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The existence of high-$\alpha$ stars with inferred ages < 6 Gyr has been confirmed recently with large spectroscopic and photometric surveys. However, stellar mergers or binary interactions can induce properties associated with young ages, such as high mass, rapid rotation, or high activity, even in old populations. Literature studies have confirmed that at least some of these apparently young stars are old merger products. However, none have ruled out the possibility of genuinely young high-$\alpha$ stars. Because cool GKM dwarfs spin down, rapid rotation can be used to indicate youth. In this paper, we provide strong evidence that truly young high-$\alpha$ stars exist by studying high-$\alpha$ rotators in the Kepler and K2 field with abundance measurements from GALAH and APOGEE. After excluding close binaries using radial velocity (RV) measurements from Gaia DR3 and multi-epoch RVs from APOGEE, we find a total of 70 high-$\alpha$ rapid rotators with periods ~10-30 days, 29 of which have lithium measurements from GALAH, indicating that they have not gone through past mass transfer or stellar merger events. We identify 10 young high-$\alpha$ candidates with no signs of merger-induced mixing or close companions. One clear example is a G dwarf with a measurable rotation and an age of 1.98$^{+0.12}_{-0.28}$ Gyr that is likely a single star with multiple RV measurements from APOGEE, has significant lithium detection from GALAH (A(Li) = 1.79), and has no signs of planet engulfment., Comment: 21 pages, 13 figures, submitted to AJ

The infall of the LMC into the Milky Way (MW) has dynamical implications throughout the MW's dark matter halo. We study the impact of this merger on the statistical properties of populations of simulated stellar streams. Specifically, we investigate the radial and on-sky angular dependence of stream perturbations caused by the direct effect of stream-LMC interactions and/or the response of the MW dark matter halo. We use a time-evolving MW--LMC simulation described by basis function expansions to simulate streams. We quantify the degree of perturbation using a set of stream property statistics including the misalignment of proper motions with the stream track. In the outer halo, direct stream--LMC interactions produce a statistically significant effect, boosting the fraction of misaligned proper motions by ~25% compared to the model with no LMC. Moreover, there is on-sky angular dependence of stream perturbations:~the highest fractions of perturbed streams coincide with the same on-sky quadrant as the present-day LMC location. In the inner halo, the MW halo dipole response primarily drives stream perturbations, but it remains uncertain whether this is a detectable signature distinct from the LMC's influence. For the fiducial MW--LMC model, we find agreement between the predicted fraction of streams with significantly misaligned proper motions, $\bar{\vartheta}>10^{\circ}$, and Dark Energy Survey data. Finally, we predict this fraction for the Vera Rubin Large Synoptic Survey Telescope (LSST) footprint. Using LSST data will improve our constraints on dark matter models and LMC properties as it is sensitive to both., Comment: ApJ submitted. 22 pages, 8 figures

We present cogsworth, an open-source Python tool for producing self-consistent population synthesis and galactic dynamics simulations. With cogsworth one can (1) sample a population of binaries and star formation history, (2) perform rapid (binary) stellar evolution, (3) integrate orbits through the galaxy and (4) inspect the full evolutionary history of each star or compact object, as well as their positions and kinematics. We include the functionality for post-processing hydrodynamical zoom-in simulations as a basis for galactic potentials and star formation histories to better account for initial spatial stellar clustering and more complex potentials. Alternatively, several analytic models are available for both the potential and star formation history. cogsworth can transform the intrinsic simulated population to an observed population through the joint application of dust maps, bolometric correction functions and survey selection functions. We provide a detailed explanation of the functionality of cogsworth and demonstrate its capabilities through a series of use cases: (1) We predict the spatial distribution of compact objects and runaways in both dwarf and Milky-Way-like galaxies, (2) using a star cluster from a hydrodynamical simulation, we show how supernovae can change the orbits of stars in several ways, and (3) we predict the separation of disrupted binary stellar companions on the sky and create a synthetic Gaia colour-magnitude diagram. We also discuss some current limitations and plans for future developments. We designed cogsworth and its online documentation to provide a powerful tool for constraining binary evolution, but also a flexible and accessible resource for the entire community., Comment: 14 pages, 9 figures. Submitted to ApJS & JOSS. Comments welcome! Documentation at https://cogsworth.readthedocs.io

Stellar streams in the Milky Way are promising detectors of low-mass dark matter (DM) subhalos predicted by $\Lambda$CDM. Passing subhalos induce perturbations in streams that indicate the presence of the subhalos. Understanding how known DM-dominated satellites impact streams is a crucial step towards using stream perturbations to constrain the properties of dark perturbers. Here, we cross-match a $\textit{Gaia}$ EDR3 and SEGUE member catalog of the Cetus-Palca stream (CPS) with H3 for additional radial velocity measurements and fit the orbit of the CPS using this 6-D data. We demonstrate for the first time that the ultra-faint dwarf Segue 2 had a recent (77$\pm$5 Myr ago) close flyby (within the stream's 2$\sigma$ width) with the CPS. This interaction enables constraints on Segue 2's mass and density profile at larger radii ($\mathcal{O}(1)$ kpc) than are probed by its stars ($\mathcal{O}(10)$ pc). While Segue 2 is not expected to strongly affect the portion of the stream covered by our 6-D data, we predict that if Segue 2's mass within $\sim 6$ kpc is $5\times 10^9\,M_\odot$, the CPS's velocity dispersion will be $\sim 40$ km/s larger ahead of the impact site than behind it. If no such heating is detected, Segue 2's mass cannot exceed $10^9\,M_\odot$ within $\sim 6$ kpc. The proper motion distribution of the CPS near the impact site is mildly sensitive to the shape of Segue 2's density profile. This study presents a critical test for frameworks designed to constrain properties of dark subhalos from stream perturbations., Comment: 33 pages, 14 figures, 6 tables. Submitted to ApJ, comments welcome

The hierarchical model of galaxy formation predicts that the Milky Way halo is populated by tidal debris of dwarf galaxies and globular clusters. Due to long dynamical times, debris from the lowest mass objects remains coherent as thin and dynamically cold stellar streams for billions of years. The Gaia mission, providing astrometry and spectrophotometry for billions of stars, has brought three fundamental changes to our view of stellar streams in the Milky Way. First, more than a hundred stellar streams have been discovered and characterized using Gaia data. This is an order of magnitude increase in the number of known streams, thanks to Gaia's capacity for identifying comoving groups of stars among the field Milky Way population. Second, Gaia data have revealed that density variations both along and across stellar streams are common. Dark-matter subhalos, as well as baryonic structures were theoretically predicted to form such features, but observational evidence for density variations was uncertain before Gaia. Third, stream kinematics are now widely available and have constrained the streams' orbits and origins. Gaia has not only provided proper motions directly, but also enabled efficient spectroscopic follow-up of the proper-motion selected targets. These discoveries have established stellar streams as a dense web of sensitive gravitational tracers in the Milky Way halo. We expect the coming decade to bring a full mapping of the Galactic population of stellar streams, as well as develop numerical models that accurately capture their evolution within the Milky Way for a variety of cosmological models. Perhaps most excitingly, the comparison between the two will be able to reveal the presence of dark-matter subhalos below the threshold for galaxy formation (~10^6 Msun), and provide the most stringent test of the cold dark matter paradigm on small scales., Comment: Review article for New Astronomy Reviews (as part of a Special Issue: "Gaia, the first crop of discoveries"). Comments are welcome!

