of gamma rays
that occur about once a day in random locations in the sky. Since gamma rays do
not penetrate the Earth's atmosphere, they are detected by satellites, which
automatically trigger ground-based telescopes for follow-up observations at
longer wavelengths. In this introduction to Gamma Ray Bursts we review how
building a multi-wavelength picture of these events has revealed that they are
the most energetic explosions since the Big Bang and are connected with stellar
deaths in other galaxies. However, in spite of exceptional observational and
theoretical progress in the last 15 years, recent observations raise many
questions which challenge our understanding of these elusive phenomena. Gamma
Ray Bursts therefore remain one of the hottest topics in modern astrophysics. | astro-ph |
Extremely Red Galaxies: Preliminary results of a project aiming at unveiling the nature of the
extremely red galaxies (ERGs) (objects with colours R-K>6 and I-K>5) found in
deep optical-NIR surveys are presented. Very little is known about these
objects, the critical issue being whether they are old ellipticals at z>1 or
distant star-forming galaxies strongly reddened by dust extinction. We expect
to shed light onto the unknown nature of these galaxies by completing our
three-step project: (1) the construction of two very deep optical/NIR surveys
to select ERGs, (2) subsequent VLT/NIR spectroscopy; (3) observations in the
sub-mm region with SCUBA at the JCMT and with MPIfRbolo at the IRAM 30m
antenna. | astro-ph |
GD 244: asteroseismology of a pulsator in the middle of the ZZ Ceti
instability strip: We present our preliminary results on the asteroseismological investigations
of the ZZ Ceti star GD 244. We used literature values of the effective
temperature and surface gravity and utilized the White Dwarf Evolution Code of
Bischoff-Kim, Montgomery and Winget (2008, ApJ, 675, 1512) to build our model
grid for the seismological analysis. Five observed pulsational modes published
up to now were used to find acceptable model solutions. We found that the best
model fits have masses between 0.61 and 0.74 M_solar and constitute two groups
with hydrogen layer masses of either ~10^{-5} or 10^{-6} M_solar. Based on a
statistical analysis of a larger sample of possible model solutions, we assume
that the mass of the star is below ~0.68 M_solar and the oxygen content in the
centre is less than 60 percent. | astro-ph |
X-ray and Infrared Observations of Two Externally-Polluted White Dwarfs: With XMM-Newton and the Spitzer Space Telescope, we obtain upper bounds to
the X-ray fluxes from G29-38 and GD 362, and the 70 micron flux from G29-38.
These data provide indirect evidence that G29-38 is accreting from a
tidally-disrupted asteroid: it is neither accreting large amounts of hydrogen
and helium nor is its surrounding dusty disk being replenished from a reservoir
of cold grains experiencing Poynting-Robertson drag. The upper bound to the
X-ray flux from GD 362 is consistent with the estimated rate of mass accretion
required to explain its pollution by elements heavier than helium. GD 362 also
possesses 0.01 of an Earth's mass of hydrogen, an anomalously large amount for
a white dwarf with a helium-dominated atmosphere. One possibility is that
before the current disk was formed, this hydrogen was accreted from either
about 100 Ceres-like asteroids or one large object. An alternative scenario
which simultaneously explains all of GD 362's distinctive properties is that we
are witnessing the consequences of the tidal-destruction of a single parent
body that had internal water and was at least as massive as Callisto and
probably as massive as Mars. | astro-ph |
Linear stability of magnetized massive protoplanetary disks: Magneto-rotational instability (MRI) and gravitational instability (GI) are
the two principle routes to turbulent angular momentum transport in accretion
disks. Protoplanetary disks may develop both. This paper aims to reinvigorate
interest in the study of magnetized massive protoplanetary disks, starting from
the basic issue of stability. The local linear stability of a self-gravitating,
uniformly magnetized, differentially rotating, three-dimensional stratified
disk subject to axisymmetric perturbations is calculated numerically. The
formulation includes resistivity. It is found that the reduction in the disk
thickness by self-gravity can decrease MRI growth rates; the MRI becomes global
in the vertical direction, and MRI modes with small radial length scales are
stabilized. The maximum vertical field strength that permits the MRI in a
strongly self-gravitating polytropic disk with polytropic index $\Gamma=1$ is
estimated to be $B_{z,\mathrm{max}} \simeq c_{s0}\Omega\sqrt{\mu_0/16\pi G} $,
where $c_{s0}$ is the midplane sound speed and $\Omega$ is the angular
velocity. In massive disks with layered resistivity, the MRI is not
well-localized to regions where the Elsasser number exceeds unity. For MRI
modes with radial length scales on the order of the disk thickness,
self-gravity can enhance density perturbations, an effect that becomes
significant in the presence of a strong toroidal field, and which depends on
the symmetry of the underlying MRI mode. In gravitationally unstable disks
where GI and MRI growth rates are comparable, the character of unstable modes
can transition smoothly between MRI and GI. Implications for non-linear
simulations are discussed briefly. | astro-ph |
Density PDFs of diffuse gas in the Milky Way: The probability distribution functions (PDFs) of the average densities of the
diffuse ionized gas (DIG) and the diffuse atomic gas are close to lognormal,
especially when lines of sight at |b|<5 degree and |b|>5 degree are considered
separately. Our results provide strong support for the existence of a lognormal
density PDF in the diffuse ISM, consistent with a turbulent origin of density
structure in the diffuse gas. | astro-ph |
Disentangling Morphology, Star Formation, Stellar Mass, and Environment
in Galaxy Evolution: We present a study of the spectroscopic and photometric properties of
galaxies in six nearby clusters. We perform a partial correlation analysis on
our dataset to investigate whether the correlation between star formation rates
in galaxies and their environment is merely another aspect of correlations of
morphology, stellar mass, or mean stellar age with environment, or whether star
formation rates vary independently of these other correlations. We find a
residual correlation of ongoing star formation with environment, indicating
that even galaxies with similar morphologies, stellar masses, and mean stellar
ages have lower star formation rates in denser environments. Thus, the current
star formation gradient in clusters is not just another aspect of the
morphology-density, stellar mass-density, or mean stellar age-density
relations. Furthermore, the star formation gradient cannot be solely the result
of initial conditions, but must partly be due to subsequent evolution through a
mechanism (or mechanisms) sensitive to environment. Our results constitute a
true ``smoking gun'' pointing to the effect of environment on the later
evolution of galaxies. | astro-ph |
The effects of self-interacting dark matter on the stripping of galaxies
that fall into clusters: We use the Cluster-EAGLE (C-EAGLE) hydrodynamical simulations to investigate
the effects of self-interacting dark matter (SIDM) on galaxies as they fall
into clusters. We find that SIDM galaxies follow similar orbits to their Cold
Dark Matter (CDM) counterparts, but end up with ${\sim}$25 per cent less mass
by the present day. One in three SIDM galaxies are entirely disrupted, compared
to one in five CDM galaxies. However, the excess stripping will be harder to
observe than suggested by previous DM-only simulations because the most
stripped galaxies form cores and also lose stars: the most discriminating
objects become unobservable. The best test will be to measure the
stellar-to-halo mass relation (SHMR) for galaxies with stellar mass
$10^{10-11}\,\mathrm{M}_{\odot}$. This is 8 times higher in a cluster than in
the field for a CDM universe, but 13 times higher for an SIDM universe. Given
intrinsic scatter in the SHMR, these models could be distinguished with
noise-free galaxy-galaxy strong lensing of ${\sim}32$ cluster galaxies. | astro-ph |
J-state interference signatures in the Second Solar Spectrum: Modeling
the Cr I triplet at 5204-5208 A: The scattering polarization in the solar spectrum is traditionally modeled
with each spectral line treated separately, but this is generally inadequate
for multiplets where J-state interference plays a significant role. Through
simultaneous observations of all the 3 lines of a Cr I triplet, combined with
realistic radiative transfer modeling of the data, we show that it is necessary
to include J-state interference consistently when modeling lines with partially
interacting fine structure components. Polarized line formation theory that
includes J-state interference effects together with partial frequency
redistribution for a two-term atom is used to model the observations.
