text: 267,129,1334130228512354304,314014164,Adrian Price-Whelan,"Paper day! <LINK> ""Orbital Torus Imaging"" is a new method for dynamical inference (measuring the Galactic mass / dark matter distribution, etc.) that exploits the existence of element-abundance gradients, like these from @APOGEEsurvey: <LINK> Conceptually, the method words because the element-abundance contours ""show"" us the shapes of orbits, demonstrated here with vertical kinematics of stars (in z–vz) <LINK> You can think of ""Orbital Torus Imaging"" as a replacement for Jeans modeling that will be more precise and requires fewer assumptions in practice: We only require that orbits are phase-mixed locally in action-space We do a simple demonstration in this paper using @APOGEEsurvey data: Using just 8 element abundances, and only modeling the vertical kinematics of stars (but don't assume separability) we get constraints on the disk mass and scale height that are precise to a few percent <LINK> Plus, now we finally have a (dynamical) use for all of those millions x 30 element abundances that spectroscopic surveys like @APOGEEsurvey, @galahsurvey, LAMOST, etc. are delivering :) As usual in Galactic dynamics, there are many caveats and we make many assumptions, but I'm excited to see what we will learn when we apply this to larger data sets and with more ambitious / flexible models of the Galactic mass! Thanks to everyone that contributed and helped with this project along the way! @davidwhogg @kvj_astro @melissakness @FritzZwicky @rareflwr41 + many others",https://arxiv.org/abs/2012.00015,"Many approaches to galaxy dynamics assume that the gravitational potential is simple and the distribution function is time-invariant. Under these assumptions there are traditional tools for inferring potential parameters given observations of stellar kinematics (e.g., Jeans models). However, spectroscopic surveys measure many stellar properties beyond kinematics. Here we present a new approach for dynamical inference, Orbital Torus Imaging, which makes use of kinematic measurements and element abundances (or other invariant labels). We exploit the fact that, in steady state, stellar labels vary systematically with orbit characteristics (actions), yet must be invariant with respect to orbital phases (conjugate angles). The orbital foliation of phase space must therefore coincide with surfaces along which all moments of all stellar label distributions are constant. Both classical-statistics and Bayesian methods can be built on this; these methods will be more robust and require fewer assumptions than traditional tools because they require no knowledge of the (spatial) survey selection function and they do not involve second moments of velocity distributions. We perform a classical-statistics demonstration with red giant branch stars from the APOGEE surveys: We model the vertical orbit structure in the Milky Way disk to constrain the local disk mass, scale height, and the disk--halo mass ratio (at fixed local circular velocity). We find that the disk mass can be constrained (na\""ively) at the few-percent level with Orbital Torus Imaging using only eight element-abundance ratios, demonstrating the promise of combining stellar labels with dynamical invariants. ","Orbital Torus Imaging: Using Element Abundances to Map Orbits and Mass