Using APOGEE Wide Binaries to Test Chemical Tagging with Dwarf Stars

Stars of a common origin are thought to have similar, if not nearly identical, chemistry. Chemical tagging seeks to exploit this fact to identify Milky Way subpopulations through their unique chemical fingerprints. In this work, we compare the chemical abundances of dwarf stars in wide binaries to test the abundance consistency of stars of a common origin. Our sample of 31 wide binaries is identified from a catalog produced by cross-matching APOGEE stars with UCAC5 astrometry, and we confirm the fidelity of this sample with precision parallaxes from Gaia DR2. For as many as 14 separate elements, we compare the abundances between components of our wide binaries, finding they have very similar chemistry (typically within 0.1 dex). This level of consistency is more similar than can be expected from stars with different origins (which show typical abundance differences of 0.3-0.4 dex within our sample). For the best measured elements, Fe, Si, K, Ca, Mn, and Ni, these differences are reduced to 0.05-0.08 dex when selecting pairs of dwarf stars with similar temperatures. Our results suggest that APOGEE dwarf stars may currently be used for chemical tagging at the level of $\sim$0.1 dex or at the level of $\sim$0.05 dex when restricting for the best-measured elements in stars of similar temperatures. Larger wide binary catalogs may provide calibration sets, in complement to open cluster samples, for on-going spectroscopic surveys.Comment: 21 pages, 14 figures, accepted for publication in Ap

Constraining the Mass Profiles of Stellar Systems: Schwarzschild Modeling of Discrete Velocity Datasets

(ABRIDGED) We present a new Schwarzschild orbit-superposition code designed to model discrete datasets composed of velocities of individual kinematic tracers in a dynamical system. This constitutes an extension of previous implementations that can only address continuous data in the form of (the moments of) velocity distributions, thus avoiding potentially important losses of information due to data binning. Furthermore, the code can handle any combination of available velocity components, i.e., only line-of-sight velocities, only proper motions, or a combination of both. It can also handle a combination of discrete and continuous data. The code finds the distribution function (DF, a function of the three integrals of motion E, Lz, and I3) that best reproduces the available kinematic and photometric observations in a given axisymmetric gravitational potential. The fully numerical approach ensures considerable freedom on the form of the DF f(E,Lz,I3). This allows a very general modeling of the orbital structure, thus avoiding restrictive assumptions about the degree of (an)isotropy of the orbits. We describe the implementation of the discrete code and present a series of tests of its performance based on the modeling of simulated datasets generated from a known DF. We find that the discrete Schwarzschild code recovers the original orbital structure, M/L ratios, and inclination of the input datasets to satisfactory accuracy, as quantified by various statistics. The code will be valuable, e.g., for modeling stellar motions in Galactic globular clusters, and those of individual stars, planetary nebulae, or globular clusters in nearby galaxies. This can shed new light on the total mass distributions of these systems, with central black holes and dark matter halos being of particular interest.Comment: ApJ, in press; 51 pages, 11 figures; manuscript revised following comments by refere

Made-to measure galaxy models - I Methodology

We re-derive the made-to-measure method of Syer and Tremaine 1996, for modelling stellar systems and individual galaxies, and demonstrate how extensions to the made-to-measure method may be implemented and used. We illustrate the enhanced made-to-measure method by determining the mass-to-light ratio of a galaxy modelled as a Plummer sphere. From the standard galactic observables of surface brightness and line-of-sight velocity dispersion together with the h_4 Gauss-Hermite coefficient of the line-of-sight velocity distribution, we successfully recover the true mass-to-light ratio of our toy galaxy. Using kinematic data from Kleyna et al 2002, we then estimate the mass-to-light ratio of the dwarf spheroidal galaxy Draco achieving a V-band value of 539 \pm 136 M_{\odot} / L_{\odot}. We describe the main aspects of creating a made-to-measure galaxy model and show how the key modelling parameters may be determined.Comment: 18 pages, 15 figure

Magnetic Mixing in Red Giant and Asymptotic Giant Branch Stars

The available information on isotopic abundances in the atmospheres of low-mass Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars requires that episodes of extensive mixing occur below the convective envelope, reaching down to layers close to the hydrogen burning shell (Cool Bottom Processing). Recently \cite{Busso:2007jw} suggested that dynamo-produced buoyant magnetic flux tubes could provide the necessary physical mechanisms and also supply sufficient transport rates. Here, we present an $\alpha-\Omega$ dynamo in the envelope of an RGB/AGB star in which shear and rotation drain via turbulent dissipation and Poynting flux. In this context, if the dynamo is to sustain throughout either phase, convection must resupply shear. Under this condition, volume-averaged, peak toroidal field strengths of $\simeq3\times10^3$ G (RGB) and $\simeq5\times10^3$ G (AGB) are possible at the base of the convection zone. If the magnetic fields are concentrated in flux tubes, the corresponding field strengths are comparable to those required by Cool Bottom Processing.Comment: Replaced to correct small error in published version: In \S 2.1, paragraphs 2 and 3 incorrectly refer to the poloidal field when qualitatively discussing magnetic diffusion in the shear zone. The correct physical interpretation is that the toroidal field diffuses through the shear zone consistent with the value of $\beta_\phi