Collisional frequency redistribution is also accounted for. We show that the
resonance polarization in the Cr I triplet is strongly affected by the partial
frequency redistribution effects in the line core and near wing peaks. The Cr I
triplet is quite sensitive to the temperature structure of the photospheric
layers. Our complete frequency redistribution calculations in semi-empirical
models of the solar atmosphere cannot reproduce the observed near wing
polarization or the cross-over of the Stokes Q/I line polarization about the
continuum polarization level that is due to the J-state interference. When
however partial frequency redistribution is included, a good fit to these
features can be achieved. Further, to obtain a good fit to the far wings, a
small temperature enhancement of the FALF model in the photospheric layers is
necessary. | astro-ph |
Spectral and Temporal Properties of MAXI J1836-194 during 2011 Outburst: We study black hole candidate (BHC) MAXI~J1836-194 during its 2011 outburst
with Two Component Advective Flow (TCAF) model using RXTE/PCU2 data in
$2.5-25$~keV band. From spectral fit, accretion flow parameters such as
Keplerian disk rate ($\dot{m_d}$), sub-Keplerian halo rate ($\dot{m_h}$), shock
location ($X_{s}$) and compression ratio (R) are extracted directly. During the
entire phase of the outburst, quasi-periodic oscillations (QPOs) are observed
sporadically. From the nature of the variation of accretion rate ratio
(ARR=$\dot{m_h}$ / $\dot{m_d}$) and QPOs, entire period of the outburst is
classified in two spectral states, such as, hard (HS), hard-intermediate
(HIMS). Unlike other transient BHCs, no signature of soft (SS) and
soft-intermediate (SIMS) spectral states are observed during entire phase of
the outburst | astro-ph |
Optical Design and Active Optics Methods in Astronomy: Optical designs for astronomy involve implementation of active optics and
adaptive optics from X-ray to the infrared. Developments and results of active
optics methods for telescopes, spectrographs and coronagraph planet finders are
presented. The high accuracy and remarkable smoothness of surfaces generated by
active optics methods also allow elaborating new optical design types with high
aspheric and/or non-axisymmetric surfaces. Depending on the goal and
performance requested for a deformable optical surface analytical
investigations are carried out with one of the various facets of elasticity
theory: small deformation thin plate theory, large deformation thin plate
theory, shallow spherical shell theory, weakly conical shell theory. The
resulting thickness distribution and associated bending force boundaries can be
refined further with finite element analysis. Keywords: active optics, optical
design, elasticity theory, astronomical optics, diffractive optics, X-ray
optics | astro-ph |
The impact of interactions, bars, bulges, and AGN on star formation
efficiency in local massive galaxies: Using observations from the GASS and COLD GASS surveys and complementary data
from SDSS and GALEX, we investigate the nature of variations in gas depletion
time observed across the local massive galaxy population. The large and
unbiased COLD GASS sample allows us to assess the relative importance of galaxy
interactions, bar instabilities, morphologies and the presence of AGN in
regulating star formation efficiency. Both the H2 mass fraction and depletion
time vary as a function of the distance of a galaxy from the main sequence in
the SFR-M* plane. The longest gas depletion times are found in below-main
sequence bulge-dominated galaxies that are either gas-poor, or else on average
less efficient than disk-dominated galaxy at converting into stars any cold gas
they may have. We find no link between AGN and these long depletion times. The
galaxies undergoing mergers or showing signs of morphological disruptions have
the shortest molecular gas depletion times, while those hosting strong stellar
bars have only marginally higher global star formation efficiencies as compared
to matched control samples. Our interpretation is that depletion time
variations are caused by changes in the ratio between the gas mass traced by
the CO(1-0) observations, and the gas mass in high density star-forming cores,
with interactions, mergers and bar instabilities able to locally increase
pressure and raise the ratio of efficiently star-forming gas to CO-detected
gas. Building a sample representative of the local massive galaxy population,
we derive a global Kennicutt-Schmidt relation of slope 1.18+/-0.24, and observe
structure within the scatter around this relation, with galaxies having low
(high) stellar mass surface densities lying systematically above (below) the
mean relation, suggesting that gas surface density is not the only parameter
driving the global star formation ability of a galaxy. | astro-ph |
Disentangling jet and disc emission from the 2005 outburst of XTE
J1118+480: The black hole X-ray transient, XTE J1118+480, has now twice been observed in
outburst - 2000 and 2005 - and on both occasions remained in the low/hard X-ray
spectral state. Here we present radio, infrared, optical, soft X-ray and hard
X-ray observations of the more recent outburst. We find that the lightcurves
have very different morphologies compared with the 2000 event and the optical
decay is delayed relative to the X-ray/radio. We attribute this lesser degree
of correlation to contributions of emission from multiple components, in
particular the jet and accretion disc. Whereas the jet seemed to dominate the
broadband spectrum in 2000, in 2005 the accretion disc seems to be more
prominent and we use an analysis of the lightcurves and spectra to distinguish
between the jet and disc emission. There also appears to be an optically thin
component to the radio emission in the 2005 data, possibly associated with
multiple ejection events and decaying as the outburst proceeds. These results
add to the discussion that the term "low/hard state'" covers a wider range of
properties than previously thought, if it is to account for XTE J1118+480
during these two outbursts. | astro-ph |
Foreground influence on primordial non-Gaussianity estimates: needlet
analysis of WMAP 5-year data: We constrain the amplitude of primordial non-Gaussianity in the CMB data
taking into account the presence of foreground residuals in the maps. We
generalise the needlet bispectrum estimator marginalizing over the amplitudes
of thermal dust, free-free and synchrotron templates. We apply our procedure to
WMAP 5 year data, finding fNL= 38\pm 47 (1 \sigma), while the analysis without
marginalization provides fNL= 35\pm 42. Splitting the marginalization over each
foreground separately, we found that the estimates of fNL are positively cross
correlated of 17%, 12% with the dust and synchrotron respectively, while a
negative cross correlation of about -10% is found for the free-free component. | astro-ph |
One-loop graviton corrections to the curvature perturbation from
inflation: We compute one-loop corrections to the power spectrum of the curvature
perturbation in single-field slow-roll inflation arising from gravitons and
inflaton interactions. The quantum corrections due to gravitons to the power
spectrum of the inflaton field are computed around the time of horizon crossing
and their effect on the curvature perturbation is obtained on superhorizon
scales through the delta-N formalism. We point out that one-loop corrections
from the tensor modes are of the same magnitude as those coming from scalar
self-interactions, therefore they cannot be neglected in a self-consistent
