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 an analysis of spectroscopic and astrometric data from APOGEE-2 and Gaia DR2 to identify structures toward the Orion Complex. By applying a hierarchical clustering algorithm to the ...six-dimensional stellar data, we identify spatially and/or kinematically distinct groups of young stellar objects with ages ranging from 1 to 12 Myr. We also investigate the star-forming history within the Orion Complex and identify peculiar subclusters. With this method we reconstruct the older populations in the regions that are currently largely devoid of molecular gas, such as Orion C (which includes the Ori cluster) and Orion D (the population that traces Ori OB1a, OB1b, and Orion X). We report on the distances, kinematics, and ages of the groups within the Complex. The Orion D group is in the process of expanding. On the other hand, Orion B is still in the process of contraction. In λ Ori the proper motions are consistent with a radial expansion due to an explosion from a supernova; the traceback age from the expansion exceeds the age of the youngest stars formed near the outer edges of the region, and their formation would have been triggered when they were halfway from the cluster center to their current positions. We also present a comparison between the parallax and proper-motion solutions obtained by Gaia DR2 and those obtained toward star-forming regions by the Very Long Baseline Array.
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.
Abstract
Diffuse ionized gas (DIG) is prevalent in star-forming galaxies. Using a sample of 365 nearly face-on star-forming galaxies observed by Mapping Nearby Galaxies at APO, we demonstrate how DIG ...in star-forming galaxies impacts the measurements of emission-line ratios, hence the interpretation of diagnostic diagrams and gas-phase metallicity measurements. At fixed metallicity, DIG-dominated low ΣHα regions display enhanced S ii/Hα, N ii/Hα, O ii/Hβ and O i/Hα. The gradients in these line ratios are determined by metallicity gradients and ΣHα. In line ratio diagnostic diagrams, contamination by DIG moves H ii regions towards composite or low-ionization nuclear emission-line region (LI(N)ER)-like regions. A harder ionizing spectrum is needed to explain DIG line ratios. Leaky H ii region models can only shift line ratios slightly relative to H ii region models, and thus fail to explain the composite/LI(N)ER line ratios displayed by DIG. Our result favours ionization by evolved stars as a major ionization source for DIG with LI(N)ER-like emission. DIG can significantly bias the measurement of gas metallicity and metallicity gradients derived using strong-line methods. Metallicities derived using N2O2 are optimal because they exhibit the smallest bias and error. Using O3N2, R
23, N2 = N ii/Hα and N2S2Hα to derive metallicities introduces bias in the derived metallicity gradients as large as the gradient itself. The strong-line method of Blanc et al. (IZI hereafter) cannot be applied to DIG to get an accurate metallicity because it currently contains only H ii region models that fail to describe the DIG.
ABSTRACT
Studies of the kinematics and chemical compositions of Galactic globular clusters (GCs) enable the reconstruction of the history of star formation, chemical evolution, and mass assembly of ...the Galaxy. Using the latest data release (DR16) of the SDSS/APOGEE survey, we identify 3090 stars associated with 46 GCs. Using a previously defined kinematic association, we break the sample down into eight separate groups and examine how the kinematics-based classification maps into chemical composition space, considering only α (mostly Si and Mg) elements and Fe. Our results show that (i) the loci of both in situ and accreted subgroups in chemical space match those of their field counterparts; (ii) GCs from different individual accreted subgroups occupy the same locus in chemical space. This could either mean that they share a similar origin or that they are associated with distinct satellites which underwent similar chemical enrichment histories; (iii) the chemical compositions of the GCs associated with the low orbital energy subgroup defined by Massari and collaborators is broadly consistent with an in situ origin. However, at the low-metallicity end, the distinction between accreted and in situ populations is blurred; (iv) regarding the status of GCs whose origin is ambiguous, we conclude the following: the position in Si–Fe plane suggests an in situ origin for Liller 1 and a likely accreted origin for NGC 5904 and NGC 6388. The case of NGC 288 is unclear, as its orbital properties suggest an accretion origin, its chemical composition suggests it may have formed in situ.
Abstract
We derive distances and masses of stars from the Sloan Digital Sky Survey (SDSS) Apache Point Observatory Galactic Evolution Experiment Data Release 17 using simple neural networks. Training ...data for distances comes from Gaia EDR3, supplemented by literature distances for star clusters. For masses, the network is trained using asteroseismic masses for evolved stars and isochrone masses for main-sequence stars. The models are trained on effective temperature, surface gravity, metallicity, and carbon and nitrogen abundances. We found that our distance predictions have median fractional errors that range from ≈20% at low log
g
and ≈10% at higher log
g
with a standard deviation of ≈11%. The mass predictions have a standard deviation of ±12%. Using the masses, we derive ages for evolved stars based on the correspondence between mass and age for giant stars given by isochrones. The results are compiled into a Value Added Catalog called DistMass that contains distances and masses for 733,901 independent spectra, plus ages for 396,548 evolved stars.
