Multi-epoch radial velocity measurements of stars can be used to identify stellar, substellar, and planetary-mass companions. Even a small number of observation epochs can be informative about ...companions, though there can be multiple qualitatively different orbital solutions that fit the data. We have custom-built a Monte Carlo sampler (The Joker) that delivers reliable (and often highly multimodal) posterior samplings for companion orbital parameters given sparse radial velocity data. Here we use The Joker to perform a search for companions to 96,231 red giant stars observed in the APOGEE survey (DR14) with ≥3 spectroscopic epochs. We select stars with probable companions by making a cut on our posterior belief about the amplitude of the variation in stellar radial velocity induced by the orbit. We provide (1) a catalog of 320 companions for which the stellar companion's properties can be confidently determined, (2) a catalog of 4898 stars that likely have companions, but would require more observations to uniquely determine the orbital properties, and (3) posterior samplings for the full orbital parameters for all stars in the parent sample. We show the characteristics of systems with confidently determined companion properties and highlight interesting systems with candidate compact object companions.
We present a new N-body model for the tidal disruption of the Sagittarius (Sgr) dwarf that is capable of simultaneously satisfying the majority of angular position, distance, and radial velocity ...constraints imposed by current wide-field surveys of its dynamically young ({approx_lt}3 Gyr) tidal debris streams. In particular, this model resolves the conflicting angular position and radial velocity constraints on the Sgr leading tidal stream that have been highlighted in recent years. While the model does not reproduce the apparent bifurcation observed in the leading debris stream, recent observational data suggest that this bifurcation may represent a constraint on the internal properties of the Sgr dwarf rather than the details of its orbit. The key element in the success of this model is the introduction of a non-axisymmetric component to the Galactic gravitational potential that can be described in terms of a triaxial dark matter halo whose minor/major axis ratio (c/a){sub {Phi}} = 0.72 and intermediate/major axis ratio (b/a){sub {Phi}} = 0.99 at radii 20 kpc < r < 60 kpc. The minor/intermediate/major axes of this halo lie along the directions (l, b) = (7{sup 0}, 0{sup 0}), (0{sup 0}, 90{sup 0}), and (97{sup 0}, 0{sup 0}) respectively, corresponding to a nearly oblate ellipsoid whose minor axis is contained within the Galactic disk plane. This particular disk/halo orientation is difficult to reconcile within the general context of galactic dynamics (and cold dark matter models in particular), suggesting either that the orientation may have evolved significantly with time or that inclusion of other non-axisymmetric components (such as the gravitational influence of the Magellanic Clouds) in the model may obviate the need for triaxiality in the dark matter halo. The apparent proper motion of Sgr in this model is estimated to be ({mu}{sub l}cos b, {mu}{sub b}) = (-2.16, 1.73) mas yr{sup -1}, corresponding to a Galactocentric space velocity (U, V, W) = (230, -35, 195) km s{sup -1} . Based on the velocity dispersion in the stellar tidal streams, we estimate that Sgr has a current bound mass M{sub Sgr} = 2.5{sup +1.3}{sub -1.0} x 10{sup 8} M{sub sun}. We demonstrate that with simple assumptions about the star formation history of Sgr, tidal stripping models naturally give rise to gradients in the metallicity distribution function (MDF) along the stellar debris streams similar to those observed in recent studies. These models predict a strong evolution in the MDF of the model Sgr dwarf with time, indicating that the chemical abundances of stars in Sgr at the present day may be significantly different than the abundances of those already contributed to the Galactic stellar halo. We conclude by using the new N-body model to re-evaluate previous claims of the association of miscellaneous halo substructure with the Sgr dwarf.
The spectral analysis and data products in Data Release 16 (DR16; 2019 December) from the high-resolution near-infrared Apache Point Observatory Galactic Evolution Experiment (APOGEE)-2/Sloan Digital ...Sky Survey (SDSS)-IV survey are described. Compared to the previous APOGEE data release (DR14; 2017 July), APOGEE DR16 includes about 200,000 new stellar spectra, of which 100,000 are from a new southern APOGEE instrument mounted on the 2.5 m du Pont telescope at Las Campanas Observatory in Chile. DR16 includes all data taken up to 2018 August, including data released in previous data releases. All of the data have been re-reduced and re-analyzed using the latest pipelines, resulting in a total of 473,307 spectra of 437,445 stars. Changes to the analysis methods for this release include, but are not limited to, the use of MARCS model atmospheres for calculation of the entire main grid of synthetic spectra used in the analysis, a new method for filling "holes" in the grids due to unconverged model atmospheres, and a new scheme for continuum normalization. Abundances of the neutron-capture element Ce are included for the first time. A new scheme for estimating uncertainties of the derived quantities using stars with multiple observations has been applied, and calibrated values of surface gravities for dwarf stars are now supplied. Compared to DR14, the radial velocities derived for this release more closely match those in the Gaia DR2 database, and a clear improvement in the spectral analysis of the coolest giants can be seen. The reduced spectra as well as the result of the analysis can be downloaded using links provided on the SDSS DR16 web page.