calculation. | astro-ph |
Properties of Original Impactors Estimated from Three-Dimensional
Analysis of Whole Stardust Tracks: The Stardust mission captured comet Wild 2 particles in aerogel at 6.1
km/sec. We performed high resolution three-dimensional imaging and X-ray
fluorescence mapping of whole cometary tracks in aerogel. We present the
results of a survey of track structures using Laser Scanning Confocal
Microscopy, including measurements of track volumes, entry hole size and
cross-sectional profiles. We compare various methods for measuring track
parameters. We demonstrate a methodology for discerning hypervelocity particle
ablation rates using synchrotron-based X-ray fluorescence, combined with mass
and volume estimates of original impactors derived from measured track
properties. Finally, we present a rough framework for reconstruction of
original impactor size, and volume of volatilized material, using our measured
parameters. The bulk of this work is in direct support of non-destructive
analysis and identification of cometary grains in whole tracks, and its
eventual application to the reconstruction of the size, shape, porosity and
chemical composition of whole Stardust impactors. | astro-ph |
ISO observations of the reflection nebula Ced 201: evolution of
carbonaceous dust: We present spectrophotometric imaging mid-IR observations of the reflection
nebula Ced201. Ced201 is a part of a molecular cloud illuminated by a B9.5V
star moving through it at more than 12 km/s. The spectra of Ced201 give
evidence for transformation of very small carbonaceous grains into the carriers
of the Aromatic Infrared Bands (AIBs), due to the radiation field of the
illuminating star and/or to shock waves created by its motion. These very small
grains emit mainly very broad bands and a continuum. We suggest that they are
present everywhere in the interstellar medium but can only be detected in the
mid-IR under special circumstances such as those prevailing in this reflection
nebula. The efficiency of energy conversion of stellar light into mid-infrared
emission is 7.5% for both the very small grains and the AIB carriers, and the
fraction of interstellar carbon locked in these emitters is approximately 15%. | astro-ph |
Hydrodynamic simulations of merging clusters of galaxies: We present the results of high-resolution AP3M+SPH simulations of merging
clusters of galaxies. We find that the compression and shocking of the core gas
during a merger can lead to large increases in bolometric X-ray luminosities
and emission-weighted temperatures of clusters. Cooling flows are completely
disrupted during equal-mass mergers, with the mass deposition rate dropping to
zero as the cores of the clusters collide. The large increase in the cooling
time of the core gas strongly suggests that cooling flows will not recover from
such a merger within a Hubble time. Mergers with subclumps having 1/8th of the
mass of the main cluster are also found to disrupt a cooling flow if the merger
is head-on. However, in this case the entropy injected into the core gas is
rapidly radiated away and the cooling flow restarts within a few Gyr of the
merger. Mergers in which the subcluster has an impact parameter of 500kpc do
not disrupt the cooling flow, although the mass deposition rate is reduced by
\~30%. Finally, we find that equal mass, off-centre mergers can effectively mix
gas in the cores of clusters, while head on mergers lead to very little mixing.
Gas stripped from the outer layers of subclumps results in parts of the outer
layers of the main cluster being well mixed, although they have little effect
on the gas in the core of the cluster. None of the mergers examined here
resulted in the ICM being well mixed globally. | astro-ph |
Orbital and escape dynamics in barred galaxies -- IV. Heteroclinic
connections: Continuing the series of papers on a new model for a barred galaxy, we
investigate the heteroclinic connections between the two normally hyperbolic
invariant manifolds sitting over the two index-1 saddle points of the effective
potential. The heteroclinic trajectories and the nearby periodic orbits of
similar shape populate the bar region of the galaxy and a neighbourhood of its
nucleus. Thereby we see a direct relation between the important structures of
the interior region of the galaxy and the projection of the heteroclinic tangle
into the position space. As a side result, we obtain a detailed picture of the
primary heteroclinic intersection surface in the phase space. | astro-ph |
Is there a circumbinary planet around NSVS 14256825?: The cyclic behaviour of (O-C) residuals of eclipse timings in the sdB+M
eclipsing binary NSVS 14256825 was previously attributed to one or two
Jovian-type circumbinary planets. We report 83 new eclipse timings that not
only fill in the gaps in those already published but also extend the time span
of the (O-C) diagram by three years. Based on the archival and our new data
spanning over more than 17 years we re-examined the up to date system (O-C).
The data revealed systematic, quasi-sinusoidal variation deviating from an
older linear ephemeris by about 100 s. It also exhibits a maximum in the (O-C)
near JD 2,456,400 that was previously unknown. We consider two most credible
explanations of the (O-C) variability: the light propagation time due to the
presence of an invisible companion in a distant circumbinary orbit, and
magnetic cycles reshaping one of the binary components, known as the Applegate
or Lanza-Rodono effect. We found that the latter mechanism is unlikely due to
the insufficient energy budget of the M-dwarf secondary. In the framework of
the third-body hypothesis, we obtained meaningful constraints on the Keplerian
parameters of a putative companion and its mass. Our best-fitting model
indicates that the observed quasi-periodic (O-C) variability can be explained
by the presence of a brown dwarf with the minimal mass of 15 Jupiter masses
rather than a planet, orbiting the binary in a moderately elliptical orbit (~
0.175) with the period of ~ 10 years. Our analysis rules out two planets model
proposed earlier. | astro-ph |
Axion-Like Particles, Cosmic Magnetic Fields and Gamma-Ray Astrophysics: Axion-Like Particles (ALPs) are predicted by many extensions of the Standard
Model and give rise to characteristic dimming and polarization effects in a
light beam travelling in a magnetic field. In this Letter, we demonstrate that
photon-ALP mixing in cosmic magnetic fields produces an observable distortion
in the energy spectra of distant gamma-ray sources (like AGN) for ranges of the
ALP parameters allowed by all available constraints. The resulting effect is
expected to show up in the energy band 100 MeV - 100 GeV, and so it can be
serched with the upcoming GLAST mission. | astro-ph |
A Search for Intrinsic Polarization in O Stars with Variable Winds: New observations of 9 of the brightest northern O stars have been made with
the Breger polarimeter on the 0.9~m telescope at McDonald Observatory and the
AnyPol polarimeter on the 0.4~m telescope at Limber Observatory, using the
Johnson-Cousins UBVRI broadband filter system. Comparison with earlier
measurements shows no clearly defined long-term polarization variability. For
all 9 stars the wavelength dependence of the degree of polarization in the
optical range can be fit by a normal interstellar polarization law. The
polarization position angles are practically constant with wavelength and are
consistent with those of neighboring stars. Thus the simplest conclusion is
that the polarization of all the program stars is primarily interstellar.
