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1. The K2 Galactic Archaeology Program: Overview, target selection, and survey properties

   https://doi.org/10.1093/mnras/stac2031  

   Sharma, Sanjib ; Stello, Dennis ; Zinn, Joel C. ; Reyes, Claudia ; Hon, Marc ; Bland-Hawthorn, Joss ( July 2022 , Monthly Notices of the Royal Astronomical Society)

   K2 was a community-driven NASA mission where all targets were proposed through guest observer programmes. Here we provide an overview of one of the largest of these endeavours, the K2 Galactic Archaeology Programme (K2GAP), with about 25 per cent of the observed targets being allocated to this programme. K2GAP provides asteroseismic parameters for about 23 000 giant stars across the Galaxy, which together with spectroscopic stellar parameters can give age and masses of stars. We discuss in detail the target selection procedure and provide a python program that implements the selection function (github.com/sanjibs/k2gap). Broadly speaking, the targets were selected on 2MASS colour J − Ks > 0.5, with finely tuned adjustments for each campaign. We discuss the detection completeness of the asteroseismic parameters νmax and Δν. About 14 per cent of giants were found to miss νmax detections and it was difficult to detect Δν for RC stars. Making use of the selection function, we compare the observed distribution of asteroseismic masses to theoretical predictions. The median asteroseismic mass is higher by about 4 per cent compared to predictions. We provide a selection-function-matched mock catalogue of stars based on a synthetic model of the Galaxy for the community to use in subsequent analyses of the K2GAP data set (physics.usyd.edu.au/k2gap/download/).

    

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2. M giants with IGRINS: I. Stellar parameters and α -abundance trends of the solar neighborhood population

   https://doi.org/10.1051/0004-6361/202346149  

   Nandakumar, G. ; Ryde, N. ; Casagrande, L. ; Mace, G. ( July 2023 , Astronomy & Astrophysics)

   Context. Cool stars, such as M giants, can only be analyzed in the near-infrared (NIR) regime due to the ubiquitous titanium oxide features in optical spectra of stars with T eff  < 4000 K. In dust-obscured regions, the inner bulge and Galactic center region, the intrinsically bright M giants observed in the NIR are an optimal option for studying stellar abundances and the chemical evolution of stellar populations. Because of the uncertainties in photometric methods, a method for determining the stellar parameters for M giants from the NIR spectra themselves is needed. Aims. We develop a method for determining the stellar parameters for M giants from the NIR spectra. We validate the method by deriving the stellar parameters for nearby well-studied M giants with spectra from the spectral library of the Immersion GRating INfrared Spectrograph (IGRINS). We demonstrate the accuracy and precision of our method by determining the stellar parameters and α -element trends versus metallicity for solar neighborhood M giants. Methods. We carried out new observations of 44 M giant stars with IGRINS mounted on the Gemini South telescope. We also obtained the full H and K band IGRINS spectra of six nearby well-studied M giants at a spectral resolving power of R  = 45 000 from the IGRINS spectral library. We used the tool called spectroscopy made easy in combination with one-dimensional (1D) model atmospheres in a radiative and convective scheme (MARCS) stellar atmosphere models to model the synthetic spectrum that fits the observed spectrum best. Results. The effective temperatures that we derive from our new method (tested for 3400 ≲  T eff  ≲ 4000 K here) agree excellently with those of the six nearby well-studied M giants, which indicates that the accuracy is indeed high. For the 43 solar neighborhood M giants, our T eff , log g , [Fe/H], ξ micro , [C/Fe], [N/Fe], and [O/Fe] agree with APOGEE with mean differences and a scatter (our method – APOGEE) of −67±33 K, −0.31±0.15 dex, 0.02±0.05 dex, 0.22±0.13 km s −1 , −0.05±0.06 dex, 0.06±0.06 dex, and 0.02±0.09 dex, respectively. Furthermore, the tight offset with a small dispersion compared to the APOGEE T eff indicates a high precision in our derived temperatures and those derived from the APOGEE pipeline. The typical uncertainties in the stellar parameters are found to be ±100 K in T eff , ±0.2 dex in log g , ±0.1 dex in [Fe/H], and ±0.1 km s −1 in ξ micro . The α -element trends versus metallicity for Mg, Si, Ca, and Ti are consistent with the APOGEE DR17 trends for the same stars and with the GILD optical trends. We also find a clear enhancement in the abundances for thick-disk stars. 

