The APO-K2 Catalog. I. 7,673 Red Giants with Fundamental Stellar Parameters from APOGEE DR17 Spectroscopy and K2-GAP Asteroseismology

We present a catalog of fundamental stellar properties for 7,673 evolved stars, including stellar radii and masses, determined from the combination of spectroscopic observations from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), part of the Sloan Digital Sky Survey IV (SDSS), and asteroseimology from K2. The resulting APO-K2 catalog provides spectroscopically derived temperatures, and metallicities, as well as asteroseismic global parameters, evolutionary states, and asteroseismic masses and radii. Additionally, we include kinematic information from Gaia. We investigate the multi-dimensional 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 (~320 at [M/H] ≤ -1), and their asteroseismic masses are larger than astrophysical priors for them. We argue that this may reflect offsets in the adopted fundamental temperature scale for metal-poor stars, rather than metallicity dependent issues with the interpretation of 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, ν_max, color, and magnitude with reference to Galactic simulations and target selection criteria, to enable robust statistical inferences with the catalog.