Apache Point Observatory Galactic Evolution Research Articles

Tonalli: an asexual genetic code to characterise APOGEE-2 stellar spectra. I. Validation with synthetic and solar spectra.

Abstract We present tonalli, a spectroscopic analysis python code that efficiently predicts effective temperature, stellar surface gravity, metallicity, α-element abundance, and rotational and radial velocities for stars with effective temperatures between 3200 and 6250 K, observed with the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2). tonalli implements an asexual genetic algorithm to optimise the finding of the best comparison between a target spectrum and the continuum-normalised synthetic spectra library from the Model Atmospheres with a Radiative and Convective Scheme (MARCS), which is interpolated in each generation. Using simulated observed spectra and the APOGEE-2 solar spectrum of Vesta, we study the performance, limitations, accuracy and precision of our tool. Finally, a Monte Carlo realisation was implemented to estimate the uncertainties of each derived stellar parameter. The ad hoc continuum-normalised library is publicly available on Zenodo (DOI 10.5281/zenodo.12736546).

Near-infrared Spectroscopy of the Sun and Solar Analog Star HD 76151: Compiling an Extensive Line List in the Y, J, H, and K Bands

Determining the physical nature of a star requires precise knowledge of its stellar atmospheric parameters, including effective temperature, surface gravity, and metallicity. This study presents a new atomic line list covering a broad spectral range (1–2.5 μm; YJHK bands) for iron (Fe) and α-elements (Ca, Mg, Ti, Si) to improve stellar parameter determination using near-infrared (NIR) spectroscopy. We highlight the limitations of existing line lists, stemming primarily from inconsistencies in oscillator strengths for ionized iron lines within the Apache Point Observatory Galactic Evolution Experiment DR17. The line list was validated using the high-resolution and high-quality disk-center NIR spectra of the Sun and its solar analog HD 76151. As a result of the spectroscopic analyses, the effective temperature of HD 76151 was calculated as 5790 ± 170 K, surface gravity as 4.35 ± 0.18 cgs, metal abundance as 0.24 ± 0.09 dex, and microturbulence velocity of 0.30−0.3+0.5 km s−1 by combining the optical and NIR line lists. A comparison of the model atmospheric parameters calculated for HD 76151 with the PARSEC isochrones resulted in a stellar mass of 1.053−0.068+0.056M⊙ , radius 1.125−0.011+0.035R⊙ , and an age of 5.5−2.1+2.5 Gyr. For the first time, kinematic and dynamical orbital analyses of HD 76151 using a combination of Gaia astrometric and spectroscopic data showed that the star was born in a metal-rich region within the solar circle and is a member of the thin disk population. Thus, the slightly metal-rich nature of the star, as reflected in its spectroscopic analysis, was confirmed by dynamical orbital analysis.

Chemical abundances for a sample of FGK dwarfs in the Pleiades open cluster from APOGEE

ABSTRACT This paper presents chemical abundances of 12 elements (C, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, and Fe) for 80 FGK dwarfs in the Pleiades open cluster, which span a temperature range of $\sim$2000 K in T$_{\rm eff}$, using the high-resolution (R$\sim$22 500) near-infrared SDSS (Sloan Digital Sky Survey)-IV/APOGEE (Apache Point Observatory Galactic Evolution Experiment) spectra ($\lambda$1.51–1.69 $\mu$m). Using a 1D local thermodynamic equilibrium abundance analysis, we determine an overall metallicity of [Fe/H] = +0.03 $\pm$ 0.04 dex, with the elemental ratios [$\alpha$/Fe] = +0.01 $\pm$ 0.05, [odd-z/Fe] = –0.04 $\pm$ 0.08, and [iron peak/Fe] = –0.02 $\pm$ 0.08. These abundances for the Pleiades are in line with the abundances of other open clusters at similar galactocentric distances as presented in the literature. Examination of the abundances derived from each individual spectral line revealed that several of the stronger lines displayed trends of decreasing abundance with decreasing $T_{\rm eff}$. The list of spectral lines that yield abundances that are independent of $T_{\rm eff}$ are presented and used for deriving the final abundances. An investigation into possible causes of the temperature-dependent abundances derived from the stronger lines suggests that the radiative codes and the APOGEE line list we employ may inadequately model van der Waals broadening, in particular in the cooler K dwarfs.