Chemical cartography of the Galactic disk provides insights to its structure and assembly history over cosmic time. In this work, we use chemical cartography to explore chemical gradients and azimuthal substructure in the Milky Way disk with giant stars from APOGEE DR17. We confirm the existence of a radial metallicity gradient in the disk of $\Delta$[Fe/H]/$\Delta$R $\sim -0.066 \pm 0.0004$ dex/kpc and a vertical metallicity gradient of $\Delta$[Fe/H]/$\Delta$Z $\sim -0.164 \pm 0.001$ dex/kpc. We find azimuthal variations ($\pm0.1$ dex) on top of the radial metallicity gradient that have been previously established with other surveys. The APOGEE giants show strong correlations with stellar age and the intensity of azimuthal variations in iron; older stellar populations show the largest deviations from the radial metallicity gradient. Beyond iron, we show that other elements (e.g., Mg, O) display azimuthal variations at the $\pm0.05$ dex-level across the Galactic disk. We illustrate that moving into the orbit-space could help constrain the mechanisms producing these azimuthal metallicity variations. These results suggest that the spiral arms of the Galaxy are not solely responsible for azimuthal metallicity variations and other Galactic processes are at play., Comment: 19 pages, 11 figures

Gaia can not individually resolve very close binary systems, however, the collected data can still be used to identify them. A powerful indicator of stellar multiplicity is the sources reported Renormalized Unit Weight Error (ruwe), which effectively captures the astrometric deviations from single-source solutions. We aim to characterise the imprints left on ruwe caused by binarity. By flagging potential binary systems based on ruwe, we aim to characterise which of their properties will contribute the most to their detectability. We develop a model to estimate ruwe values for observations of Gaia sources, based on the biases to the single-source astrometric track arising from the presence of an unseen companion. Then, using the recipes from previous GaiaUnlimited selection functions, we estimate the selection probability of sources with high ruwe, and discuss what binary properties contribute to increasing the sources ruwe. We compute the maximum ruwe value which is compatible with single-source solutions as a function of their location on-sky. We see that binary systems selected as sources with a ruwe higher than this sky-varying threshold have a strong detectability window in their orbital period distribution, which peaks at periods equal to the Gaia observation time baseline. We demonstrate how our sky-varying ruwe threshold provides a more complete sample of binary systems when compared to single sky-averaged values by studying the unresolved binary population in the Gaia Catalogue of Nearby Stars. We provide the code and tools used in this study, as well as the sky-varying ruwe threshold through the GaiaUnlimited Python package, Comment: 11 pages, 10 figures. Accepted for publication in A&A

Stellar age measurements are fundamental to understanding a wide range of astronomical processes, including galactic dynamics, stellar evolution, and planetary system formation. However, extracting age information from Main Sequence stars is complicated, with techniques often relying on age proxies in the absence of direct measurements. The Gaia data releases have enabled detailed studies of the dynamical properties of stars within the Milky Way, offering new opportunities to understand the relationship between stellar age and dynamics. In this study, we leverage high-precision astrometric data from Gaia DR3 to construct a stellar age prediction model based only on stellar dynamical properties; namely, the vertical action. We calibrate two distinct, hierarchical stellar age--vertical action relations, first employing asteroseismic ages for red giant branch stars, then isochrone ages for main-sequence turn-off stars. We describe a framework called "zoomies" based on this calibration, by which we can infer ages for any star given its vertical action. This tool is open-source and intended for community use. We compare dynamical age estimates from "zoomies" with ages derived from other techniques for a sample of open clusters and main-sequence stars with asteroseismic age measurements. We also compare dynamical age estimates for stellar samples from the Kepler, K2, and TESS exoplanet transit surveys. While dynamical age relations are associated with large uncertainty, they are generally mass-independent and depend on homogeneously measured astrometric data. These age predictions are uniquely useful for large-scale demographic investigations, especially in disentangling the relationship between planet occurrence, metallicity, and age for low-mass stars., Comment: 23 pages, 12 figures, 2 tables. Submitted to AAS Journals

Reconstructing the mass assembly history of the Milky Way relies on obtaining detailed measurements of the properties of many stars in the Galaxy, especially in the stellar halo. One of the most constraining quantities is stellar age, as it can shed light on the accretion time and quenching of star formation in merging satellites. However, obtaining reliable age estimates for large samples of halo stars is difficult. We report published ages of 120 subgiant halo stars with highly-radial orbits that likely belong to the debris of the \textsl{Gaia-Enceladus/Sausage}~(GES) galaxy. The majority of these halo stars are old, with an age distribution characterized by a median of 11.6~Gyr and 16$^{\rm th}$(84$^{\rm th}$) percentile of 10.5~(12.7)~Gyr. However, the distribution is skewed, with a tail of younger stars that span ages down to $\sim6$--$9$ Gyr. All highly-radial halo stars have chemical and kinematic/orbital quantities that associate them with the GES debris. Initial results suggest that these intermediate-age stars are not a product of mass transfer and/or stellar mergers, which can bias their age determination low. If this conclusion is upheld by upcoming spectro-photometric studies, then the presence of these stars will pose an important challenge for constraining the properties of the GES merger and the accretion history of the Galaxy., Comment: Accepted for publication to ApJ after minor edits to the manuscript. 18 pages, 20 figures, 1 Table

Stars formed with initial mass over 50 Msun are very rare today, but they are thought to be more common in the early universe. The fates of those early, metal-poor, massive stars are highly uncertain. Most are expected to directly collapse to black holes, while some may explode as a result of rotationally powered engines or the pair-creation instability. We present the chemical abundances of J0931+0038, a nearby low-mass star identified in early followup of SDSS-V Milky Way Mapper, which preserves the signature of unusual nucleosynthesis from a massive star in the early universe. J0931+0038 has relatively high metallicity ([Fe/H] = -1.76 +/- 0.13) but an extreme odd-even abundance pattern, with some of the lowest known abundance ratios of [N/Fe], [Na/Fe], [K/Fe], [Sc/Fe], and [Ba/Fe]. The implication is that a majority of its metals originated in a single extremely metal-poor nucleosynthetic source. An extensive search through nucleosynthesis predictions finds a clear preference for progenitors with initial mass > 50 Msun, making J0931+0038 one of the first observational constraints on nucleosynthesis in this mass range. However the full abundance pattern is not matched by any models in the literature. J0931+0038 thus presents a challenge for the next generation of nucleosynthesis models and motivates study of high-mass progenitor stars impacted by convection, rotation, jets, and/or binary companions. Though rare, more examples of unusual early nucleosynthesis in metal-poor stars should be found in upcoming large spectroscopic surveys., Comment: 11 pages + 22 page appendix, accepted to ApJL

Under the assumption of a simple and time-invariant gravitational potential, many Galactic dynamics techniques infer the Milky Way's mass and dark matter distribution from stellar kinematic observations. These methods typically rely on parameterized potential models of the Galaxy and must take into account non-trivial survey selection effects, because they make use of the density of stars in phase space. Large-scale spectroscopic surveys now supply information beyond kinematics in the form of precise stellar label measurements (especially element abundances). These element abundances are known to correlate with orbital actions or other dynamical invariants. Here, we use the Orbital Torus Imaging (OTI) framework that uses abundance gradients in phase space to map orbits. In many cases these gradients can be measured without detailed knowledge of the selection function. We use stellar surface abundances from the APOGEE survey combined with kinematic data from the Gaia mission. Our method reveals the vertical ($z$-direction) orbit structure in the Galaxy and enables empirical measurements of the vertical acceleration field and orbital frequencies in the disk. From these measurements, we infer the total surface mass density, $\Sigma$, and midplane volume density, $\rho_0$, as a function of Galactocentric radius and height. Around the Sun, we find $\Sigma_{\odot}(z=1.1$ kpc)$=72^{+6}_{-9}$M$_{\odot}$pc$^{-2}$ and $\rho_{\odot}(z=0)=0.081^{+0.015}_{-0.009}$ M$_{\odot}$pc$^{-3}$ using the most constraining abundance ratio, [Mg/Fe]. This corresponds to a dark matter contribution in surface density of $\Sigma_{\odot,\mathrm{DM}}(z=1.1$ kpc)$=24\pm4$ M$_{\odot}$pc$^{-2}$, and in total volume mass density of $\rho_{\odot,\mathrm{DM}}(z=0)=0.011\pm0.002$ M$_{\odot}$pc$^{-3}$. Moreover, using these mass density values we estimate the scale length of the low-$\alpha$ disc to be $h_R=2.24\pm0.06$kpc., Comment: Accepted for publication in ApJ. 19 pages, 11 figures, 3 Tables