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3. The K2 Galactic Archaeology Program Data Release 3: Age-abundance Patterns in C1–C8 and C10–C18

   https://doi.org/10.3847/1538-4357/ac2c83  

   Zinn, Joel C. ; Stello, Dennis ; Elsworth, Yvonne ; García, Rafael A. ; Kallinger, Thomas ; Mathur, Savita ; Mosser, Benoît ; Hon, Marc ; Bugnet, Lisa ; Jones, Caitlin ; et al ( February 2022 , The Astrophysical Journal)

   We present the third and final data release of the K2 Galactic Archaeology Program (K2 GAP) for Campaigns C1–C8 and C10–C18. We provide asteroseismic radius and mass coefficients,κ_Randκ_M, for ∼19,000 red giant stars, which translate directly to radius and mass given a temperature. As such, K2 GAP DR3 represents the largest asteroseismic sample in the literature to date. K2 GAP DR3 stellar parameters are calibrated to be on an absolute parallactic scale based on Gaia DR2, with red giant branch and red clump evolutionary state classifications provided via a machine-learning approach. Combining these stellar parameters with GALAH DR3 spectroscopy, we determine asteroseismic ages with precisions of ∼20%–30% and compare age-abundance relations to Galactic chemical evolution models among both low- and high-αpopulations forα, light, iron-peak, and neutron-capture elements. We confirm recent indications in the literature of both increased Ba production at late Galactic times as well as significant contributions tor-process enrichment from prompt sources associated with, e.g., core-collapse supernovae. With an eye toward other Galactic archeology applications, we characterize K2 GAP DR3 uncertainties and completeness using injection tests, suggesting that K2 GAP DR3 is largely unbiased in mass/age, with uncertainties of 2.9% (stat.) ± 0.1% (syst.) and 6.7% (stat.) ± 0.3% (syst.) inκ_Randκ_Mfor red giant branch stars and 4.7% (stat.) ± 0.3% (syst.) and 11% (stat.) ± 0.9% (syst.) for red clump stars. We also identify percent-level asteroseismic systematics, which are likely related to the time baseline of the underlying data, and which therefore should be considered in TESS asteroseismic analysis.

    

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4. The Galactic chemical evolution of phosphorus observed with IGRINS

   https://doi.org/10.1051/0004-6361/202244724  

   Nandakumar, G. ; Ryde, N. ; Montelius, M. ; Thorsbro, B. ; Jönsson, H. ; Mace, G. ( December 2022 , Astronomy & Astrophysics)