The Brown Dwarf Kinematics Project (BDKP). VI. Ultracool Dwarf Radial and Rotational Velocities from SDSS/APOGEE High-resolution Spectroscopy

We present precise measurements of radial (RV) and projected rotational ( vsini ) velocities of a sample of 258 M6 to L2 dwarfs with multiepoch, high-resolution (λ/Δλ = 22,500), near-infrared (1.514–1.696 μm) spectroscopic observations reported in the Apache Point Observatory Galactic Evolution Experiment Data Release 17. The spectra were modeled using a Markov Chain Monte Carlo forward-modeling method, which achieved median precisions of σ RV = 0.4 km s−1 and σvsini = 1.1 km s−1. One-half of our sample (138 sources) are previously known members of nearby young clusters and moving groups, and we identified three new kinematic members of the Argus or Carina Near moving groups, 2MASS J05402570+2448090, 2MASS J14093200+4138080, and 2MASS J21272531+5553150. Excluding these sources, we find that the majority of our sample have kinematics consistent with the Galactic thin disk, and 11 sources are associated with the intermediate thin/thick disk. The field sample has a velocity dispersion of 38.2 ± 0.3 km s−1, equivalent to an age of 3.30 ± 0.19 Gyr based on empirical age–velocity dispersion relations, and a median vsini of 17 km s−1. For 172 sources with multiepoch observations, we identified 37 as having significant RV variations, and determined preliminary orbit parameters for 26 sources with four or more epochs, nine of which are short-period binary candidates. For 40 sources with photometric variability periods from the literature less than 5 days and vsini > 20 km s−1, we find a decline in projected radii ( Rsini ) with age congruent with evolutionary models. Finally, we also present multiepoch RV and vsini measurements for additional 444 candidate ultracool dwarfs.

Data-driven Stellar Intrinsic Colors and Dust Reddenings for Spectrophotometric Data: From the Blue-edge Method to a Machine Learning Approach

Intrinsic colors (ICs) of stars are essential for studies on both stellar physics and dust reddening. In this work, we developed an XGBoost model to predict ICs with the atmospheric parameters T eff, logg , and [M/H]. The model was trained and tested for three colors at Gaia and Two Micron All Sky Survey bands with 1,040,446 low-reddening sources. The atmospheric parameters were determined by the Gaia DR3 General Stellar Parameterizer from Photometry (GSP-phot) module and were validated by comparing with the Apache Point Observatory Galactic Evolution Experiment and Large Sky Area Multi-Object fiber Spectroscopic Telescope. We further confirmed that the biases in GSP-phot parameters, especially for [M/H], do not present a significant impact on the IC prediction. The generalization error of the model estimated by the test set is 0.014 mag for (GBP−GRP)0 , 0.050 mag for (GBP−KS)0 , and 0.040 mag for (J−KS)0 . The model was applied to a sample containing 5,714,528 reddened stars with stellar parameters from R. Andrae et al. to calculate ICs and reddenings. The high consistency in the comparison of E(J − K S) between our results and literature values further validates the accuracy of the XGBoost model. The variation of E(G BP − K S)/E(G BP − G RP), a representation of the extinction law, with Galactic longitude is found on large scales. This work preliminarily presents the feasibility and accuracy of the machine learning approach for IC and dust reddening calculation, whose products could be widely applied to spectrophotometric data. The data sets and trained model can be accessed via Zenodo, doi:10.5281/zenodo.12787594. Models for more bands will be completed in following work.