Understanding the physical mechanism behind the formation of a co-rotating thin plane of satellite galaxies, like the one observed around the Milky Way (MW), has been challenging. The perturbations induced by a massive satellite galaxy, like the Large Magellanic Cloud (LMC) provide valuable insight into this problem. The LMC induces an apparent co-rotating motion in the outer halo by displacing the inner regions of the halo with respect to the outer halo. Using the Latte suite of FIRE-2 cosmological simulations of MW-mass galaxies, we confirm that the apparent motion of the outer halo induced by the infall of a massive satellite changes the observed distribution of orbital poles of outer-halo tracers, including satellites. We quantify the changes in the distribution of orbital poles using the two-point angular correlation function and find that all satellites induce changes. However, the most massive satellites with pericentric passages between 30-100kpc induce the largest changes. The best LMC-like satellite analog shows the largest change in orbital pole distribution. The dispersion of orbital poles decreases by 20{\deg} during the first two pericentric passages. Even when excluding the satellites brought in with the LMC-like satellite, there is clustering of orbital poles. These results suggest that in the MW, the recent pericentric passage of the LMC should have changed the observed distribution of orbital poles of all other satellites. Therefore, studies of kinematically-coherent planes of satellites that seek to place the MW in a cosmological context should account for the existence of a massive satellite like the LMC., Comment: 20 pages, 10 figures. ApJ submitted, Comments are welcome

The stellar stream connected to the Sagittarius (Sgr) dwarf galaxy is the most massive tidal stream that has been mapped in the Galaxy, and is the dominant contributor to the outer stellar halo of the Milky Way. We present metallicity maps of the Sgr stream, using 34,240 red giant branch stars with inferred metallicities from Gaia BP/RP spectra. This sample is larger than previous samples of Sgr stream members with chemical abundances by an order of magnitude. We measure metallicity gradients with respect to Sgr stream coordinates $(\Lambda, B)$, and highlight the gradient in metallicity with respect to stream latitude coordinate $B$, which has not been observed before. We find $\nabla \mathrm{[M/H]} = -2.48 \pm 0.08 \times 10^{-2}$ dex/deg above the stream track ($B>B_0$ where $B_0=1.5$ deg is the latitude of the Sgr remnant) and $\nabla \mathrm{[M/H]} =- 2.02 \pm 0.08 \times 10^{-2}$ dex/deg below the stream track ($B

As our nearest large neighbor, the Andromeda Galaxy provides a unique laboratory for investigating galaxy formation and the distribution and substructure properties of dark matter in a Milky Way-like galaxy. Here, we propose an initial 2-epoch ($\Delta t\approx 5$yr), 2-band Roman survey of the entire halo of Andromeda, covering 500 square degrees, which will detect nearly every red giant star in the halo (10$\sigma$ detection in F146, F062 of 26.5, 26.1AB mag respectively) and yield proper motions to $\sim$25 microarcsec/year (i.e., $\sim$90 km/s) for all stars brighter than F146 $\approx 23.6$ AB mag (i.e., reaching the red clump stars in the Andromeda halo). This survey will yield (through averaging) high-fidelity proper motions for all satellites and compact substructures in the Andromeda halo and will enable statistical searches for clusters in chemo-dynamical space. Adding a third epoch during the extended mission will improve these proper motions by $\sim t^{-1.5}$, to $\approx 11$ km/s, but this requires obtaining the first epoch in Year 1 of Roman operations. In combination with ongoing and imminent spectroscopic campaigns with ground-based telescopes, this Roman survey has the potential to yield full 3-d space motions of $>$100,000 stars in the Andromeda halo, including (by combining individual measurements) robust space motions of its entire globular cluster and most of its dwarf galaxy satellite populations. It will also identify high-velocity stars in Andromeda, providing unique information on the processes that create this population. These data offer a unique opportunity to study the immigration history, halo formation, and underlying dark matter scaffolding of a galaxy other than our own., Comment: Submitted in response to the call for Roman Space Telescope Core Community Survey white papers

The Nancy Grace Roman Space Telescope is capable of delivering an unprecedented all-sky, high-spatial resolution, multi-epoch infrared map to the astronomical community. This opportunity arises in the midst of numerous ground- and space-based surveys that will provide extensive spectroscopy and imaging together covering the entire sky (such as Rubin/LSST, Euclid, UNIONS, SPHEREx, DESI, SDSS-V, GALAH, 4MOST, WEAVE, MOONS, PFS, UVEX, NEO Surveyor, etc.). Roman can uniquely provide uniform high-spatial-resolution (~0.1 arcsec) imaging over the entire sky, vastly expanding the science reach and precision of all of these near-term and future surveys. This imaging will not only enhance other surveys, but also facilitate completely new science. By imaging the full sky over two epochs, Roman can measure the proper motions for stars across the entire Milky Way, probing 100 times fainter than Gaia out to the very edge of the Galaxy. Here, we propose NANCY: a completely public, all-sky survey that will create a high-value legacy dataset benefiting innumerable ongoing and forthcoming studies of the universe. NANCY is a pure expression of Roman's potential: it images the entire sky, at high spatial resolution, in a broad infrared bandpass that collects as many photons as possible. The majority of all ongoing astronomical surveys would benefit from incorporating observations of NANCY into their analyses, whether these surveys focus on nearby stars, the Milky Way, near-field cosmology, or the broader universe., Comment: Submitted to the call for white papers for the Roman Core Community Survey (June 16th, 2023), and to the Bulletin of the AAS

Stellar streams form through the tidal disruption of satellite galaxies or globular clusters orbiting a host galaxy. Globular cluster streams are exciting since they are thin (dynamically cold) and, therefore sensitive to perturbations from low-mass subhalos. Since the subhalo mass function differs depending on the dark matter composition, these gaps can provide unique constraints on dark matter models. However, current samples are limited to the Milky Way. With its large field of view, deep imaging sensitivity, and high angular resolution, the upcoming Nancy Grace Roman Space Telescope (Roman) presents a unique opportunity to increase the number of observed streams and gaps significantly. This paper presents a first exploration of the prospects for detecting gaps in streams in M31 and other nearby galaxies with resolved stars. We simulate the formation of gaps in a Palomar-5-like stream and generate mock observations of these gaps with background stars in M31 and the foreground Milky Way stellar fields. We assess Roman's ability to detect gaps out to 10 Mpc through visual inspection and with the gap-finding tool ${\texttt{FindTheGap}}$. We conclude that gaps of $\approx 1.5$ kpc in streams that are created from subhalos of masses $\geq5 \times 10^6$M$_{\odot}$ are detectable within a 2-3 Mpc volume in exposures of 1000s to 1 hour. This volume contains $\approx 150$ galaxies, including $\approx 8$ galaxies with luminosities $>10^{9}~$L$_{\odot}$. Large samples of stream gaps in external galaxies will open up a new era of statistical analyses of gap characteristics in stellar streams and help constrain dark matter models., Comment: ApJ version

The recent discovery of a spiral pattern in the vertical kinematic structure in the solar neighborhood provides a prime opportunity to study non-equilibrium dynamics in the Milky Way from local stellar kinematics. Furthermore, results from simulations indicate that even in a limited volume, differences in stellar orbital histories allow us to trace variations in the initial perturbation across large regions of the disk. We present $\texttt{ESCARGOT}$, a novel algorithm for studying these variations in both simulated and observed data sets. $\texttt{ESCARGOT}$ automatically extracts key quantities from the structure of a given phase spiral, including the time since perturbation and the perturbation mode. We test $\texttt{ESCARGOT}$ on simulated data and show that it is capable of accurately recovering information about the time since the perturbation occurred as well as subtle differences in phase spiral morphology due to stellar location in the disk at the time of perturbation. We apply $\texttt{ESCARGOT}$ to kinematic data from data release 3 of the ${\it Gaia}$ mission in bins of guiding radius. We show that similar structural differences in morphology occur in the ${\it Gaia}$ phase spirals as a function of stellar orbital history. These results indicate that the phase spirals are the product of a complex dynamical response in the disk with large-scale coupling between different regions of phase space., Comment: 20 pages, 12 figures

Recent observations have revealed a trove of unexpected morphological features in many of the Milky Way's stellar streams. Explanations for such features include time-dependent deformations of the Galactic gravitational potential, local disruptions induced by dark matter substructure, and special configurations of the streams' progenitors. In this paper, we study how these morphologies can also arise in certain static, non-spherical gravitational potentials that host a subset of resonantly-trapped orbit families. The transitions, or separatrices, between these orbit families mark abrupt discontinuities in the orbital structure of the potential. We develop a novel numerical approach for measuring the libration frequencies of resonant and near-resonant orbits, and apply it to study the evolution of stellar streams on these orbits. We reveal two distinct morphological features that arise in streams on near-resonant orbits: fans, that come about due to a large spread in the libration frequencies near a separatrix; and bifurcations, that arise when a separatrix splits the orbital distribution of the stellar stream between two (or more) distinct orbit families. We demonstrate that these effects can arise in some Milky Way streams for certain choices of the dark matter halo potential, and discuss how this might be used to probe and constrain the global shape of the Milky Way's gravitational potential., Comment: 24 pages, 8 figures. Submitted to ApJ; comments welcome!