   Context. Phosphorus (P) is considered to be one of the key elements for life, making it an important element to look for in the abundance analysis of spectra of stellar systems. Yet, only a select number of spectroscopic studies exist to estimate the phosphorus abundances and investigate its trend across a range of metallicities. This is due to the lack of good phosphorus lines in the optical wavelength region and the requirement of careful manual analysis of the blended phosphorus lines in near-infrared H-band spectra obtained with individual observations and surveys such as the Apache Point Observatory Galactic Evolution Experiment (APOGEE). Aims. Based on a consistent and systematic analysis of high-resolution, near-infrared Immersion GRating INfrared Spectrograph (IGRINS) spectra of 38 K giant stars in the Solar neighborhood, we present and investigate the phosphorus abundance trend in the metallicity range of −1.2 dex < [Fe/H] < 0.4 dex. Furthermore, we compare this trend with the available chemical evolution models to shed some light on the origin and evolution of phosphorus. Methods. We have observed full H - and K -band spectra at a spectral resolving power of R = 45 000 with IGRINS mounted on the Gemini South telescope, the Discovery Channel Telescope, and the Harlan J Smith Telescope at McDonald Observatory. Abundances were determined from spectral lines by modeling the synthetic spectrum that best matches the observed spectrum by χ 2 minimization. For this task, we used the Spectroscopy Made Easy (SME) tool in combination with one-dimensional (1D) Model Atmospheres in a Radiative and Convective Scheme (MARCS) stellar atmosphere models. The investigated sample of stars have reliable stellar parameters estimated using optical FIber-fed Echelle Spectrograph (FIES) spectra obtained in a previous study of a set of stars called Giants in the Local Disk (GILD). In order to determine the phosphorus abundances from the 16482.92 Å phosphorus line, we needed to take special care blending the CO( v = 7−4) line. With the stellar parameters known, we thus determined the C, N, and O abundances from atomic carbon and a range of nonblended molecular lines (CO, CN, and OH) which are plentiful in the H-band region of K giant stars, assuring an appropriate modeling of the blending CO( v = 7−4) line. Results. We present the [P/Fe] versus [Fe/H] trend for K giant stars in the metallicity range of −1.2 dex < [Fe/H] < 0.4 dex and enhanced phosphorus abundances for two metal-poor s-rich stars. We find that our trend matches well with the compiled literature sample of prominently dwarf stars and the limited number of giant stars. Our trend is found to be higher by ~0.05−0.1 dex compared to the theoretical chemical evolution trend resulting from the core collapse supernova (type II) of massive stars with the phosphorus yields arbitrarily increased by a factor of 2.75. Thus the enhancement factor might need to be ~0.05−0.1 dex higher to match our trend. We also find an empirically determined primary behavior for phosphorus. Furthermore, the phosphorus abundance is found to be elevated by ~0.6−0.9 dex in the two s-enriched stars compared to the theoretical chemical evolution trend. 

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5. The Open Cluster Chemical Abundances and Mapping Survey. VII. APOGEE DR17 [C/N]–Age Calibration

   https://doi.org/10.3847/1538-3881/ac5d53  

   Spoo, Taylor ; Tayar, Jamie ; Frinchaboy, Peter M. ; Cunha, Katia ; Myers, Natalie ; Donor, John ; Majewski, Steven R. ; Bizyaev, Dmitry ; García-Hernández, D. A. ; Jönsson, Henrik ; et al ( April 2022 , The Astronomical Journal)

   Large-scale surveys open the possibility to investigate Galactic evolution both chemically and kinematically; however, reliable stellar ages remain a major challenge. Detailed chemical information provided by high-resolution spectroscopic surveys of the stars in clusters can be used as a means to calibrate recently developed chemical tools for age-dating field stars. Using data from the Open Cluster Abundances and Mapping survey, based on the Sloan Digital Sky Survey/Apache Point Observatory Galactic Evolution Experiment 2 survey, we derive a new empirical relationship between open cluster stellar ages and the carbon-to-nitrogen ([C/N]) abundance ratios for evolved stars, primarily those on the red giant branch. With this calibration, [C/N] can be used as a chemical clock for evolved field stars to investigate the formation and evolution of different parts of our Galaxy. We explore how mixing effects at different stellar evolutionary phases, like the red clump, affect the derived calibration. We have established the [C/N]–age calibration for APOGEE Data Release 17 (DR17) giant star abundances to be$\log {[\mathrm{Age}(\mathrm{yr})]}_{\mathrm{DR}17}=10.14(\pm 0.08)+2.23(\pm 0.19)[\mathrm{C}/\mathrm{N}]$, usable for$8.62\le \log (\mathrm{Age}[\mathrm{yr}])\le 9.82$, derived from a uniform sample of 49 clusters observed as part of APOGEE DR17 applicable primarily to metal-rich, thin- and thick-disk giant stars. This measured [C/N]–age APOGEE DR17 calibration is also shown to be consistent with asteroseismic ages derived from Kepler photometry.

    

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