Testing the sources of the peculiar abundances in globular clusters

This work aims to analyze some of the polluters proposed in the self-enrichment scenarios put forward to explain the multiple populations in globular clusters (GCs), extending previous studies. Three scenarios with different polluter stars were tested: asymptotic giant branch stars (AGBs), high-mass interacting binaries (IBs), and fast rotating massive stars (FRMSs). With abundance data available from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey and ΔY estimates from precise Hubble Space Telescope (HST) photometry, twenty-six clusters were studied, increasing the number of clusters in previous studies by more than a factor of three. We also included the study of the abundances of N, C, Mg, and Al, extending previous studies that mainly focused on the abundances of He, O, and Na. In addition, we constructed an empirical model to test whether one could explain the chemical signatures of the “enriched” population of GC stars with a fixed source and dilution process based on empirical data. In agreement with work by other authors, we found that the proposed polluters can generally predict the qualitative abundance patterns in GC stars and in some cases quantitatively predict some elements, but in most cases when we compare the model yields with the observations, we find that they cannot explain the entire set of observed abundance patterns. The empirical model succeeds in reproducing the abundances of Al for a given ΔY (and vice versa), showing that there is a direct relationship between Al and He, with one increasing proportionally to the other. However, the empirical model fails to reproduce the observed abundances of Na and N, in agreement with the results of previous works. The observed decoupling between the maximum abundances of CNO-cycle elements such as N and Na with those of Al and He provides new information and constraints for future models and could take us a step closer to understanding the origin of the peculiar abundance variations of GC stars.

Stellar Characterization and Chemical Abundances of Exoplanet-hosting M Dwarfs from APOGEE Spectra: Future JWST Targets

Exoplanets hosting M dwarfs are the best targets to characterize Earth-like or super-Earth planetary atmospheres with the James Webb Space Telescope (JWST). We determine detailed stellar parameters (T eff, logg, and ξ) and individual abundances of 12 elements for four cool M dwarfs hosting exoplanets TOI-1685, GJ 436, GJ 3470, and TOI-2445, scheduled for future observations by the JWST. The analysis utilizes high-resolution near-infrared spectra from the Sloan Digital Sky Survey IV APOGEE survey between 1.51 and 1.69 μm. Based on 1D LTE plane-parallel models, we find that TOI-2445 is slightly metal poor ([Fe/H] = −0.16 ± 0.09 dex), while TOI-1685, GJ 436, and GJ 3470 are more metal rich ([Fe/H] = 0.06 ± 0.18, 0.10 ± 0.20, and 0.25 ± 0.15 dex, respectively). The derived C/O ratios for TOI-2445, TOI-1685, GJ 436, and GJ 3470 are 0.526 ± 0.027, 0.558 ± 0.097, 0.561 ± 0.029, and 0.638 ± 0.015, respectively. From the results for 28 M dwarfs analyzed homogeneously from Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectra, we find exoplanet-hosting M dwarfs exhibit a C/O abundance ratio approximately 0.01–0.05 higher than those with nondetected exoplanets, at limits of a statistically significant offset. A linear regression of the [Fe/H] versus C/O distribution reveals a noticeable difference in the angular coefficient between FGK dwarfs (0.27) and M dwarfs (0.13). Assuming our abundance ratios of Ca/Mg, Si/Mg, Al/Mg, and Fe/Mg, we determine a mass of 3.276−0.419+0.448 M ⊕ for TOI-2445 b, which has a density (6.793 −0.099+0.005 g cm−3) and core mass fraction (0.329 −0.049+0.028 ) very similar to Earth’s. We also present an atlas of 113 well-defined spectral lines to analyze M dwarfs in the H band and a comprehensive evaluation of uncertainties from variations in the atmospheric parameters, signal-to-noise ratio, and pseudocontinuum.

Galactic Chemical Evolution Models Favor an Extended Type Ia Supernova Delay-time Distribution

Type Ia supernovae (SNe Ia) produce most of the Fe-peak elements in the Universe and therefore are a crucial ingredient in galactic chemical evolution models. SNe Ia do not explode immediately after star formation, and the delay-time distribution (DTD) has not been definitively determined by supernova surveys or theoretical models. Because the DTD also affects the relationship among age, [Fe/H], and [α/Fe] in chemical evolution models, comparison with observations of stars in the Milky Way is an important consistency check for any proposed DTD. We implement several popular forms of the DTD in combination with multiple star formation histories for the Milky Way in multizone chemical evolution models that include radial stellar migration. We compare our predicted interstellar medium abundance tracks, stellar abundance distributions, and stellar age distributions to the final data release of the Apache Point Observatory Galactic Evolution Experiment. We find that the DTD has the largest effect on the [α/Fe] distribution: a DTD with more prompt SNe Ia produces a stellar abundance distribution that is skewed toward a lower [α/Fe] ratio. While the DTD alone cannot explain the observed bimodality in the [α/Fe] distribution, in combination with an appropriate star formation history it affects the goodness of fit between the predicted and observed high-α sequence. Our model results favor an extended DTD with fewer prompt SNe Ia than the fiducial t −1 power law.