Massive disk galaxies like our Milky Way should host an ancient, metal-poor, and centrally concentrated stellar population. This population reflects the star formation and enrichment in the few most massive progenitor components that coalesced at high redshift to form the proto-Galaxy. While metal-poor stars are known to reside in the inner few kiloparsecs of our Galaxy, current data do not yet provide a comprehensive picture of such a metal-poor "heart" of the Milky Way. We use information from Gaia DR3, especially the XP spectra, to construct a sample of 2 million bright (BP $<15.5$ mag) giant stars within $30^\circ$ of the Galactic Center with robust [M/H] estimates, $\delta$ [M/H] $\lesssim 0.1$. For most sample members we can calculate orbits based on Gaia RVS velocities and astrometry. This sample reveals an extensive, ancient, and metal-poor population that includes $\sim 18,000$ stars with $-2.7<$ [M/H] $<-1.5$, representing a stellar mass of $\gtrsim 5\times 10^7$ M$_\odot$. The spatial distribution of these [M/H] $<-1.5$ stars has a Gaussian extent of only $\sigma_{\mathrm{R_{GC}}} \sim 2.7$ kpc around the Galactic center, with most of these orbits being confined to the inner Galaxy. At high orbital eccentricities, there is clear evidence for accreted halo stars in their pericentral orbit phase. Stars with [M/H] $< -2$ show no net rotation, whereas those with [M/H] $\sim -1$ are rotation dominated. Most of the tightly bound stars show $[\alpha/\text{Fe}]$-enhancement and [Al/Fe]-[Mn/Fe] abundance patterns expected for an origin in the more massive portions of the proto-Galaxy. These central, metal-poor stars most likely predate the oldest part of the disk ($\tau_{\text{age}}\approx 12.5$ Gyrs), which implies that they formed at $z\gtrsim 5$, forging the proto-Milky Way., Comment: 21 pages, 9 figures, submitted to ApJ

We created the APOGEE-GALEX-\emph{Gaia} catalog to study white dwarfs binaries. This database aims to create a minimally biased sample of WD binary systems identified from a combination of GALEX, {\it Gaia}, and APOGEE data to increase the number of WD binaries with orbital parameters and chemical compositions. We identify 3,414 sources as WD binary candidates, with nondegenerate companions of spectral types between F and M, including main sequence, main sequence binaries, subgiants, sub-subgiants, red giants, and red clump stars. Among our findings are (a) a total of 1,806 systems having inferred WD radii $R < 25$ R$_{\Earth}$, which constitute a more reliable group of WD binary candidates within the main sample; (b) a difference in the metallicity distribution function between WD binary candidates and the control sample of most luminous giants ($M_H < -3.0$); (c) the existence of a population of sub-subgiants with WD companions; (d) evidence for shorter periods in binaries that contain WDs compared to those that do not, as shown by the cumulative distributions of APOGEE radial velocity shifts; (e) evidence for systemic orbital evolution in a sample of 252 WD binaries with orbital periods, based on differences in the period distribution between systems with red clump, main sequence binary, and sub-subgiant companions and systems with main sequence or red giant companions; and (f) evidence for chemical enrichment during common envelope (CE) evolution, shown by lower metallicities in wide WD binary candidates ($P > 100$ days) compared to post-CE ($P < 100$ days) WD binary candidates., Comment: 22 pages, 18 figures. Accepted for publication in AJ

The Astropy Project supports and fosters the development of open-source and openly-developed Python packages that provide commonly needed functionality to the astronomical community. A key element of the Astropy Project is the core package $\texttt{astropy}$, which serves as the foundation for more specialized projects and packages. In this article, we summarize key features in the core package as of the recent major release, version 5.0, and provide major updates for the Project. We then discuss supporting a broader ecosystem of interoperable packages, including connections with several astronomical observatories and missions. We also revisit the future outlook of the Astropy Project and the current status of Learn Astropy. We conclude by raising and discussing the current and future challenges facing the Project., Comment: 43 pages, 5 figures. To appear in ApJ. The author list has two parts: the authors that made significant contributions to the writing and/or coordination of the paper, followed by maintainers of and contributors to the Astropy Project. The position in the author list does not correspond to contributions to the Astropy Project as a whole

The goal of the Open Cluster Chemical Abundances and Mapping (OCCAM) survey is to constrain key Galactic dynamic and chemical evolution parameters by the construction and analysis of a large, comprehensive, uniform data set of infrared spectra for stars in hundreds of open clusters. This sixth contribution from the OCCAM survey presents analysis of SDSS/APOGEE Data Release 17 (DR17) results for a sample of stars in 150 open clusters, 94 of which we designate to be "high quality'' based on the appearance of their color-magnitude diagram. We find the APOGEE DR17-derived [Fe/H] values to be in good agreement with those from previous high resolution spectroscopic open cluster abundance studies. Using a subset of the high quality sample, the Galactic abundance gradients were measured for 16 chemical elements, including [Fe/H], for both Galactocentric radius ($R_{GC}$) and guiding center radius ($R_{Guide}$). We find an overall Galactic [Fe/H] vs $R_{GC}$ gradient of $-0.073 \pm 0.002$ dex/kpc over the range of $6 < R_{GC} < 11.5$ kpc, and a similar gradient is found for [Fe/H] versus $R_{Guide}$. Significant Galactic abundance gradients are also noted for O, Mg, S, Ca, Mn, Na, Al, K and Ce. Our large sample additionally allows us to explore the evolution of the gradients in four age bins for the remaining 15 elements., Comment: 21 pages, 14 Figures, Astronomical Journal Accepted

Carbon-enhanced metal-poor (CEMP) stars comprise almost a third of stars with [Fe/H] < --2, although their origins are still poorly understood. It is highly likely that one sub-class (CEMP-$s$ stars) is tied to mass-transfer events in binary stars, while another sub-class (CEMP-no stars) are enriched by the nucleosynthetic yields of the first generations of stars. Previous studies of CEMP stars have primarily concentrated on the Galactic halo, but more recently they have also been detected in the thick disk and bulge components of the Milky Way. $Gaia$ DR3 has provided an unprecedented sample of over 200 million low-resolution ($R\approx$ 50) spectra from the BP and RP photometers. Training on the CEMP catalog from the SDSS/SEGUE database, we use XGBoost to identify the largest all-sky sample of CEMP candidate stars to date. In total, we find 58,872 CEMP star candidates, with an estimated contamination rate of 12%. When comparing to literature high-resolution catalogs, we positively identify 60-68% of the CEMP stars in the data, validating our results and indicating a high completeness rate. Our final catalog of CEMP candidates spans from the inner to outer Milky Way, with distances as close as $r \sim$ 0.8 kpc from the Galactic center, and as far as $r >$ 30 kpc. Future higher-resolution spectroscopic follow-up of these candidates will provide validations of their classification and enable investigations of the frequency of CEMP-$s$ and CEMP-no stars throughout the Galaxy, to further constrain the nature of their progenitors., Comment: 19 pages, 14 figures, accepted to MNRAS

{\it Gaia} Data Release 2 (DR2) revealed that the Milky Way contains significant indications of departures from equilibrium in the form of asymmetric features in the phase space density of stars in the Solar neighborhood. One such feature is the $z$--$v_z$ phase spiral, interpreted as the response of the disk to the influence of a perturbation perpendicular to the disk plane, which could be external (e.g., a satellite) or internal (e.g., the bar or spiral arms). In this work we use {\it Gaia} DR3 to dissect the phase spiral by dividing the local data set into groups with similar azimuthal actions, $J_\phi$, and conjugate angles, $\theta_\phi$, which selects stars on similar orbits and at similar orbital phases, thus having experienced similar perturbations in the past. These divisions allow us to explore areas of the Galactic disk larger than the surveyed region. The separation improves the clarity of the $z$--$v_z$ phase spiral and exposes changes to its morphology across the different action-angle groups. In particular, we discover a transition to two armed `breathing spirals' in the inner Milky Way. We conclude that the local data contains signatures of not one, but multiple perturbations with the prospect to use their distinct properties to infer the properties of the interactions that caused them., Comment: 5 pages, 3 figures, accepted in MNRAS letters