The O stars chosen for this study are generally known from ultraviolet and
optical spectroscopy to have substantial mass loss rates and variable winds, as
well as occasional circumstellar emission. Their lack of intrinsic polarization
in comparison with the similar Be stars may be explained by the dominance of
radiation as a wind driving force due to higher luminosity, which results in
lower density and less rotational flattening in the electron scattering inner
envelopes where the polarization is produced. However, time series of
polarization measurements taken simultaneously with H-alpha and UV spectroscopy
during several coordinated multiwavelength campaigns suggest two cases of
possible small-amplitude, periodic short-term polarization variability, and
therefore intrinsic polarization, which may be correlated with the more widely
recognized spectroscopic variations. | astro-ph |
The explosion energy of early stellar populations: The Fe-peak element
ratios in low metallicity damped Lyman-alpha systems: The relative abundances of the Fe-peak elements (Ti-Zn) at the lowest
metallicities are intimately linked to the physics of core-collapse supernova
explosions. With a sample of 25 very metal-poor damped Lyman-alpha systems, we
investigate the trends of the Fe-peak element ratios with metallicity. For nine
of the 25 DLAs, a direct measurement (or useful upper limit) of one or more of
the Ti,Cr,Co,Ni,Zn/Fe abundance ratios could be determined from detected
absorption lines. For the remaining systems (without detections), we devised a
new form of spectral stacking to estimate the typical Fe-peak element ratios of
the DLA population in this metallicity regime. We compare these data to
analogous measurements in metal-poor stars of the Galactic halo and to detailed
calculations of explosive nucleosynthesis in metal-free stars. We conclude that
most of the DLAs in our sample were enriched by stars that released an energy
of < 1.2 x 10^51 erg when they exploded as core-collapse supernovae. Finally,
we discuss the exciting prospect of measuring Fe-peak element ratios in damped
Lyman-alpha systems with Fe/H < 1/1000 of solar when 30-m class telescopes
become available. Only then will we be able to pin down the energy that was
released by the supernovae of the first stars. | astro-ph |
3C 273 - half a century later: We have presented an optical monitoring of 3C 273, the first quasar
discovered fifty years ago. It does not show variability both on intra-night
and long-term time scales. To facilitate the further monitoring of 3C 273, we
compiled the available calibrations of the comparison stars in its field into a
mean sequence. | astro-ph |
Low Mass Star Formation in the Gum Nebula: The CG~30/31/38 complex: We present photometric and spectroscopic results for the low mass pre-main
sequence (PMS) stars with spectral types K - M in the cometary globule (CG)
30/31/38 complex. We obtained multi-object high resolution spectra for the
targets selected as possible PMS stars from multi-wavelength photometry. We
identified 11 PMS stars brighter than V = 16.5 with ages < 5 Myr at a distance
of approximately 200 pc. The spatial distribution of the PMS stars, CG clouds,
and ionizing sources (O stars and supernova remnants) suggests a possible
triggered origin of the star formation in this region. We confirm the youth of
the photometrically selected PMS stars using the lithium abundances. The radial
velocities of the low mass PMS stars are consistent with those of the cometary
globules. Most of the PMS stars show weak Halpha emission with W(Halpha) < 10
A. Only 1 out of the 11 PMS stars shows a moderate near-IR excess, which
suggests a short survival time (t < 5 Myr) of circumstellar disks in this star
forming environment. In addition, we find five young late type stars and one Ae
star which have no obvious relation to the CG 30/31/38 complex. We also discuss
a possible scenario of star formation history in the CG 30/31/38 region. | astro-ph |
Luminous Matter Distribution, Bulk Flows and Baryon Contents in
Cosmological Models with a Local Void: First, we consider galaxy formation from the viewpoint of hierarchical
clustering theory and discuss the possibility that inhomogeneous models with a
local void may be compatible with the observed homogeneity of galactic
distributions found by recent redshift surveys, because their inhomogeneity can
be weakened by the difference in the feedback system of galaxy formation
between the inner and outer regions. Next, it is shown with the results of
numerical simulations that the observed inhomogeneity of two-point correlations
of galaxies can be accounted for by these models. Also, the natural appearance
of bulk flows for an off-central observer is shown. Finally the inhomogeneity
of baryon contents is discussed from the viewpoint of our inhomogeneous models. | astro-ph |
Hubble Space Telescope Observations of Dusty Filaments in Hercules A:
Evidence for Entrainment: We present U, V, and I-band images of the host galaxy of Hercules A (3C 348)
obtained with HST/WFC3/UVIS. We find a network of dusty filaments which are
more complex and extended than seen in earlier HST observations. The filaments
are associated with a faint blue continuum light (possibly from young stars)
and faint H-alpha emission. It seems likely that the cold gas and dust has been
stripped from a companion galaxy now seen as a secondary nucleus. There are
dusty filaments aligned with the base of the jets on both eastern and western
sides of the galaxy. The morphology of the filaments is different on the two
sides - the western filaments are fairly straight, while the eastern filaments
are mainly in two loop-like structures. We suggest that despite the difference
in morphologies, both sets of filaments have been entrained in a slow moving
boundary layer outside the relativistic flow. As suggested by Fabian et al.
(2008), magnetic fields in the filaments may stabilize them against disruption.