Chemical Doppelgangers in GALAH DR3: The Distinguishing Power of Neutron-capture Elements among Milky Way Disk Stars

The observed chemical diversity of Milky Way stars places important constraints on Galactic chemical evolution and the mixing processes that operate within the interstellar medium. Recent works have found that the chemical diversity of disk stars is low. For example, the Apache Point Observatory Galactic Evolution Experiment (APOGEE) “chemical doppelganger rate,” or the rate at which random pairs of field stars appear as chemically similar as stars born together, is high, and the chemical distributions of APOGEE stars in some Galactic populations are well-described by two-dimensional models. However, limited attention has been paid to the heavy elements (Z > 30) in this context. In this work, we probe the potential for neutron-capture elements to enhance the chemical diversity of stars by determining their effect on the chemical doppelganger rate. We measure the doppelganger rate in GALactic Archaeology with HERMES DR3, with abundances rederived using The Cannon, and find that considering the neutron-capture elements decreases the doppelganger rate from ∼2.2% to 0.4%, nearly a factor of 6, for stars with −0.1 < [Fe/H] < 0.1. While chemical similarity correlates with similarity in age and dynamics, including neutron-capture elements does not appear to select stars that are more similar in these characteristics. Our results highlight that the neutron-capture elements contain information that is distinct from that of the lighter elements and thus add at least one dimension to Milky Way abundance space. This work illustrates the importance of considering the neutron-capture elements when chemically characterizing stars and motivates ongoing work to improve their atomic data and measurements in spectroscopic surveys.

Offset of M54 from the Sagittarius dwarf spheroidal galaxy

ABSTRACT We present results from simultaneous modelling of 2D (projected along the line of sight) position, proper motion, and line-of-sight velocity for Gaia- and Apache Point Observatory Galactic Evolution Experiment (APOGEE)-observed stars near the centre of the Sagittarius (Sgr) dwarf spheroidal galaxy. We use a mixture model that allows for independent sub-populations contributed by the Sgr galaxy, its nuclear star cluster Messier 54 (M54), and the Milky Way foreground. We find an offset of $0.295\pm 0.029$ deg between the inferred centroids of Sgr and M54, corresponding to a (projected) physical separation of $0.135\pm 0.013$ kpc. The detected offset might plausibly be driven by unmodelled asymmetry in Sgr’s stellar configuration; however, standard criteria for model selection favour our symmetric model over an alternative that allows for bilateral asymmetry. We infer an offset between the proper motion centres of Sgr and M54 of $[\Delta \mu _{\alpha }\cos \delta ,\Delta \mu _{\delta }]=[4.9, -19.7] \pm [6.8, 6.2] \ \mu \mathrm{ as}\,\mathrm{ yr}^{-1}$ ($[0.61, -2.46] \pm [0.85, 0.77] \ \mathrm{ km}\,\mathrm{ s}^{-1}$), with magnitude similar to the covariance expected due to spatially correlated systematic error. We infer an offset of $4.1\pm 1.2 \ \mathrm{ km}\,\mathrm{ s}^{-1}$ in line-of-sight velocity. Using inferred values for the systemic positions and motions of Sgr and M54 as initial conditions, we calculate the recent orbital history of a simplified Sgr/M54 system, which we demonstrate to be sensitive to any line-of-sight distance offset between M54 and Sgr, and to the distribution of dark matter within Sgr.

A Lack of Mass-gap Compact Object Binaries in APOGEE

Depending principally on mass, the compact object remnant left behind after a star’s life may be a white dwarf, neutron star (NS), or black hole (BH). While we have large samples of each of these remnants, we lack knowledge of the exact conditions separating these outcomes. The boundary between low-mass BHs and massive NSs is particularly poorly understood, as few objects between 2 and 5 M ⊙ are known. To probe this regime, we search the Apache Point Observatory Galactic Evolution Experiment 17th Data Release data set of 657,000 unique stars for binary systems with one stellar component that exhibit large radial velocity shifts over multiple observations. We identify 4751 likely binary systems, and estimate a minimum mass for each system’s “invisible companion” under the assumption of tidal synchronization. Two systems have companion masses ≳2 M ⊙, although we conclude that neither are good candidates for possessing a mass-gap NS or BH companions.