Fragile Binary Candidates in the SDSS DR8 spectroscopic archive

We present a catalog of 80 very wide fragile binary candidates (projected separations > 10000 AU) from the SDSS DR8 spectral archive. The pairs were selected based on proper motion, radial velocity, metallicity and photometric parallax criteria. The angular separations of these pairs range from 3" to 250". The peak in the metallicity distribution of these pairs is about -0.5 dex of solar metallicity. Space motions and reduced proper motion diagrams indicate all these pairs are members of the disk. The chromospheric activity index SHK of each component in 38 binary candidates having spectra of high signal-to-noise ratio and member stars of three open clusters (NGC2420, M67 and NGC6791) were measured. The SHK vs. color relation for these binary candidates is consistent with the trend seen in these open clusters. The ages implied by this relation suggest that fragile wide pairs can survive longer than 8 Gyr.Comment: 17 pages, 12 figures, accepted by A

Unraveling UBC 274: a morphological, kinematical and chemical analysis of a disrupting open cluster

We do a morphological, kinematic and chemical analysis of the disrupting cluster UBC 274 (2.5 Gyr, $d=1778$ pc) to study its global properties. We use HDBSCAN to obtain a new membership list up to 50 pc from its centre and up to magnitude $G=19$ using Gaia EDR3 data. We use high resolution and high signal-to-noise spectra to obtain atmospheric parameters of 6 giants and subgiants, and individual abundances of 18 chemical species. The cluster has a highly eccentric (0.93) component, tilted $\sim$10 deg with respect to the plane of the Galaxy, which is morphologically compatible with the result of a test-particle simulation of a disrupting cluster. Our abundance analysis shows that the cluster has a subsolar metallicity of [Fe/H]$=-0.08\pm0.02$. Its chemical pattern is compatible with that of Ruprecht 147, of similar age but located closer to the Sun, with the remarkable exception of neutron-capture elements, which present an overabundance of $[n\mathrm{/Fe]}\sim0.1$. The cluster's elongated morphology is associated with the internal part of its tidal tail, following the expected dynamical process of disruption. We find a significant sign of mass segregation where the most massive stars appear 1.5 times more concentrated than other stars. The cluster's overabundance of neutron-capture elements can be related to the metallicity dependence of the neutron-capture yields due to the secondary nature of these elements, predicted by some models. UBC 274 presents a high chemical homogeneity at the level of $0.03$ dex in the sampled region of its tidal tails.Comment: Accepted by A&

Hubble Space Telescope Proper Motion (HSTPROMO) Catalogs of Galactic Globular Clusters. I. Sample Selection, Data Reduction and NGC 7078 Results

We present the first study of high-precision internal proper motions (PMs) in a large sample of globular clusters, based on Hubble Space Telescope (HST) data obtained over the past decade with the ACS/WFC, ACS/HRC, and WFC3/UVIS instruments. We determine PMs for over 1.3 million stars in the central regions of 22 clusters, with a median number of ~60,000 stars per cluster. These PMs have the potential to significantly advance our understanding of the internal kinematics of globular clusters by extending past line-of-sight (LOS) velocity measurements to two- or three-dimensional velocities, lower stellar masses, and larger sample sizes. We describe the reduction pipeline that we developed to derive homogeneous PMs from the very heterogeneous archival data. We demonstrate the quality of the measurements through extensive Monte-Carlo simulations. We also discuss the PM errors introduced by various systematic effects, and the techniques that we have developed to correct or remove them to the extent possible. We provide in electronic form the catalog for NGC 7078 (M 15), which consists of 77,837 stars in the central 2.4 arcmin. We validate the catalog by comparison with existing PM measurements and LOS velocities, and use it to study the dependence of the velocity dispersion on radius, stellar magnitude (or mass) along the main sequence, and direction in the plane of the sky (radial/tangential). Subsequent papers in this series will explore a range of applications in globular-cluster science, and will also present the PM catalogs for the other sample clusters.Comment: 34 pages, 22 figures (3 in low res), 30 tables, accepted for publication in ApJ on October 20, 201