We consider a speculative scenario to explain the relation between the radio
source and the shock and cavities in the hot ICM seen in the Chandra data
(Nulsen et al. 2005). We suggest the radio source originally (~60 Myr ago)
propagated along a position angle of ~35 degrees where it created the shock and
cavities. The radio source axis changed to its current orientation (~100
degrees) possibly due to a supermassive black hole merger and began its current
epoch of activity about 20 Myr ago. | astro-ph |
Testing LCDM with the Growth Function δ(a): Current Constraints: We have compiled a dataset consisting of 22 datapoints at a redshift range
(0.15,3.8) which can be used to constrain the linear perturbation growth rate
f=\frac{d\ln\delta}{d\ln a}. Five of these data-points constrain directly the
growth rate f through either redshift distortions or change of the power
spectrum with redshift. The rest of the datapoints constrain f indirectly
through the rms mass fluctuation \sigma_8(z) inferred from Ly-\alpha at various
redshifts. Our analysis tests the consistency of the LCDM model and leads to a
constraint of the Wang-Steinhardt growth index \gamma (defined from
f=\Omega_m^\gamma) as \gamma=0.67^{+0.20}_{-0.17}. This result is clearly
consistent at $1\sigma$ with the value \gamma={6/11}=0.55 predicted by LCDM. A
first order expansion of the index \gamma in redshift space leads to similar
results.We also apply our analysis on a new null test of LCDM which is similar
to the one recently proposed by Chiba and Nakamura (arXiv:0708.3877) but does
not involve derivatives of the expansion rate $H(z)$. This also leads to the
fact that LCDM provides an excellent fit to the current linear growth data. | astro-ph |
Radio Recombination Lines at Decametre Wavelengths: Prospects for the
Future: This paper considers the suitability of a number of emerging and future
instruments for the study of radio recombination lines (RRLs) at frequencies
below 200 MHz. These lines arise only in low-density regions of the ionized
interstellar medium, and they may represent a frequency-dependent foreground
for next-generation experiments trying to detect H I signals from the Epoch of
Reionization and Dark Ages ("21-cm cosmology"). We summarize existing
decametre-wavelength observations of RRLs, which have detected only carbon
RRLs. We then show that, for an interferometric array, the primary instrumental
factor limiting detection and study of the RRLs is the areal filling factor of
the array. We consider the Long Wavelength Array (LWA-1), the LOw Frequency
ARray (LOFAR), the low-frequency component of the Square Kilometre Array
(SKA-lo), and a future Lunar Radio Array (LRA), all of which will operate at
decametre wavelengths. These arrays offer digital signal processing, which
should produce more stable and better defined spectral bandpasses; larger
frequency tuning ranges; and better angular resolution than that of the
previous generation of instruments that have been used in the past for RRL
observations. Detecting Galactic carbon RRLs, with optical depths at the level
of 10^-3, appears feasible for all of these arrays, with integration times of
no more than 100 hr. The SKA-lo and LRA, and the LWA-1 and LOFAR at the lowest
frequencies, should have a high enough filling factor to detect lines with much
lower optical depths, of order 10^-4 in a few hundred hours. The amount of
RRL-hosting gas present in the Galaxy at the high Galactic latitudes likely to
be targeted in 21-cm cosmology studies is currently unknown. If present,
however, the spectral fluctuations from RRLs could be comparable to or exceed
the anticipated H I signals. | astro-ph |
Chemical Conditions on Hycean Worlds: Traditionally, the search for life on exoplanets has been predominantly
focused on rocky exoplanets. Hycean worlds are a class of habitable
sub-Neptunes with planet-wide oceans and H2-rich atmospheres. Their broad range
of possible sizes and temperatures lead to a wide habitable zone and high
potential for discovery and atmospheric characterization using transit
spectroscopy. Over a dozen candidate Hycean planets are already known to be
transiting nearby M dwarfs, making them promising targets for atmospheric
characterization with the James Webb Space Telescope (JWST). In this work, we
investigate possible chemical conditions on a canonical Hycean world, focusing
on (a) the present and primordial molecular composition of the atmosphere, and
(b) the inventory of bioessential elements for the origin and sustenance of
life in the ocean. Based on photochemical and kinetic modeling for a range of
conditions, we discuss the possible chemical evolution and observable
present-day composition of its atmosphere. In particular, for reduced
primordial conditions the early atmospheric evolution passes through a phase
that is rich in organic molecules that could provide important feedstock for
prebiotic chemistry. We investigate avenues for delivering bioessential metals
to the ocean, considering the challenging lack of weathering from a rocky
surface and the ocean separated from the rocky core by a thick icy mantle.
Based on ocean depths from internal structure modelling and elemental estimates
for the early Earth's oceans, we estimate the requirements for bioessential
metals in such a planet. We find that the requirements can be met for plausible
assumptions about impact history and atmospheric sedimentation, and
supplemented by other steady state sources. We discuss the observational
prospects for atmospheric characterisation of Hycean worlds. | astro-ph |
Cosmography with Galaxy Clusters: In the present work we focus on future experiments using cluster abundance
observations to constraint the Dark Energy equation of state parameter, w. To
obtain tight constraints from this kind of experiment, a reliable sample of
galaxy clusters must be obtained from deep and wide-field images. We therefore
present the computational environment (2DPHOT) that allow us to build the
galaxy catalog from the images and the Voronoi Tessellation cluster finding
algorithm that we use to identify the galaxy clusters on those catalogs. To
test our pipeline with data similar in quality to what will be gathered by
future wide field surveys, we process images from the Deep fields obtained as
part of the LEGACY Survey (four fields of one square degree each, in five
bands, with depth up to r'=25). We test our cluster finder by determining the
completeness and purity of the finder when applied to mock galaxy catalogs made
for the Dark Energy Survey cluster finder comparison project by Risa Wechsler
and Michael Busha. This procedure aims to understand the selection function of
the underlying dark matter halos. | astro-ph |
Constraining the distribution of dark matter in inner galaxy with
indirect detection signal: The case of tentative 130 gev γ-ray line: The dark matter distribution in the very inner region of our Galaxy is still
in debate. In the N-body simulations a cuspy dark matter halo density profile
is favored. Several dissipative baryonic processes however are found to be able
to significantly flatten dark matter distribution and a cored dark matter halo
density profile is possible. The baryons dominate the gravitational potential
in the inner Galaxy, hence a direct constrain on the abundance of the dark
matter particles is rather challenging. Recently, a few groups have identified
a tentative 130 GeV line signal in the Galactic center, which could be
interpreted as the signal of the dark matter annihilation. With current 130 GeV
line data and adopting the generalized Navarro-Frenk-White profile of the dark
matter halo, for local dark matter density \rho_0=0.4 GeV cm^{-3} and r_s=20
kpc we obtain a 95% confidence level lower (upper) limit on the inner slope of
dark matter density distribution \alpha = 1.06 (the cross section of dark
matter annihilation into gamma-rays <\sigma v >_{\chi\chi -> \gamma\gamma}=
1.3\times 10^{-27} cm^3 s^{-1}). Such a slope is consistent with the results of
some N-body simulations, and if the signal is due to dark matter, suggests that
baryonic processes may be unimportant. | astro-ph |
Stellar clusters in the Gaia era: Stellar clusters are important for astrophysics in many ways, for instance as
optimal tracers of the Galactic populations to which they belong or as one of
the best test bench for stellar evolutionary models. Gaia DR1, with TGAS, is
just skimming the wealth of exquisite information we are expecting from the
more advanced catalogues, but already offers good opportunities and indicates
the vast potentialities. Gaia results can be efficiently complemented by
ground-based data, in particular by large spectroscopic and photometric
surveys. Examples of some scientific results of the Gaia-ESO survey are
presented, as a teaser for what will be possible once advanced Gaia releases
and ground-based data will be combined. | astro-ph |
Improved kinematics for brown dwarfs and very low-mass stars in ChaI and
a discussion of brown dwarf formation: We present a precise kinematic study of very young brown dwarfs (BDs) in the
ChaI cloud based on radial velocities (RVs) measured with UVES / VLT. This is
compared to the kinematics of T Tauri stars (TTS) in the same field, based on
both own measurements and on RVs from the literature. More UVES spectra were
taken compared with a former paper (Joergens & Guenther 2001), and the
reduction of the spectra was improved, while studying the literature for RVs of
T Tauri stars in ChaI led to a cleaned and enlarged sample of T Tauri stars.