Derivative Spectroscopy and its Application at Detecting the Weak Emission/Absorption Lines

The development of spectroscopic survey telescopes like Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), Apache Point Observatory Galactic Evolution Experiment and Sloan Digital Sky Survey has opened up unprecedented opportunities for stellar classification. Specific types of stars, such as early-type emission-line stars and those with stellar winds, can be distinguished by the profiles of their spectral lines. In this paper, we introduce a method based on derivative spectroscopy (DS) designed to detect signals within complex backgrounds and provide a preliminary estimation of curve profiles. This method exhibits a unique advantage in identifying weak signals and unusual spectral line profiles when compared to other popular line detection methods. We validated our approach using synthesis spectra, demonstrating that DS can detect emission signals three times fainter than Gaussian fitting. Furthermore, we applied our method to 579,680 co-added spectra from LAMOST Medium-Resolution Spectroscopic Survey, identifying 16,629 spectra with emission peaks around the Hα line from 10,963 stars. These spectra were classified into three distinct morphological groups, resulting in nine subclasses as follows. (1) Emission peak above the pseudo-continuum line (single peak, double peaks, emission peak situated within an absorption line, P Cygni profile, Inverse P Cygni profile); (2) Emission peak below the pseudo-continuum line (sharp emission peak, double absorption peaks, emission peak shifted to one side of the absorption line); (3) Emission peak between the pseudo-continuum line.

Exploring the origin of the distance bimodality of stars in the periphery of the Small Magellanic Cloud with APOGEE and Gaia

ABSTRACT The Magellanic Cloud system represents a unique laboratory for study of both interacting dwarf galaxies and the ongoing process of the formation of the Milky Way and its halo. We focus on one aspect of this complex, three-body interaction – the dynamical perturbation of the Small Magellanic Cloud (SMC) by the Large Magellanic Cloud (LMC), and specifically potential tidal effects on the SMC’s eastern side. Using Gaia astrometry and the precise radial velocities (RVs) and multielement chemical abundances from Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) Data Release 17, we explore the well-known distance bimodality on the eastern side of the SMC. Through estimated stellar distances, proper motions, and RVs, we characterize the kinematics of the two populations in the bimodality and compare their properties with those of SMC populations elsewhere. Moreover, while all regions explored by APOGEE seem to show a single chemical enrichment history, the metallicity distribution function (MDF), of the ‘far’ stars on the eastern periphery of the SMC is found to resemble that for the more metal-poor fields of the western periphery, whereas the MDF for the ‘near’ stars on the eastern periphery resembles that for stars in the SMC Centre. The closer eastern periphery stars also show RVs (corrected for SMC rotation and bulk motion) that are, on average, approaching us relative to all other SMC populations sampled. We interpret these trends as evidence that the near stars on the eastern side of the SMC represent material pulled out of the central SMC as part of its tidal interaction with the LMC.

Orbital Torus Imaging: Acceleration, Density, and Dark Matter in the Galactic Disk Measured with Element Abundance Gradients

Under the assumption of a simple and time-invariant gravitational potential, many Galactic dynamics techniques infer the milky Way’s mass and dark matter distributions from stellar kinematic observations. These methods typically rely on parameterized potential models of the Galaxy and must take into account nontrivial 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 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 Apache Point Observatory Galactic Evolution Experiment survey combined with kinematic data from the Gaia mission. Our method reveals the vertical (z-direction) orbital 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, Σ, and midplane volume density, ρ 0, as a function of Galactocentric radius and height. Around the Sun, we find Σ⊙(z=1.1kpc)=72−9+6M⊙pc−2 and ρ⊙(z=0)=0.081−0.009+0.015M⊙pc−3 using the most constraining abundance ratio, [Mg/Fe]. This corresponds to a dark matter contribution in surface density of Σ⊙,DM(z = 1.1 kpc) = 24 ± 4 M ⊙ pc−2, and in total volume mass density of ρ ⊙,DM(z = 0) = 0.011 ± 0.002 M ⊙ pc−3. Moreover, using these mass density values we estimate the scale length of the low-α disk to be h R = 2.24 ± 0.06 kpc.