We present a novel method for constraining the length of the Galactic bar using 6D phase space information to directly integrate orbits. We define a pseudo-length for the Galactic bar, named $R_{Freq}$, based on the maximal extent of trapped bar orbits. We find the $R_{Freq}$ measured from orbits is consistent with the $R_{Freq}$ of the assumed potential only when the length of the bar and pattern speed of said potential is similar to the model from which the initial phase-space coordinates of the orbits are derived. Therefore, one can measure the model's or the Milky Way's bar length from 6D phase-space coordinates by determining which assumed potential leads to a self-consistent measured $R_{Freq}$. When we apply this method to $\approx$210,000 stars in APOGEE DR17 and $Gaia$ eDR3 data, we find a consistent result only for potential models with a dynamical bar length of $\approx$3.5 kpc. We find the Milky Way's trapped bar orbits extend out to only $\approx$3.5 kpc, but there is also an overdensity of stars at the end of the bar out to 4.8 kpc which could be related to an attached spiral arm. We also find that the measured orbital structure of the bar is strongly dependent on the properties of the assumed potential., Comment: 15 pages, 8 figures, 2 tables, accepted to MNRAS, comments welcome

Stellar streams in the Galactic halo are useful probes of the assembly of galaxies like the Milky Way. Many tidal stellar streams that have been found in recent years are accompanied by a known progenitor globular cluster or dwarf galaxy. However, the Orphan--Chenab (OC) stream is one case where a relatively narrow stream of stars has been found without a known progenitor. In an effort to find the parent of the OC stream, we use astrometry from the early third data release of ESA's Gaia mission (Gaia EDR3) and radial velocity information from the SDSS-IV APOGEE survey to find up to 13 stars that are likely members of the OC stream. We use the APOGEE survey to study the chemical nature (for up to 13 stars) of the OC stream in the $\alpha$ (O, Mg, Ca, Si, Ti, S), odd-Z (Al, K, V), Fe-peak (Fe, Ni, Mn, Co, Cr) and neutron capture (Ce) elemental groups. We find that the stars that make up the OC stream are not consistent with a mono-metallic population and have a median metallicity of --1.92~dex with a dispersion of 0.28 dex. Our results also indicate that the $\alpha$-elements are depleted compared to the known Milky Way populations and that its [Mg/Al] abundance ratio is not consistent with second generation stars from globular clusters. The detailed chemical pattern of these stars indicates that the OC stream progenitor is very likely to be a dwarf spheroidal galaxy with a mass of ~10$^6$ M$_\odot$., Comment: 13 Pages; 4 Figures. Submitted to AAS Journals, comments welcome

We investigate the effects of a massive ($\gtrsim4\times10^{10}M_\odot$) Sagittarius dwarf spheroidal galaxy (Sgr) on stellar streams using test particle simulations in a realistic Milky Way potential. We find that Sgr can easily disrupt streams formed more than $\sim3$ Gyr ago, while stars stripped more recently are generally unaffected. In certain realizations, Sgr is able to produce asymmetry between the leading and trailing tails of Pal 5, qualitatively similar to observations. Using data from the Gaia space telescope and elsewhere, we fit models to the GD-1 stream in the presence of a Sgr with various initial masses. While the best-fitting models do show perturbations resulting from interactions with Sgr, we find that the level of disruption is not significantly greater than in the observed stream. To investigate the general effects of Sgr on a population of streams, we generate 1000 mock streams on GD-1-like orbits with randomized orientations. Some streams show clear evidence of disruption, becoming folded on the sky or developing asymmetry betweeen their two tails. However, many survive unaffected and the peak surface brightness of stars is decreased by no more than $\sim0.3$ mag/arcsec$^2$ on average. We conclude that Sgr having an initial mass of $\gtrsim4\times10^{10}M_\odot$ is compatible with the survival and detection of streams formed more than 3 Gyr ago., Comment: 19 pages, 12 figures, published in MNRAS. Animated figures can be found at https://www.youtube.com/playlist?list=PLEleLLhXAwEMx6GcSror-iF-QsskHrXYM

In the coming decade, thousands of stellar streams will be observed in the halos of external galaxies. What fundamental discoveries will we make about dark matter from these streams? As a first attempt to look at these questions, we model Magellan/Megacam imaging of the Centaurus A's (Cen A) disrupting dwarf companion Dwarf 3 (Dw3) and its associated stellar stream, to find out what can be learned about the Cen A dark-matter halo. We develop a novel external galaxy stream-fitting technique and generate model stellar streams that reproduce the stream morphology visible in the imaging. We find that there are many viable stream models that fit the data well, with reasonable parameters, provided that Cen A has a halo mass larger than M$_{200}$ $>4.70\times 10^{12}$ M$_{\odot}$. There is a second stream in Cen A's halo that is also reproduced within the context of this same dynamical model. However, stream morphology in the imaging alone does not uniquely determine the mass or mass distribution for the Cen A halo. In particular, the stream models with high likelihood show covariances between the inferred Cen A mass distribution, the inferred Dw3 progenitor mass, the Dw3 velocity, and the Dw3 line-of-sight position. We show that these degeneracies can be broken with radial-velocity measurements along the stream, and that a single radial velocity measurement puts a substantial lower limit on the halo mass. These results suggest that targeted radial-velocity measurements will be critical if we want to learn about dark matter from extragalactic stellar streams., Comment: 11 figures, 23 pages, Accepted to ApJ

Precise Gaia measurements of positions, parallaxes, and proper motions provide an opportunity to calculate 3D positions and 2D velocities (i.e. 5D phase-space) of Milky Way stars. Where available, spectroscopic radial velocity (RV) measurements provide full 6D phase-space information, however there are now and will remain many stars without RV measurements. Without an RV it is not possible to directly calculate 3D stellar velocities, however one can infer 3D stellar velocities by marginalizing over the missing RV dimension. In this paper, we infer the 3D velocities of stars in the Kepler field in Cartesian Galactocentric coordinates (vx, vy, vz). We directly calculate velocities for around a quarter of all Kepler targets, using RV measurements available from the Gaia, LAMOST and APOGEE spectroscopic surveys. Using the velocity distributions of these stars as our prior, we infer velocities for the remaining three-quarters of the sample by marginalizing over the RV dimension. The median uncertainties on our inferred vx, vy, and vz velocities are around 4, 18, and 4 km/s, respectively. We provide 3D velocities for a total of 148,590 stars in the Kepler field. These 3D velocities could enable kinematic age-dating, Milky Way stellar population studies, and other scientific studies using the benchmark sample of well-studied Kepler stars. Although the methodology used here is broadly applicable to targets across the sky, our prior is specifically constructed from and for the Kepler field. Care should be taken to use a suitable prior when extending this method to other parts of the Galaxy., Comment: Accepted for publication in AAS Journals

The total mass of the Local Group (LG) is a fundamental quantity that enables interpreting the orbits of its constituent galaxies and placing the LG in a cosmological context. One of the few methods that allows inferring the total mass directly is the "Timing Argument," which models the relative orbit of the Milky Way (MW) and M31 in equilibrium. The MW itself is not in equilibrium, a byproduct of its merger history including the recent pericentric passage of the LMC, and recent work has found that the MW disk is moving with a lower bound "travel velocity" of $\sim 32~{\rm km}~{\rm s}^{-1}$ with respect to the outer stellar halo. Previous Timing Argument measurements attempt to account for this non-equilibrium state, but have been restricted to theoretical predictions for the impact of the LMC specifically. In this paper, we quantify the impact of a travel velocity on recovered LG mass estimates using several different compilations of recent kinematic measurements of M31. We find that incorporating the measured value of the travel velocity lowers the inferred LG mass by 10--12\% compared to a static MW halo. Measurements of the travel velocity with more distant tracers could yield even larger values, which would further decrease the inferred LG mass. Therefore, the newly measured travel velocity directly implies a lower LG mass than from a model with a static MW halo and must be considered in future dynamical studies of the Local Volume., Comment: 14 pages, 3 figures; Published in ApJ; updated to match published version