The result is an improved empirical RV distribution of BDs as well as of TTS in
ChaI. We found that nine BDs/VLMSs (M6-M8) in ChaI have a RV dispersion of 0.9
km/s measured in terms of a standard deviation. This is consistent with the
dispersion measured earlier in terms of fwhm of 2.1 km/s. The studied sample of
25 TTS (G2-M5) has a dispersion of 1.3 km/s (standard deviation). The RV
dispersion of the BDs is consistent within the errors with that of TTS, which
is in line with the finding of no mass dependence in some theoretical models of
the ejection-scenario for the formation of brown dwarfs. In contrast to current
N-body simulations, we did not find a high-velocity tail for the BDs RVs. We
found hints suggesting different kinematics for binaries compared to
predominantly single objects in ChaI. The global RV dispersion for ChaI members
(1.24 km/s) is significantly lower than for Taurus members (2.0 km/s), despite
higher stellar density in ChaI showing that a fundamental increase in velocity
dispersion with stellar density of the star-forming region is not established
observationally. The RVs of BDs observed in ChaI are less dispersed than
predicted by existing models for the ejection-scenario. | astro-ph |
Search for High Energy Neutrino Emission from Gamma-Ray Bursts with the
A ntarctic Muon and Neutrino Detector Array (AMANDA): The photo-meson production of pions by shock-accelerated protons could
generate a burst of ~10^14 eV neutrinos from gamma-ray bursts (GRBs) observable
in the Antarctic Muon and Neutrino Detector Array (AMANDA) or its larger
successors. Measurement of this flux can test the hypothesis that GRBs are the
sources of the highest-energy cosmic rays, and GRB neutrinos could permit
high-precision experiments in neutrino limiting speed, neutrino oscillations,
and the weak equivalence principle. Neutrino emission can be expected primarily
during the prompt gamma-ray flash and satellite coincidence provides a
well-defined window in position and time that can be searched for an excess of
upgoing muon events in AMANDA from bursts in the Northern Hemisphere. Using an
event quality analysis to further reduce background in a sample of 78 GRBs from
the 1997 AMANDA-B10 data set, I find a fluence limit of (E_nu)^2
(dN_nu)/(dE_nu)<3.8*10^-4 min(1,E_nu/E_break) [TeV cm^-2] per average burst,
which is orders of magnitude more stringent than in similar previous searches. | astro-ph |
Cosmological Origin for Cosmic Rays Above $10^{19}$ eV: The cosmic ray spectrum at $10^{19}{\rm eV}-10^{20}{\rm eV}$, reported by the
Fly's Eye and the AGASA experiments, is shown to be consistent with a
cosmological distribution of sources of protons, with a power law generation
spectrum ${\rm d}\ln N/{\rm d}\ln E=-2.3\pm0.5$ and energy production rate of
$4.5\pm1.5\times10^{44}{\rm erg}\ {\rm Mpc}^{-3}\ {\rm yr}^{-1}$. The two
events measured above $10^{20}{\rm eV}$ are not inconsistent with this model.
Verifying the existence of a ``black-body cutoff'', currently observed with low
significance, would require $\sim30$ observation-years with existing
experiments, but only $\sim1$ year with the proposed $\sim5000\ {\rm km}^2$
detectors. For a cosmological source distribution, no anisotropy is expected in
the angular distribution of events with energies up to $\sim5\times10^{19}{\rm
eV}$. | astro-ph |
Filling Factors and Ionized Masses in Planetary Nebulae: We calculate filling factors (${\varepsilon}$) and ionized masses (M$_{\rm
i}$) for a total of 84 galactic and extragalactic planetary nebulae (PNe) at
known distances. To do these calculations, we have chosen forbidden line
electron densities, observed angular diameters, and H${\beta}$ fluxes, from the
most recent measurements available in the literature. Statistical analysis on
the distributions of ${\varepsilon}$ and ${\rm M_i}$ show that (1) the ranges
of values of these parameters is wider than what was previously found; (2) the
mean value of the filling factor is between $0.3$ and $0.4$, for the different
sets; (3) the mean value of the ionized mass is between $0.1$ and $0.25$
$M_{\odot}$; (4) a clear correlation between the filling factors and the
dimensions of the PNe was not found when distance-independent sets of PNe were
used; (5) for extragalactic PNe, where distance errors are not a factor, the
filling factors and the ionized masses anticorrelate tightly with the electron
densities. The results indicate that the modified Shklowsky distance method is
correct. | astro-ph |
Spatial Correlations in the Helium-Ionizing Background: After quasars ionize intergalactic HeII at z~3, a large radiation field
builds up above the HeII ionization edge. Unlike the background responsible for
HI ionizations, this field should be highly variable, thanks to the scarcity of
bright quasars and the relatively short attenuation lengths (~50 Mpc) of these
high-energy photons. Recent observations of the HeII and HI Lyman-alpha forests
show that this background does indeed vary strongly, with substantial
fluctuations on scales as small as ~2 Mpc. Here we show that such spatial
fluctuation scales are naturally expected in any model in which the sources are
as rare as bright quasars, so long as the attenuation length is relatively
small. The correlation length itself is comparable to the attenuation length
(~10 Mpc) for the most plausible physical scenarios, but we find
order-of-magnitude fluctuations on all scales smaller than ~6 Mpc. Moreover,
aliasing along the one-dimensional skewers probed by the HeII and HI
Lyman-alpha forests exaggerates these variations, so that order-of-magnitude
fluctuations should be observed on all scales smaller than ~20 Mpc. Complex
radiative transfer is therefore not required to explain the observed
fluctuations, at least at the level of current data. | astro-ph |
Periodic and Phase-locked Modulation in PSR B1929+10 Observed with FAST: We present a detailed single-pulse analysis for PSR B1929+10 based on
observations with the Five-hundred-meter Aperture Spherical radio Telescope
(FAST). The main pulse and interpulse are found to be modulated with a
periodicity of $\sim12$ times the pulsar's rotational period ($P$). The
$\sim12P$ modulation is confirmed as a periodic amplitude modulation instead of
systematic drifting. The periodic amplitude modulation in the IP is found to be
anti-correlated with that in the weak preceding component of the MP (MP_I), but
correlated with that in the first two components of the MP (MP_II), which
implies that the modulation patterns in the IP and the MP are phase-locked.