TESS Stellar Rotation up to 80 Days in the Southern Continuous Viewing Zone

The Transiting Exoplanet Survey Satellite (TESS) mission delivers time-series photometry for millions of stars across the sky, offering a probe into stellar astrophysics, including rotation, on a population scale. However, light-curve systematics related to the satellite’s 13.7 day orbit have prevented stellar rotation searches for periods longer than 13 days, putting the majority of stars beyond reach. Machine-learning methods have the ability to identify systematics and recover robust signals, enabling us to recover rotation periods up to 35 days for GK dwarfs and 80 days for M dwarfs. We present a catalog of 7245 rotation periods for cool dwarfs in the Southern Continuous Viewing Zone, estimated using convolutional neural networks. We find evidence for structure in the period distribution consistent with prior Kepler and K2 results, including a gap in 10–20 day cool-star periods thought to arise from a change in stellar spin-down or activity. Using a combination of spectroscopic and gyrochronologic constraints, we fit stellar evolution models to estimate masses and ages for stars with rotation periods. We find strong correlations between the detectability of rotation in TESS and the effective temperature, age, and metallicity of the stars. Finally, we investigate the relationships between rotation and newly obtained spot filling fractions estimated from Apache Point Observatory Galactic Evolution Experiment spectra. Field starspot filling fractions are elevated in the same temperature and period regime where open clusters’ magnetic braking stalls, lending support to an internal shear mechanism that can produce both phenomena.

Stellar Atmospheric Parameters for Cool Dwarfs in Gaia Data Release 3

We provide a catalog of atmospheric parameters for 1,806,921 cool dwarfs from Gaia Data Release 3 (DR3) that lie within the range covered by LAMOST cool dwarf spectroscopic parameters: 3200 K < T eff < 4300 K, −0.8 < [M/H] < 0.2 dex, and 4.5 < log g < 5.5 dex. Our values are derived based on machine-learning models trained with multiband photometry corrected for dust. The photometric data comprise optical data from the Sloan Digital Sky Survey r, i, and z bands, near-infrared data from the Two Micron All Sky Survey J, H, and K bands, and mid-infrared data from the ALLWISE W1 and W2 bands. We used both random forest and light gradient boosting machine machine-learning models and found similar results from both, with an error dispersion of 68 K, 0.22 dex, and 0.05 dex for T eff, [M/H], and log g, respectively. Assessment of the relative feature importance of different photometric colors indicated W1 − W2 as most sensitive to both T eff and log g, with J − H being most sensitive to [M/H]. We find that our values show a good agreement with the Apache Point Observatory Galactic Evolution Experiment, but are significantly different to those provided as part of Gaia DR3.

Spectroscopic Distance, Mass, and Age Estimations for APOGEE DR17

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.

The APO-K2 Catalog. I. ∼7500 Red Giants with Fundamental Stellar Parameters from APOGEE DR17 Spectroscopy and K2-GAP Asteroseismology

We present a catalog of fundamental stellar properties for ∼7500 evolved stars, including stellar radii and masses, determined from the combination of spectroscopic observations from the Apache Point Observatory Galactic Evolution Experiment, part of the Sloan Digital Sky Survey IV, and asteroseismology from K2. The resulting APO-K2 catalog provides spectroscopically derived temperatures and metallicities, asteroseismic global parameters, evolutionary states, and asteroseismically derived masses and radii. Additionally, we include kinematic information from Gaia. We investigate the multidimensional space of abundance, stellar mass, and velocity with an eye toward applications in Galactic archaeology. The APO-K2 sample has a large population of low-metallicity stars (∼288 with [M/H] ≤ −1), and their asteroseismic masses are larger than astrophysical estimates. We argue that this may reflect offsets in the adopted fundamental temperature scale for metal-poor stars rather than metallicity-dependent issues with interpreting asteroseismic data. We characterize the kinematic properties of the population as a function of α enhancement and position in the disk and identify those stars in the sample that are candidate components of the Gaia-Enceladus merger. Importantly, we characterize the selection function for the APO-K2 sample as a function of metallicity, radius, mass, νmax , color, and magnitude referencing Galactic simulations and target selection criteria to enable robust statistical inferences with the catalog.