We analyze two binary systems containing giant stars, V723 Mon ("the Unicorn") and 2M04123153+6738486 ("the Giraffe"). Both giants orbit more massive but less luminous companions, previously proposed to be mass-gap black holes. Spectral disentangling reveals luminous companions with star-like spectra in both systems. Joint modeling of the spectra, light curves, and spectral energy distributions robustly constrains the masses, temperatures, and radii of both components: the primaries are luminous, cool giants ($T_{\rm eff,\,giant} = 3,800\,\rm K$ and $4,000\,\rm K$, $R_{\rm giant}= 22.5\,R_{\odot}$ and $25\,R_{\odot}$) with exceptionally low masses ($M_{\rm giant} \approx 0.4\,M_{\odot}$) that likely fill their Roche lobes. The secondaries are only slightly warmer subgiants ($T_{\rm eff,\,2} = 5,800\,\rm K$ and $5,150\,\rm K$, $R_2= 8.3\,R_{\odot}$ and $9\,R_{\odot}$) and thus are consistent with observed UV limits that would rule out main-sequence stars with similar masses ($M_2 \approx 2.8\,M_{\odot}$ and $\approx 1.8\,M_{\odot}$). In the Unicorn, rapid rotation blurs the spectral lines of the subgiant, making it challenging to detect even at wavelengths where it dominates the total light. Both giants have surface abundances indicative of CNO processing and subsequent envelope stripping. The properties of both systems can be reproduced by binary evolution models in which a $1-2\,M_{\odot}$ primary is stripped by a companion as it ascends the giant branch. The fact that the companions are also evolved implies either that the initial mass ratio was very near unity, or that the companions are temporarily inflated due to rapid accretion. The Unicorn and Giraffe offer a window into into a rarely-observed phase of binary evolution preceding the formation of wide-orbit helium white dwarfs, and eventually, compact binaries containing two helium white dwarfs., Comment: 20 pages, 16 figures. Accepted to MNRAS

To understand the formation of the Milky Way's prominent bar it is important to know whether stars in the bar differ in the chemical element composition of their birth material as compared to disk stars. This requires stellar abundance measurements for large samples across the Milky Way's body. Such samples, e.g. luminous red giant stars observed by SDSS's Apogee survey, will inevitably span a range of stellar parameters; as a consequence, both modelling imperfections and stellar evolution may preclude consistent and precise estimates of their chemical composition at a level of purported bar signatures, which has left current analyses of a chemically distinct bar inconclusive. Here, we develop a new self-calibration approach to eliminate both modelling and astrophysical abundance systematics among red giant branch (RGB) stars of different luminosities (and hence surface gravity $\log g$). We apply our method to $48,853$ luminous Apogee DR16 RGB stars to construct spatial abundance maps of $20$ chemical elements near the Milky Way's mid-plane, covering Galactocentric radii of $0\,{\rm kpc}

This paper documents the seventeenth data release (DR17) from the Sloan Digital Sky Surveys; the fifth and final release from the fourth phase (SDSS-IV). DR17 contains the complete release of the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, which reached its goal of surveying over 10,000 nearby galaxies. The complete release of the MaNGA Stellar Library (MaStar) accompanies this data, providing observations of almost 30,000 stars through the MaNGA instrument during bright time. DR17 also contains the complete release of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2) survey which publicly releases infra-red spectra of over 650,000 stars. The main sample from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), as well as the sub-survey Time Domain Spectroscopic Survey (TDSS) data were fully released in DR16. New single-fiber optical spectroscopy released in DR17 is from the SPectroscipic IDentification of ERosita Survey (SPIDERS) sub-survey and the eBOSS-RM program. Along with the primary data sets, DR17 includes 25 new or updated Value Added Catalogs (VACs). This paper concludes the release of SDSS-IV survey data. SDSS continues into its fifth phase with observations already underway for the Milky Way Mapper (MWM), Local Volume Mapper (LVM) and Black Hole Mapper (BHM) surveys., Comment: 40 pages, 8 figures, 6 tables. In press at ApJSS (arxiv v2 corrects some minor typos and updates references)

Stellar abundances and ages afford the means to link chemical enrichment to galactic formation. In the Milky Way, individual element abundances show tight correlations with age, which vary in slope across ([Fe/H]-[$\alpha$/Fe]). Here, we step from characterising abundances as measures of age, to understanding how abundances trace properties of stellar birth-environment in the disk over time. Using measurements from $\sim$27,000 APOGEE stars (R=22,500, SNR$>$200), we build simple local linear models to predict a sample of elements (X = Si, O, Ca, Ti, Ni, Al, Mn, Cr) using (Fe, Mg) abundances alone, as fiducial tracers of supernovae production channels. Given [Fe/H] and [Mg/H], we predict these elements, [X/H], to about double the uncertainty of their measurements. The intrinsic dispersion, after subtracting measurement errors in quadrature is $\approx 0.015-0.04$~dex. The residuals of the prediction (measurement $-$ model) for each element demonstrate that each element has an individual link to birth properties at fixed (Fe, Mg). Residuals from primarily massive-star supernovae (i.e. Si, O, Al) partially correlate with guiding radius. Residuals from primarily supernovae Ia (i.e. Mn, Ni) partially correlate with age. A fraction of the intrinsic scatter that persists at fixed (Fe, Mg), however, after accounting for correlations, does not appear to further discriminate between birth properties that can be traced with present-day measurements. Presumably, this is because the residuals are also, in part, a measure of the typical (in)-homogeneity of the disk's stellar birth environments, previously inferred only using open-cluster systems. Our study implies at fixed birth radius and time, there is a median scatter of $\approx 0.01-0.015$ dex in elements generated in supernovae sources., Comment: submitted to ApJ, 16 pages, 12 figures

APOGEE-2 is a dual-hemisphere, near-infrared (NIR), spectroscopic survey with the goal of producing a chemo-dynamical mapping of the Milky Way Galaxy. The targeting for APOGEE-2 is complex and has evolved with time. In this paper, we present the updates and additions to the initial targeting strategy for APOGEE-2N presented in Zasowski et al. (2017). These modifications come in two implementation modes: (i) "Ancillary Science Programs" competitively awarded to SDSS-IV PIs through proposal calls in 2015 and 2017 for the pursuit of new scientific avenues outside the main survey, and (ii) an effective 1.5-year expansion of the survey, known as the Bright Time Extension, made possible through accrued efficiency gains over the first years of the APOGEE-2N project. For the 23 distinct ancillary programs, we provide descriptions of the scientific aims, target selection, and how to identify these targets within the APOGEE-2 sample. The Bright Time Extension permitted changes to the main survey strategy, the inclusion of new programs in response to scientific discoveries or to exploit major new datasets not available at the outset of the survey design, and expansions of existing programs to enhance their scientific success and reach. After describing the motivations, implementation, and assessment of these programs, we also leave a summary of lessons learned from nearly a decade of APOGEE-1 and APOGEE-2 survey operations. A companion paper, Santana et al. (submitted), provides a complementary presentation of targeting modifications relevant to APOGEE-2 operations in the Southern Hemisphere., Comment: 59 pages; 11 Figures; 7 Tables; 2 Appendices; Submitted to Journal and Under Review; Posting to accompany papers using the SDSS-IV/APOGEE-2 Data Release 17 scheduled for December 2021

APOGEE is a high-resolution (R sim 22,000), near-infrared, multi-epoch, spectroscopic survey of the Milky Way. The second generation of the APOGEE project, APOGEE-2, includes an expansion of the survey to the Southern Hemisphere called APOGEE-2S. This expansion enabled APOGEE to perform a fully panoramic mapping of all the main regions of the Milky Way; in particular, by operating in the H-band, APOGEE is uniquely able to probe the dust-hidden inner regions of the Milky Way that are best accessed from the Southern Hemisphere. In this paper we present the targeting strategy of APOGEE-2S, with special attention to documenting modifications to the original, previously published plan. The motivation for these changes is explained as well as an assessment of their effectiveness in achieving their intended scientific objective. In anticipation of this being the last paper detailing APOGEE targeting, we present an accounting of all such information complete through the end of the APOGEE-2S project; this includes several main survey programs dedicated to exploration of major stellar populations and regions of the Milky Way, as well as a full list of programs contributing to the APOGEE database through allocations of observing time by the Chilean National Time Allocation Committee (CNTAC) and the Carnegie Institution for Science (CIS). This work was presented along with a companion article, R. Beaton et al. (submitted; AAS29028), presenting the final target selection strategy adopted for APOGEE-2 in the Northern Hemisphere., Comment: 43 pages; 5 figures; 6 Tables; 1 Appendix; Submitted to Journal and Under Review; Posting to accompany papers using the SDSS-IV/APOGEE-2 Data Release 17 scheduled for December 2021