What is more interesting is that the modulation in MP_II is delayed that in the
IP by about 1P. Furthermore, high sensitivity observations by FAST reveal that
weak emission exists between the MP and the IP. In addition, we confirm that
the separation between the IP and the MP is independent of radio frequency. The
above results are a conundrum for pulsar theories and cannot be satisfactorily
explained by the current pulsar models. Therefore, our results observed with
FAST provide an opportunity to probe the structure of pulsar emission and the
neutron star's magnetosphere. | astro-ph |
The Gould's Belt distance survey: Very Long Baseline Interferometry (VLBI) observations can provide the
position of compact radio sources with an accuracy of order 50
micro-arcseconds. This is sufficient to measure the trigonometric parallax and
proper motions of any object within 500 pc of the Sun to better than a few
percent. Because they are magnetically active, young stars are often associated
with compact radio emission detectable using VLBI techniques. Here we will show
how VLBI observations have already constrained the distance to the most often
studied nearby regions of star-formation (Taurus, Ophiuchus, Orion, etc.) and
have started to provide information on their internal structure and kinematics.
We will then briefly describe a large project (called The Gould's Belt Distance
Survey) designed to provide a detailed view of star-formation in the Solar
neighborhood using VLBI observations. | astro-ph |
Annual and daily ideal periods for deliquescence at the landing site of
InSight based on GCM model calculations: Liquid water is one of the key elements in the search for possible life
outside of the Earth and has a wide range of consequences on various chemical
and geological processes. The InSight probe landed on Mars with a special
equipment dedicated to examine geophysical characteristics and internal heat
flow of the planet and some meteorological instruments also included in the
payload. We examine the annual and daily variations of near-surface relative
humidity and surface temperature calculated from the General Circulation Model
(GCM) at Elysium Planitia, the landing site of InSight and search for possible
ideal times for deliquescence. We inspect three different hygroscopic salts,
but find that out of the three only calcium-perchlorate could liquify at the
environment of InSight. We find that nighttime ideal periods could occur in a
limited window between approximately Ls 90 and 150 at the late evening hours
centered around 9 PM. In our daily studies we find no instances where the whole
night could be ideal for deliquescence. This is mostly due to the temperatures
dropping below eutectic level leading to a 0.5 - 2 hour long presumed ideal
period before midnight. On multiple occasions the temperature is just a few
degrees below the necessary limit while relative humidity is high enough,
therefore the precise temperature measurements of InSight could be critical in
determining ideal periods for deliquescence. | astro-ph |
Is Spectral Width a Reliable Measure of GRB Emission Physics?: The spectral width and sharpness of unfolded, observed GRB spectra have been
presented as a new tool to infer physical properties about GRB emission via
spectral fitting of empirical models. Following the tradition of the
'line-of-death', the spectral width has been used to rule out synchrotron
emission in a majority of GRBs. This claim is investigated via reexamination of
previously reported width measures. Then, a sample of peak-flux GRB spectra are
fit with an idealized, physical synchrotron model. It is found that many
spectra can be adequately fit by this model even when the width measures would
reject it. Thus, the results advocate for fitting a physical model to be the
sole tool for testing that model. Finally, a smoothly-broken power law is fit
to these spectra allowing for the spectral curvature to vary during the fitting
process in order to understand why the previous width measures poorly predict
the spectra. It is found that the failing of previous width measures is due to
a combination of inferring physical parameters from unfolded spectra as well as
the presence of multiple widths in the data beyond what the Band function can
model. | astro-ph |
Asteroseismology of 1523 misclassified red giants using
$\textit{Kepler}$ data: We analysed solar-like oscillations in 1523 $\textit{Kepler}$ red giants
which have previously been misclassified as subgiants, with predicted $\nu_{\rm
max}$ values (based on the Kepler Input Catalogue) between 280$\mu$Hz to
700$\mu$Hz. We report the discovery of 626 new oscillating red giants in our
sample, in addition to 897 oscillators that were previously characterized by
Hekker et al. (2011) from one quarter of $\textit{Kepler}$ data. Our sample
increases the known number of oscillating low-luminosity red giants by $26\%$
(up to $\sim$ 1900 stars). About three quarters of our sample are classified as
ascending red-giant-branch stars, while the remainder are red-clump stars. A
novel scheme was applied to determine $\Delta \nu$ for 108 stars with $\nu_{\rm
max}$ close to the Nyquist frequency (240$\mu$Hz < $\nu_{\rm max}$ <
320$\mu$Hz). Additionally, we identified 47 stars oscillating in the
super-Nyquist frequency regime, up to 387$\mu$Hz, using long-cadence light
curves. We show that the misclassifications are most likely due to large
uncertainties in KIC surface gravities, and do not result from the absence of
broadband colors or from different physical properties such as reddening,
spatial distribution, mass or metallicity. The sample will be valuable to study
oscillations in low-luminosity red giants and to characterize planet candidates
around those stars. | astro-ph |
3D spectroscopic surveys: Exploring galaxy evolution mechanisms: I review the major surveys of high redshift galaxies observed using integral
field spectroscopy techniques in the visible and in the infrared. The
comparison of various samples has to be done with care since they have
different properties linked to their parent samples, their selection criteria
and the methods used to study them. I present the various kinematic types of
galaxies that are identified within these samples (rotators, mergers, etc.) and
summarize the discussions on the mass assembly processes at various redshifts
deduced from these classifications: at intermediate redshift (z~0.6) merger may
be the main mass assembly process whereas the role of cold gas accretion along
cosmic web filaments may increase with redshift. The baryonic Tully-Fisher
relation is also discussed. This relation seems to be already in place 3 Gyr
after the Big-Bang and is then evolving until the present day. This evolution
is interpreted as an increase of the stellar mass content of dark matter haloes
of a given mass. The discovery of positive abundance gradients in MASSIV and
LSD/AMAZE samples is highlighted. At z~3 this discovery might be linked to cold
gas accretion along cosmic filaments toward the centre whereas at lower
redshift (z~1.3), this may be mainly due to accretion of gas from outer
reservoirs toward the centre via tidal tails due to interactions. | astro-ph |
Wave reflection and transmission at interface of convective and stably
stratified regions in a rotating star or planet: We use a simplified model to study wave reflection and transmission at
interface of convective region and stably stratified region (e.g. radiative
zone in star or stratification layer in gaseous planet). Inertial wave in
convective region and gravito-inertial wave in stably stratified region are
considered. We begin with polar area and then extend to any latitude. Six cases
are discussed (see Table 1), and in Case 2 both waves co-exist in both regions.