Investigating Gaia EDR3 parallax systematics using asteroseismology of cool giant stars observed by Kepler, K2, and TESS

We analyse Gaia EDR3 parallax systematics as a function of magnitude and sky location using a recently published catalogue of 12 500 asteroseismic red-giant star distances. We selected ∼3500 red clump (RC) stars of similar chemical composition as the optimal subsample for this purpose because (1) their similar luminosity allows for straightforward interpretation of trends with apparent magnitude; (2) RC stars are the most distant stars in our sample at a given apparent magnitude, so uncertainties related to asteroseismic radii and distances are the smallest; (3) and they provide the largest sample of intrinsically similar stars. We performed a detailed assessment of systematic uncertainties relevant for parallax offset estimation based on the asteroseismic distances. Specifically, we investigated (1) the impact of measuring the basic asteroseismic quantities νmax and ⟨Δν⟩ using different pipelines, (2) uncertainties related to extinction, (3) the impact of adopting spectroscopic information from different surveys, and (4) blending issues related to photometry. Following this assessment, we adopted for our baseline analysis the asteroseismic parameters measured in Elsworth et al. (2020, Res. Notes Am. Astron. Soc., 4, 177) and spectroscopy from the Apache Point Observatory Galactic Evolution Experiment (DR17), and we further restricted the sample to low-extinction (AV ≤ 0.5 mag) RC stars with quality astrometric solutions from Gaia EDR3, as indicated by RUWE &lt; 1.4. We then investigated both the parallax offset relative to the published Gaia EDR3 parallaxes and the residual parallax offset after correcting Gaia EDR3 parallaxes following Lindegren et al. (2021, A&amp;A, 649, A4). We found residual parallax offsets very close to zero (−1.6 ± 0.5 (stat.)±10 (syst.) μas) for stars fainter than G &gt; 11 mag in the initial Kepler field, suggesting that the Lindegren parallax offset corrections are adequate in this magnitude range. For 17 K2 campaigns in the same magnitude range, the residual parallax offset is +16.5 ± 1.7 (stat.)±10 (syst.) μas. At brighter magnitudes (G ≤ 11 mag), we found inconsistent residual parallax offsets between the Kepler field, 17 K2 campaigns, and the TESS southern continuous viewing zone, with differences of up to 60 μas. This contradicts the studies that suggest a monotonic trend between magnitude and residual parallax offsets and instead suggests a significant dependence on sky location at bright magnitudes due to a lack of bright physical pairs being available to determine the parallax offset corrections. Inspection of the 17 K2 campaigns allowed for investigation of parallax offsets as a function of ecliptic longitude and revealed a possible signal. Finally, we estimated the absolute magnitude of the red clump and obtained MKsRC = −1.650 ± 0.025 mag in the 2MASS Ks band and MGRC = (0.432 ± 0.004) − (0.821 ± 0.033) · (Teff [K]−4800 K)/1000 K [mag] in the Gaia G-band.

NLTE Analysis of High-Resolution H-Band Spectra, V: Neutral Sodium

In order to derive sodium abundances and investigate the effects of non-local thermodynamic equilibrium (NLTE) on the formation of H-band Na I lines, we update the sodium atomic model by incorporating collision rates with hydrogen from new quantum-mechanical calculations. The differential Na abundances for 13 sample stars are obtained by analyzing high-resolution H-band spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and optical spectra under both local thermodynamic equilibrium (LTE) and NLTE conditions. Consistent abundances from both bands suggest that our updated atomic model is valid for studying the formation of H-band Na I lines. Our calculations show that, in our stellar parameter space, NLTE effects are negative and can result in corrections larger than −0.4 dex on optical lines. The corrections on H-band Na I lines are typically small, within about 0.05 dex, but not negligible if accurate sodium abundance is desired. We note that the [Na/Fe] ratios favor the theoretical galactic chemical model.