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Abstract Known sources of lithium (Li) in the universe include the Big Bang, novae, asymptotic giant branch stars, and cosmic-ray spallation. During their longer-lived evolutionary phases, stars are not expected to add to the Li budget of the Galaxy, but to largely deplete it. In this context, recent analyses of Li data from GALAH and LAMOST for field red clump (RC) stars have concluded that there is the need for a new production channel of Li, ubiquitous among low-mass stars, and that would be triggered on the upper red giant branch (RGB) or at helium ignition. This is distinct from the Li-rich giant problem and reflects bulk RC star properties. We provide an analysis of the GALAH Li data that accounts for the distribution of progenitor masses of field RC stars observed today. Such progenitors are different than today’s field RGB stars. Using standard post-main-sequence stellar evolution, we show that the distribution of Li among field RC giants as observed by GALAH is consistent with standard model predictions, and does not require new Li production mechanisms. Our model predicts a large fraction of very low Li abundances from low-mass progenitors, with higher abundances from higher mass ones. Moreover, there should be a large number of upper limits for RC giants, and higher abundances should correspond to higher masses. The most recent GALAH data indeed confirm the presence of large numbers of upper limits, and a much lower mean Li abundance in RC stars, in concordance with our interpretation. 

ABSTRACT Precise asteroseismic parameters can be used to quickly estimate radius and mass distributions for large samples of stars. A number of automated methods are available to calculate the frequency of maximum acoustic power (νmax) and the frequency separation between overtone modes (Δν) from the power spectra of red giants. However, filtering through the results requires manual vetting, elaborate averaging across multiple methods or sharp cuts in certain parameters to ensure robust samples of stars free of outliers. Given the importance of ensemble studies for Galactic archaeology and the surge in data availability, faster methods for obtaining reliable asteroseismic parameters are desirable. We present a neural network classifier that vets Δν by combining multiple features from the visual Δν vetting process. Our classifier is able to analyse large numbers of stars, determining whether their measured Δν are reliable and thus delivering clean samples of oscillating stars with minimal effort. Our classifier is independent of the method used to obtain νmax and Δν, and therefore can be applied as a final step to any such method. Tests of our classifier’s performance on manually vetted Δν measurements reach an accuracy of 95 per cent. We apply the method to giants observed by the K2 Galactic Archaeology Program and find that our results retain stars with astrophysical oscillation parameters consistent with the parameter distributions already defined by well-characterized Kepler red giants. 

ABSTRACT 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/). 

Precise Gaia measurements of positions, parallaxes, and proper motions provide an opportunity to calculate 3D positions and 2D velocities (i.e., 5D phase-space) of Milky Way stars. Where available, spectroscopic radial velocity (RV) measurements provide full 6D phase-space information, however there are now and will remain many stars without RV measurements. Without an RV it is not possible to directly calculate 3D stellar velocities; however, one can infer 3D stellar velocities by marginalizing over the missing RV dimension. In this paper, we infer the 3D velocities of stars in the Kepler field in Cartesian Galactocentric coordinates (vx, vy, vz). We directly calculate velocities for around a quarter of all Kepler targets, using RV measurements available from the Gaia, LAMOST, and APOGEE spectroscopic surveys. Using the velocity distributions of these stars as our prior, we infer velocities for the remaining three quarters of the sample by marginalizing over the RV dimension. The median uncertainties on our inferred vx, vy, and vz velocities are around 4, 18, and 4 km/s, respectively. We provide 3D velocities for a total of 148,590 stars in the Kepler field. These 3D velocities could enable kinematic age-dating, Milky Way stellar population studies, and other scientific studies using the benchmark sample of well-studied Kepler stars. Although the methodology used here is broadly applicable to targets across the sky, our prior is specifically constructed from and for the Kepler field. Care should be taken to use a suitable prior when extending this method to other parts of the Galaxy. 

ABSTRACT The detailed age-chemical abundance relations of stars measure time-dependent chemical evolution. These trends offer strong empirical constraints on nucleosynthetic processes, as well as the homogeneity of star-forming gas. Characterizing chemical abundances of stars across the Milky Way over time has been made possible very recently, thanks to surveys like Gaia, APOGEE, and Kepler. Studies of the low-α disc have shown that individual elements have unique age–abundance trends and the intrinsic dispersion around these relations is small. In this study, we examine and compare the age distribution of stars across both the high and low-α disc and quantify the intrinsic dispersion of 16 elements around their age–abundance relations at [Fe/H] = 0 using APOGEE DR16. We examine the age–metallicity relation and visualize the temporal and spatial distribution of disc stars in small chemical cells. We find: (1) the high-α disc has shallower age–abundance relations compared to the low-α disc, but similar median intrinsic dispersions of ∼0.03 dex; (2) turnover points in the age-[Fe/H] relations across radius for both the high- and low-α disc. The former constrains the mechanisms that set similar intrinsic dispersions, regardless of differences in the enrichment history, for stars in both disc, and the latter indicates the presence of radial migration in both disc. Our study is accompanied by an age catalogue for 64 317 stars in APOGEE derived using the cannon with a median uncertainty of 1.5 Gyr (26 per cent; APO-CAN stars), and a red clump catalogue of 22 031 stars with a contamination rate of 2.7 per cent. 

By measuring the elemental abundances of a star, we can gain insight into the composition of its initial gas cloud— the formation site of the star and its planets. Planet formation requires metals, the availability of which is determined by the elemental abundance. In the case where metals are extremely deficient, planet formation can be stifled. To investigate such a scenario requires a large sample of metal-poor stars and a search for planets therein. This paper focuses on the selection and validation of a halo star sample. We select ∼17,000 metal-poor halo stars based on their Galactic kinematics, and confirm their low metallicities ([Fe/H] < −0.5), using spectroscopy from the literature. Furthermore, we perform high-resolution spectroscopic observations using LBT/PEPSI and conduct detailed metallicity ([Fe/H]) analyses on a sample of 13 previously-known halo stars that also have hot kinematics. We can use the halo star sample presented here to measure the frequency of planets and to test planet formation in extremely metal-poor environments. The result of the planet search and its implications will be presented and discussed in a companion paper by Boley et al.