We report the first APOGEE metallicities and -element abundances measured for 3600 red giant stars spanning a large radial range of both the Large (LMC) and Small Magellanic Clouds, the largest Milky ...Way (MW) dwarf galaxies. Our sample is an order of magnitude larger than that of previous studies and extends to much larger radial distances. These are the first results presented that make use of the newly installed southern APOGEE instrument on the du Pont telescope at Las Campanas Observatory. Our unbiased sample of the LMC spans a large range in metallicity, from Fe/H = −0.2 to very metal-poor stars with Fe/H −2.5, the most metal-poor Magellanic Cloud (MC) stars detected to date. The LMC /Fe-Fe/H distribution is very flat over a large metallicity range but rises by ∼0.1 dex at −1.0 < Fe/H −0.5. We interpret this as a sign of the known recent increase in MC star formation activity and are able to reproduce the pattern with a chemical evolution model that includes a recent "starburst." At the metal-poor end, we capture the increase of /Fe with decreasing Fe/H and constrain the " -knee" to Fe/H −2.2 in both MCs, implying a low star formation efficiency of ∼0.01 Gyr−1. The MC knees are more metal-poor than those of less massive MW dwarf galaxies such as Fornax, Sculptor, or Sagittarius. One possible interpretation is that the MCs formed in a lower-density environment than the MW, a hypothesis that is consistent with the paradigm that the MCs fell into the MW's gravitational potential only recently.
The data and analysis methodology used for the SDSS/APOGEE Data Releases 13 and 14 are described, highlighting differences from the DR12 analysis presented in Holtzman et al. Some improvement in the ...handling of telluric absorption and persistence is demonstrated. The derivation and calibration of stellar parameters, chemical abundances, and respective uncertainties are described, along with the ranges over which calibration was performed. Some known issues with the public data related to the calibration of the effective temperatures (DR13), surface gravity (DR13 and DR14), and C and N abundances for dwarfs (DR13 and DR14) are highlighted. We discuss how results from a data-driven technique, The Cannon, are included in DR14 and compare those with results from the APOGEE Stellar Parameters and Chemical Abundances Pipeline. We describe how using The Cannon in a mode that restricts the abundance analysis of each element to regions of the spectrum with known features from that element leads to Cannon abundances can lead to significantly different results for some elements than when all regions of the spectrum are used to derive abundances.
Numerical models of the tidal disruption of the Sagittarius (Sgr) dwarf galaxy have recently been developed that for the first time simultaneously satisfy most observational constraints on the ...angular position, distance, and radial velocity trends of both leading and trailing tidal streams emanating from the dwarf. We use these dynamical models in combination with extant three-dimensional position and velocity data for Galactic globular clusters and dSph galaxies to identify those Milky Way satellites that are likely to have originally formed in the gravitational potential well of the Sgr dwarf, and have been stripped from Sgr during its extended interaction with the Milky Way. We conclude that the globular clusters Arp 2, M 54, NGC 5634, Terzan 8, and Whiting 1 are almost certainly associated with the Sgr dwarf, and that Berkeley 29, NGC 5053, Pal 12, and Terzan 7 are likely to be as well (albeit at lower confidence). The initial Sgr system therefore may have contained five to nine globular clusters, corresponding to a specific frequency S{sub N} = 5-9 for an initial Sgr luminosity M{sub V} = -15.0. Our result is consistent with the 8 {+-} 2 genuine Sgr globular clusters expected on the basis of statistical modeling of the Galactic globular cluster distribution and the corresponding false-association rate due to chance alignments with the Sgr streams. The globular clusters identified as most likely to be associated with Sgr are consistent with previous reconstructions of the Sgr age-metallicity relation, and show no evidence for a second-parameter effect shaping their horizontal branch morphologies. We find no statistically significant evidence to suggest that any of the recently discovered population of ultrafaint dwarf galaxies are associated with the Sgr tidal streams, but are unable to rule out this possibility conclusively for all systems.