A significant fraction of Milky Way (MW) satellites exhibit phase-space properties consistent with a coherent orbital plane. Using tailored N--body simulations of a spherical MW halo that recently captured a massive (1.8$\times 10^{11}$M$\odot$) LMC-like satellite, we identify the physical mechanisms that may enhance the clustering of orbital poles of objects orbiting the MW. The LMC deviates the orbital poles of MW dark matter (DM) particles from the present-day random distribution. Instead, the orbital poles of particles beyond $R\approx 50$kpc cluster near the present-day orbital pole of the LMC along a sinusoidal pattern across the sky. The density of orbital poles is enhanced near the LMC by a factor $\delta \rho_{max}$=30\%(50\%) with respect to underdense regions, and $\delta \rho_{iso}$=15\%(30\%) relative to the isolated MW simulation (no LMC) between 50-150 kpc (150-300 kpc). The clustering appears after the LMC's pericenter ($\approx$ 50 Myr ago, 49 kpc) and lasts for at least 1 Gyr. Clustering occurs because of three effects: 1) the LMC shifts the velocity and position of the central density of the MW's halo and disk; 2) the DM dynamical friction wake and collective response induced by the LMC changes the kinematics of particles; 3) observations of particles selected within spatial planes suffer from a bias, such that measuring orbital poles in a great circle in the sky enhances the probability of their orbital poles being clustered. This scenario should be ubiquitous in hosts that recently captured a massive satellite (at least $\approx$ 1:10 mass ratio), causing the clustering of orbital poles of halo tracers., Comment: Submitted to ApJ. 23 pages and 16 figures. Comments are welcome

APOGEE spectra offer $\lesssim$1 km s$^{-1}$ precision in the measurement of stellar radial velocities (RVs). This holds even when multiple stars are captured in the same spectrum, as happens most commonly with double-lined spectroscopic binaries (SB2s), although random line of sight alignments of unrelated stars can also occur. We develop a code that autonomously identifies SB2s and higher order multiples in the APOGEE spectra, resulting in 7273 candidate SB2s, 813 SB3s, and 19 SB4s. We estimate the mass ratios of binaries, and for a subset of these systems with sufficient number of measurements we perform a complete orbital fit, confirming that most systems with period $<$10 days have circularized. Overall, we find a SB2 fraction ($F_{SB2}$) $\sim$3\% among main sequence dwarfs, and that there is not a significant trend in $F_{SB2}$ with temperature of a star. We are also able to recover a higher $F_{SB2}$ in sources with lower metallicity, however there are some observational biases. We also examine light curves from TESS to determine which of these spectroscopic binaries are also eclipsing. Such systems, particularly those that are also pre- and post-main sequence, are good candidates for a follow-up analysis to determine their masses and temperatures., Comment: Accepted to AJ. 23 pages, 17 figures

Signatures of vertical disequilibrium have been observed across the Milky Way's disk. These signatures manifest locally as unmixed phase-spirals in $z$--$v_z$ space ("snails-in-phase") and globally as nonzero mean $z$ and $v_z$ which wraps around as a physical spiral across the $x$--$y$ plane ("snails-in-space"). We explore the connection between these local and global spirals through the example of a satellite perturbing a test-particle Milky Way (MW)-like disk. We anticipate our results to broadly apply to any vertical perturbation. Using a $z$--$v_z$ asymmetry metric we demonstrate that in test-particle simulations: (a) multiple local phase-spiral morphologies appear when stars are binned by azimuthal action $J_\phi$, excited by a single event (in our case, a satellite disk-crossing); (b) these distinct phase-spirals are traced back to distinct disk locations; and (c) they are excited at distinct times. Thus, local phase-spirals offer a global view of the MW's perturbation history from multiple perspectives. Using a toy model for a Sagittarius (Sgr)-like satellite crossing the disk, we show that the full interaction takes place on timescales comparable to orbital periods of disk stars within $R \lesssim 10$ kpc. Hence such perturbations have widespread influence which peaks in distinct regions of the disk at different times. This leads us to examine the ongoing MW-Sgr interaction. While Sgr has not yet crossed the disk (currently, $z_{Sgr} \approx -6$ kpc, $v_{z,Sgr} \approx 210$ km/s), we demonstrate that the peak of the impact has already passed. Sgr's pull over the past 150 Myr creates a global $v_z$ signature with amplitude $\propto M_{Sgr}$, which might be detectable in future spectroscopic surveys.

Statistical studies of astronomical data sets, in particular of cataloged properties for discrete objects, are central to astrophysics. One cannot model those objects' population properties or incidences without a quantitative understanding of the conditions under which these objects ended up in a catalog or sample, the sample's selection function. As systematic and didactic introductions to this topic are scarce in the astrophysical literature, we aim to provide one, addressing generically the following questions: What is a selection function? What arguments $\vec{q}$ should a selection function depend on? Over what domain must a selection function be defined? What approximations and simplifications can be made? And, how is a selection function used in `modelling'? We argue that volume-complete samples, with the volume drastically curtailed by the faintest objects, reflect a highly sub-optimal selection function that needlessly reduces the number of bright and usually rare objects in the sample. We illustrate these points by a worked example, deriving the space density of white dwarfs (WD) in the Galactic neighbourhood as a function of their luminosity and Gaia color, $\Phi_0(M_G,B-R)$ in [mag$^{-2}$pc$^{-3}$]. We construct a sample of $10^5$ presumed WDs through straightforward selection cuts on the Gaia EDR3 catalog, in magnitude, color, parallax, and astrometric fidelity $\vec{q}=(m_G,B-R,\varpi,p_{af})$. We then combine a simple model for $\Phi_0$ with the effective survey volume derived from this selection function $S_C(\vec{q})$ to derive a detailed and robust estimate of $\Phi_0(M_G,B-R)$. This resulting white dwarf luminosity-color function $\Phi_0(M_G,B-R)$ differs dramatically from the initial number density distribution in the luminosity-color plane: by orders of magnitude in density and by four magnitudes in density peak location.

The vast majority of Milky Way stellar halo stars were likely accreted from a small number ($\lesssim$3) of relatively large dwarf galaxy accretion events. However, the timing of these events is poorly constrained, relying predominantly on indirect dynamical mixing arguments or imprecise age measurements of stars associated with debris structures. Here, we aim to infer robust stellar ages for stars associated with galactic substructures to more directly constrain the merger history of the Galaxy. By combining kinematic, asteroseismic, and spectroscopic data where available, we infer stellar ages for a sample of 10 red giant stars that were kinematically selected to be associated with the stellar halo, a subset of which are associated with the Gaia-Enceladus-Sausage halo substructure, and compare their ages to 3 red giant stars in the Galactic disk. Despite systematic differences in both absolute and relative ages determined by this work, age rankings of stars in this sample are robust. Passing the same observable inputs to multiple stellar age determination packages, we measure a weighted average age for the Gaia-Enceladus-Sausage stars in our sample of 8 $\pm$ 3 (stat.) $\pm$ 1 (sys.) Gyr. We also determine hierarchical ages for the populations of Gaia-Enceladus-Sausage, in situ halo and disk stars, finding a Gaia-Enceladus-Sausage population age of 8.0$^{+3.2}_{-2.3}$ Gyr. Although we cannot distinguish hierarchical population ages of halo or disk structures with our limited data and sample of stars, this framework should allow distinct characterization of Galactic substructures using larger stellar samples and additional data available in the near future., Comment: 28 pages, 21 figures. Published by AAS Journals