We investigate aluminum abundance variations in the stellar populations of globular clusters using both literature measurements of sodium and aluminum and APOGEE measurements of nitrogen and aluminum ...abundances. For the latter, we show that the Payne is the most suitable of the five available abundance pipelines for our purposes. Our combined sample of 42 globular clusters spans approximately 2 dex in Fe/H and 1.5 dex in . We find no fewer than five globular clusters with significant internal variations in nitrogen and/or sodium with little to no corresponding variation in aluminum, and that the minimum present-day cluster mass for aluminum enrichment in metal-rich systems is . We demonstrate that the slopes of the Al/Fe versus Na/Fe and Al/Fe versus N/Fe relations for stars without field-like abundances are approximately log-linearly dependent on both the metallicity and the stellar mass of the globular clusters. In contrast, the relationship between Na/Fe and N/Fe shows no evidence of such dependencies. This suggests that there were (at least) two classes of non-supernova chemical polluters that were common in the early universe, and that their relative contributions within globular clusters somehow scaled with the metallicity and mass of globular clusters. The first of these classes is predominantly responsible for the CNO and NeNa abundance variations, and likewise the second for the MgAl abundance variations. Particularly striking examples of this dichotomy include 47 Tuc and M4. As an auxiliary finding, we argue that abundance variations among Terzan 5 stars are consistent with it being a normal globular cluster.
We report on magnetic field measurements of 157 chemically peculiar A/B stars (Ap/Bp) based on resolved, magnetically split absorption lines present in H-band spectra provided by the Sloan Digital ...Sky Survey (SDSS)/Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. These stars represent the extreme magnetic end of a still-growing sample of >900 Ap/Bp stars selected among the APOGEE telluric standard stars as those with Ce iii absorption lines and/or literature Ap/Bp classifications. The lines most frequently resolved into their split components for these stars in the H-band pertain primarily pertain to Ce iii, Cr ii, Fe i, Mn ii, Si i, and Ca ii, in addition to one or more unidentified ions. Using mean magnetic field modulus ( ) estimates for transitions with known Landé factors, we estimate effective Landé factors for 5 Ce iii lines and 15 unknown lines and proceed to measure of 157 stars, only 3 of which have previous literature estimates of . This 183% increase in the number of Ap/Bp stars for which has been measured is a result of the large number of stars observed by SDSS/APOGEE, extension of high-resolution Ap/Bp star observations to fainter magnitudes, and the advantages of long wavelengths for resolving magnetically split lines. With ∼ 25 kG, the star 2MASS J02563098+4534239 is currently the most magnetic star of the SDSS/APOGEE sample. Effective Landé factors, representative line profiles, and magnetic field moduli are presented. The validity of the results is supported using optical, high-resolution, follow-up spectra for 29 of the stars.
Abstract
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 Sloan Digital Sky Survey (SDSS)-IV Apache Point Observatory Galactic Evolution Experiment (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 10 stars) of the OC stream in the
α
(O, Mg, Ca, Si, Ti, and S), odd-
Z
(Al, K, and V), Fe-peak (Fe, Ni, Mn, Co, and Cr), and neutron-capture (Ce) elemental groups. We find that the stars that make up the OC stream are not consistent with a monometallic population and have a median metallicity of −1.92 dex with a dispersion of 0.28 dex. Our results also indicate that the α 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, namely the
α
/Fe–Fe/H plane and the metallicity distribution, indicates that the OC stream progenitor is very likely to be a dwarf spheroidal galaxy with a mass of ∼10
6
M
⊙
.
Abstract
We report the characterization of 28 low-mass (0.02
M
⊙
≤
M
2
≤ 0.25
M
⊙
) companions to Kepler objects of interest (KOIs), eight of which were previously designated confirmed planets. These ...objects were detected as transiting companions to Sunlike stars (G and F dwarfs) by the Kepler mission and are confirmed as single-lined spectroscopic binaries in the current work using the northern multiplexed Apache Point Observatory Galactic Evolution Experiment near-infrared spectrograph (APOGEE-N) as part of the third and fourth Sloan Digital Sky Surveys. We have observed hundreds of KOIs using APOGEE-N and collected a total of 43,175 spectra with a median of 19 visits and a median baseline of ∼1.9 yr per target. We jointly model the Kepler photometry and APOGEE-N radial velocities to derive fundamental parameters for this subset of 28 transiting companions. The radii for most of these low-mass companions are overinflated (by ∼10%) when compared to theoretical models. Tidally locked M dwarfs on short-period orbits show the largest amount of inflation, but inflation is also evident for companions that are well separated from the host star. We demonstrate that APOGEE-N data provide reliable radial velocities when compared to precise high-resolution spectrographs that enable detailed characterization of individual systems and the inference of orbital elements for faint (
H
> 12) KOIs. The data from the entire APOGEE-KOI program are public and present an opportunity to characterize an extensive subset of the binary population observed by Kepler.
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