The Milky Way (MW) remains a primary laboratory for understanding the structure and evolution of spiral galaxies, but typically we are denied clear views of MW stellar populations at low Galactic ...latitudes because of extinction by interstellar dust. However, the combination of Two Micron All-Sky Survey (2MASS) near-infrared (NIR) and Spitzer-IRAC mid-infrared (MIR) photometry enables a powerful method for determining the line-of-sight reddening to any star: the sampled wavelengths lie in the Rayleigh-Jeans part of the spectral energy distribution of most stars, where, to first order, all stars have essentially the same intrinsic color. Thus, changes in stellar NIR-MIR colors due to interstellar reddening are readily apparent, and (under an assumed extinction law) the observed colors and magnitudes of stars can be easily and accurately restored to their intrinsic values, greatly increasing their usefulness for Galactic structure studies. In this paper, we explore this 'Rayleigh-Jeans Color Excess' (RJCE) method and demonstrate that use of even a simple variant of the RJCE method based on a single reference color, (H --4.5 Delta *m), can rather accurately remove dust effects from previously uninterpretable 2MASS color-magnitude diagrams of stars in fields along the heavily reddened Galactic midplane, with results far superior to those derived from application of other dereddening methods. We also show that 'total' Galactic midplane extinction looks rather different from that predicted using 100 Delta *m emission maps from the IRAS/ISSA and COBE/DIRBE instruments as presented by Schlegel et al. Instead, the Galactic midplane extinction strongly resembles the distribution of 13CO (J = 1->0) emission. Future papers will focus on refining the RJCE method and applying the technique to understand better not only dust and its distribution but also the distribution of stars intermixed with the dust in the low-latitude Galaxy.
Abstract
We present the first high-resolution abundance analysis of the globular cluster VVV CL001, which resides in a region dominated by high interstellar reddening toward the Galactic bulge. Using
...H
-band spectra acquired by the Apache Point Observatory Galactic Evolution Experiment, we identified two potential members of the cluster, and estimated from their Fe
i
lines that the cluster has an average metallicity of Fe/H = −2.45 with an uncertainty due to systematics of 0.24 dex. We find that the light-(N),
α
-(O, Mg, Si), and Odd-
Z
(Al) elemental abundances of the stars in VVV CL001 follow the same trend as other Galactic metal-poor globular clusters. This makes VVV CL001 possibly the most metal-poor globular cluster identified so far within the Sun’s galactocentric distance and likely one of the most metal-deficient clusters in the Galaxy after ESO280-SC06. Applying statistical isochrone fitting, we derive self-consistent age, distance, and reddening values, yielding an estimated age of
11.9
−
4.05
+
3.12
Gyr at a distance of
8.22
−
1.93
+
1.84
kpc, revealing that VVV CL001 is also an old GC in the inner Galaxy. The Galactic orbit of VVV CL001 indicates that this cluster lies on a halo-like orbit that appears to be highly eccentric. Both chemistry and dynamics support the hypothesis that VVV CL001 could be an ancient fossil relic left behind by a massive merger event during the early evolution of the Galaxy, likely associated with either the Sequoia or the Gaia–Enceladus–Sausage structures.
We use the multi-epoch radial velocities acquired by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey to perform a large-scale statistical study of stellar multiplicity for ...field stars in the Milky Way, spanning the evolutionary phases between the main sequence (MS) and the red clump. We show that the distribution of maximum radial velocity shifts (ΔRVmax) for APOGEE targets is a strong function of log g, with MS stars showing ΔRVmax as high as ∼300 , and steadily dropping down to ∼30 for log g ∼ 0, as stars climb up the red giant branch (RGB). Red clump stars show a distribution of ΔRVmax values comparable to that of stars at the tip of the RGB, implying they have similar multiplicity characteristics. The observed attrition of high ΔRVmax systems in the RGB is consistent with a lognormal period distribution in the MS and a multiplicity fraction of 0.35, which is truncated at an increasing period as stars become physically larger and undergo mass transfer after Roche Lobe overflow during H-shell burning. The ΔRVmax distributions also show that the multiplicity characteristics of field stars are metallicity-dependent, with metal-poor (Fe/H −0.5) stars having a multiplicity fraction a factor of 2-3 higher than metal-rich (Fe/H 0.0) stars. This has profound implications for the formation rates of interacting binaries observed by astronomical transient surveys and gravitational wave detectors, as well as the habitability of circumbinary planets.
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