"exoplanet" is a toolkit for probabilistic modeling of astronomical time series data, with a focus on observations of exoplanets, using PyMC3 (Salvatier et al., 2016). PyMC3 is a flexible and high-performance model-building language and inference engine that scales well to problems with a large number of parameters. "exoplanet" extends PyMC3's modeling language to support many of the custom functions and probability distributions required when fitting exoplanet datasets or other astronomical time series. While it has been used for other applications, such as the study of stellar variability, the primary purpose of "exoplanet" is the characterization of exoplanets or multiple star systems using time-series photometry, astrometry, and/or radial velocity. In particular, the typical use case would be to use one or more of these datasets to place constraints on the physical and orbital parameters of the system, such as planet mass or orbital period, while simultaneously taking into account the effects of stellar variability., Comment: Published in the Journal of Open Source Software. Comments (still) welcome. Software available at https://docs.exoplanet.codes

We calculated physical and orbital properties for 2M06464003+0109157, a 1.06 day eclipsing double lined spectroscopic binary. We modelled the system's ASAS-SN and TESS light curves, measuring the system's inclination and radii of each component. Extracting radial velocities for each component from 6 SDSS/APOGEE spectra, we measured the system's mass ratio and performed a full orbital fit. Our analysis indicates that 2M06464003+0109157 has components with nearly equal masses ($m_1/m_2 =0.99 \pm 0.01$; $M_1{_,}{_2} = 0.57 \pm 0.015$ M$_\odot$) and comparable radii ($R_1 = 0.66 \pm 0.05$ R$_\odot$, $R_2 = 0.57 \pm 0.06$ R$_\odot$). The solution required two star spots to incorporate the out of eclipse variation that is seen in the light curve. We report our full characterization of this system, and prospects for similar analyses using survey data to measure precise physical and orbital properties for EBs., Comment: 11 pages, 6 figures, accepted for publication to PASP

We present the kinematic and chemical profiles of red giant stars observed by the APOGEE-2 survey in the direction of the Jhelum stellar stream, a Milky Way substructure located in the inner halo of the Milky Way at a distance from the Sun of $\approx$ 13 kpc. From the six APOGEE-2 Jhelum pointings, we isolate stars with log($g$) $<$ 3.5, leaving a sample of 289 red giant stars. From this sample of APOGEE giants, we identified seven stars that are consistent with the astrometric signal from $Gaia$ DR2 for this stream. Of these seven, one falls onto the RGB along the same sequence as the Jhelum stars presented by \cite{ji20}. This new Jhelum member has [Fe/H]=-2.2 and is at the tip of the red giant branch. By selecting high orbital eccentricity, metal-rich stars, we identify red giants in our APOGEE sample that are likely associated with the $Gaia$-Enceladus-Sausage (GES) merger. We compare the abundance profiles of the Jhelum stars and GES stars and find similar trends in $\alpha$-elements, as expected for low-metallicity populations. However, we find that the orbits for GES and Jhelum stars are not generally consistent with a shared origin. The chemical abundances for the APOGEE Jhelum star and other confirmed members of the stream are similar to stars in known stellar streams and thus are consistent with an accreted dwarf galaxy origin for the progenitor of the stream, although we cannot rule out a globular cluster origin., Comment: Accepted to ApJ

The gas content of the complete compilation of Local Group dwarf galaxies (119 within 2 Mpc) is presented using HI survey data. Within the virial radius of the Milky Way (224 kpc here), 53 of 55 dwarf galaxies are devoid of gas to limits of M$_{\rm HI}<10^4$ M$_\odot$. Within the virial radius of M31 (266 kpc), 27 of 30 dwarf galaxies are devoid of gas (with limits typically $<10^5$ M$_\odot$). Beyond the virial radii of the Milky Way and M31, the majority of the dwarf galaxies have detected HI gas and have HI masses higher than the limits. When the relationship between gas content and distance is investigated using a Local Group virial radius, more of the non-detected dwarf galaxies are within this radius (85$\pm1$ of the 93 non-detected dwarf galaxies) than within the virial radii of the Milky Way and M31. Using the Gaia proper motion measurements available for 38 dwarf galaxies, the minimum gas density required to completely strip them of gas is calculated. Halo densities between $10^{-5}$ and $5 \times 10^{-4}$ cm$^{-3}$ are typically required for instantaneous stripping at perigalacticon. When compared to halo density with radius expectations from simulations and observations, 80% of the dwarf galaxies with proper motions are consistent with being stripped by ram pressure at Milky Way pericenter. The results suggest a diffuse gaseous galactic halo medium is important in quenching dwarf galaxies, and that a Local Group medium also potentially plays a role., Comment: 26 pages, 7 figures, 3 tables, Accepted by ApJ

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., Comment: 34 pages, 8 figures. Published in ApJ

Flattened axisymmetric galactic potentials are known to host minor orbit families surrounding orbits with commensurable frequencies. The behavior of orbits that belong to these orbit families is fundamentally different than that of typical orbits with non-commensurable frequencies. We investigate the evolution of stellar streams on orbits near the boundaries between orbit families (separatrices) in a flattened axisymmetric potential. We demonstrate that the separatrix divides these streams into two groups of stars that belong to two different orbit families, and that as a result, these streams diffuse more rapidly than streams that evolve elsewhere in the potential. We utilize Hamiltonian perturbation theory to estimate both the timescale of this effect and the likelihood of a stream evolving close enough to a separatrix to be affected by it. We analyze two prior reports of stream-fanning in simulations with triaxial potentials, and conclude that at least one of them is caused by separatrix divergence. These results lay the foundation for a method of mapping the orbit families of galactic potentials using the morphology of stellar streams. Comparing these predictions with the currently known distribution of streams in the Milky Way presents a new way of constraining the shape of our Galaxy's potential and distribution of dark matter., Comment: 12 pages, 7 figures. Accepted for publication in MNRAS

New facilities of the 2020s, such as the High Luminosity Large Hadron Collider (HL-LHC), will be relevant through at least the 2030s. This means that their software efforts and those that are used to analyze their data need to consider sustainability to enable their adaptability to new challenges, longevity, and efficiency, over at least this period. This will help ensure that this software will be easier to develop and maintain, that it remains available in the future on new platforms, that it meets new needs, and that it is as reusable as possible. This report discusses a virtual half-day workshop on "Software Sustainability and High Energy Physics" that aimed 1) to bring together experts from HEP as well as those from outside to share their experiences and practices, and 2) to articulate a vision that helps the Institute for Research and Innovation in Software for High Energy Physics (IRIS-HEP) to create a work plan to implement elements of software sustainability. Software sustainability practices could lead to new collaborations, including elements of HEP software being directly used outside the field, and, as has happened more frequently in recent years, to HEP developers contributing to software developed outside the field rather than reinventing it. A focus on and skills related to sustainable software will give HEP software developers an important skill that is essential to careers in the realm of software, inside or outside HEP. The report closes with recommendations to improve software sustainability in HEP, aimed at the HEP community via IRIS-HEP and the HEP Software Foundation (HSF)., Comment: A report from the "Sustainable Software in HEP" IRIS-HEP blueprint workshop: https://indico.cern.ch/event/930127/

Indications of disequilibrium throughout the Milky Way (MW) highlight the need for compact,flexible, non-parametric descriptions of phase--space distributions of galaxies. We present a new representation of the current Dark Matter (DM) distribution and potential derived from N-body simulations of the Milky Way and Large Magellanic Cloud (LMC) system using Basis Function Expansions (BFEs). We incorporate methods to maximize the physical signal in the representation. As a result, the simulations of $10^8$ DM particles representing the MW--LMC system can be described by 354 coefficients. We find that the LMC induces asymmetric perturbations (odd l, m) to the MW's halo, which are not well-described by oblate, prolate, or triaxial halos. Furthermore, the energy in high-order even modes (l,m $\geq$ 2) is similar to average triaxial halos found in cosmological simulations. As such, the response of the MW's halo to the LMC must be accounted for in order to recover the imprints of its assembly history. The LMC causes the outer halo ($\geq$ 30 kpc) to shift from the disk center of mass (COM) by $\sim$15-25 kpc at present day, manifesting as a dipole in the BFE and in the radial velocities of halo stars. The shift depends on the LMC's infall mass, the distortion of the LMC's halo and the MW halo response. Within 30 kpc, halo tracers are expected to orbit the COM of the MW's disk, regardless of LMC infall mass. The LMC's halo is also distorted by MW tides, we discuss the implications for its mass loss and the subsequent effects on current Magellanic satellites., Comment: 35 pages, 25 figures, Accepted for publication in ApJ. 3d movie can be found here: https://vimeo.com/546207117