Four configurations are further discussed for Case 2. The angles and energy
ratios of wave reflection and transmission are calculated. It is found that
wave propagation and transmission depend on the ratio of buoyancy frequency to
rotational frequency. In a rapidly rotating star or planet wave propagates
across interface and most of energy of incident wave is transmitted to the
other region, but in a slowly rotating star or planet wave transmission is
inhibited. | astro-ph |
A simple model for the evolution of super-massive black holes and the
quasar population: An empirically motivated model is presented for accretion-dominated growth of
the super massive black holes (SMBH) in galaxies, and the implications are
studied for the evolution of the quasar population in the universe. We
investigate the core aspects of the quasar population, including space density
evolution, evolution of the characteristic luminosity, plausible minimum masses
of quasars, the mass function of SMBH and their formation epoch distribution.
Our model suggests that the characteristic luminosity in the quasar luminosity
function arises primarily as a consequence of a characteristic mass scale above
which there is a systematic separation between the black hole and the halo
merging rates. At lower mass scales, black hole merging closely tracks the
merging of dark halos. When combined with a declining efficiency of black hole
formation with redshift, the model can reproduce the quasar luminosity function
over a wide range of redshifts. The observed space density evolution of quasars
is well described by formation rates of SMBH above $\sim 10^8 M_\odot$. The
inferred mass density of SMBH agrees with that found independently from
estimates of the SMBH mass function derived empirically from the quasar
luminosity function. | astro-ph |
Constraints on the explosion mechanism and progenitors of type Ia
supernovae: Observations of SN 2011fe at early times reveal an evolution analogous to a
fireball model of constant color. In contrast, our unmixed delayed detonations
of Chandrasekhar-mass white dwarfs (DDC series) exhibit a faster brightening
concomitant with a shift in color to the blue. In this paper, we study the
origin of these discrepancies. We find that strong chemical mixing largely
resolves the photometric mismatch at early times, but it leads to an enhanced
line broadening that contrasts, for example, with the markedly narrow SiII6355A
line of SN 2011fe. We also explore an alternative configuration with
pulsational-delayed detonations (PDDEL model series). Because of the pulsation,
PDDEL models retain more unburnt carbon, have little mass at high velocity, and
have a much hotter outer ejecta after the explosion. The pulsation does not
influence the inner ejecta, so PDDEL and DDC models exhibit similar radiative
properties beyond maximum. However, at early times, PDDEL models show bluer
optical colors and a higher luminosity, even for weak mixing. Their early-time
radiation is derived primarily from the initial shock-deposited energy in the
outer ejecta rather than radioactive decay heating. Furthermore, PDDEL models
show short-lived CII lines, reminiscent of SN 2013dy. They typically exhibit
lines that are weaker, narrower, and of near-constant width, reminiscent of SN
2011fe. In addition to multi-dimensional effects, varying configurations for
such ``pulsations" offer a source of spectral diversity amongst SNe Ia. PDDEL
and DDC models also provide one explanation for low- and high-velocity gradient
SNe Ia. | astro-ph |
Large Tensor Non-Gaussianity from Axion-Gauge Fields Dynamics: We show that an inflation model in which a spectator axion field is coupled
to an SU(2) gauge field produces a large three-point function (bispectrum) of
primordial gravitational waves, $B_{h}$, on the scales relevant to the cosmic
microwave background experiments. The amplitude of the bispectrum at the
equilateral configuration is characterized by
$B_{h}/P_h^2=\mathcal{O}(10)\times \Omega_A^{-1}$, where $\Omega_A$ is a
fraction of the energy density in the gauge field and $P_h$ is the power
spectrum of gravitational waves produced by the gauge field. | astro-ph |
Detecting the Glint of Starlight on the Oceans of Distant Planets: We propose that astronomers will be eventually be able to discriminate
between extrasolar Earth-like planets with surface oceans and those without
using the shape of phase light curves in the visible and near-IR spectrum. We
model the visible light curves of planets having Earth-like surfaces, seasons,
and optically-thin atmospheres with idealized diffuse-scattering clouds. We
show that planets partially covered by water will appear measurably brighter
near crescent phase (relative to Lambertian planets) because of the efficient
specular reflection (i.e., glint) of starlight incident on their surfaces at a
highly oblique angle. Planets on orbits within 30 degrees of edge-on
orientation (half of all planets) will show pronounced glint over a sizeable
range of orbital longitudes, from quadrature to crescent, all outside the glare
of their parent stars. Also, water-covered planets will appear darker than a
Lambertian disk near full illumination. Finally, we show that planets with a
mixed land/water surface will polarize the reflected signal by as much as 30-70
percent. These results suggest several new ways of directly identifying water
on distant planets. | astro-ph |
VLBI Scrutiny of a New Neutrino-Blazar Multiwavelength-Flare Coincidence: In the past years, evidence has started piling up that some high-energy
cosmic neutrinos can be associated with blazars in flaring states. On February
26, 2022, a new blazar-neutrino coincidence has been reported: the track-like
neutrino event IC220225A detected by IceCube is spatially coincident with the
flat-spectrum radio quasar PKS 0215+015. Like previous associations, this
source was found to be in a high optical and ${\gamma}$-ray state. Moreover,
the source showed a bright radio outburst, which substantially increases the
probability of a true physical association. We have performed six observations
with the VLBA shortly after the neutrino event with a monthly cadence and are
monitoring the source with the Effelsberg 100m-Telescope, and with the
Australia Compact Telescope Array. Here, we present first results on the
contemporary parsec-scale jet structure of PKS 0215+015 in total intensity and
polarization to constrain possible physical processes leading to neutrino
emission in blazars. | astro-ph |
The Galactic WN stars: Spectral analyses with line-blanketed model
atmospheres versus stellar evolution models with and without rotation: CONTEXT: Very massive stars pass through the Wolf-Rayet (WR) stage before
they finally explode. Details of their evolution have not yet been safely
established, and their physics are not well understood. Their spectral analysis
requires adequate model atmospheres, which have been developed step by step
during the past decades and account in their recent version for line blanketing
by the millions of lines from iron and iron-group elements. However, only very
few WN stars have been re-analyzed by means of line-blanketed models yet.
AIMS: The quantitative spectral analysis of a large sample of Galactic WN
stars with the most advanced generation of model atmospheres should provide an
empirical basis for various studies about the origin, evolution, and physics of
the Wolf-Rayet stars and their powerful winds.
METHODS: We analyze a large sample of Galactic WN stars by means of the
Potsdam Wolf-Rayet (PoWR) model atmospheres, which account for iron line
blanketing and clumping. The results are compared with a synthetic population,
generated from the Geneva tracks for massive star evolution. RESULTS: We obtain
a homogeneous set of stellar and atmospheric parameters for the Galactic WN
stars, partly revising earlier results.
CONCLUSIONS: Comparing the results of our spectral analyses of the Galactic
WN stars with the predictions of the Geneva evolutionary calculations, we
conclude that there is rough qualitative agreement. However, the quantitative
discrepancies are still severe, and there is no preference for the tracks that
account for the effects of rotation. It seems that the evolution of massive
stars is still not satisfactorily understood. | astro-ph |
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