The APOKASC Catalog: An Asteroseismic and Spectroscopic Joint Survey of Targets in the Kepler Fields

Abstract: We present the first APOKASC catalog of spectroscopic and asteroseismic properties of 1916 red giants observed in the Kepler fields. The spectroscopic parameters provided from the Apache Point Observatory Galactic Evolution Experiment project are complemented with asteroseismic surface gravities, masses, radii, and mean densities determined by members of the Kepler Asteroseismology Science Consortium. We assess both random and systematic sources of error and include a discussion of sample selection for giants in the Kepler fields. Total uncertainties in the main catalog properties are of order 80 K in Teff , 0.06 dex in [M/H], 0.014 dex in log g, and 12% and 5% in mass and radius, respectively; these reflect a combination of systematic and random errors. Asteroseismic surface gravities are substantially more precise and accurate than spectroscopic ones, and we find good agreement between their mean values and the calibrated spectroscopic surface gravities. There are, however, systematic underlying trends with Teff and log g. Our effective temperature scale is between 0-200 K cooler than that expected from the Infrared Flux Method, depending on the adopted extinction map, which provides evidence for a lower value on average than that inferred for the Kepler Input Catalog (KIC). We find a reasonable correspondence between the photometric KIC and spectroscopic APOKASC metallicity scales, with increased dispersion in KIC metallicities as the absolute metal abundance decreases, and offsets in Teff and log g consistent with those derived in the literature. We present mean fitting relations between APOKASC and KIC observables and discuss future prospects, strengths, and limitations of the catalog data.

• The paper combines Kepler asteroseismology with APOGEE spectroscopy to derive precise effective temperatures, gravities, metallicities, and mass-radius estimates for 1916 red giants.
• It employs the Infrared Flux Method and correction factors to calibrate stellar parameters, reducing discrepancies with the Kepler Input Catalog.
• The study’s calibrated relations enhance stellar modeling and offer valuable insights for mapping Galactic structure and advancing stellar evolution research.

An Examination of the APOKASC Catalog Providing Asteroseismic and Spectroscopic Data from the Kepler Fields

The paper "The APOKASC Catalog: An Asteroseismic and Spectroscopic Joint Survey of Targets in the Kepler Fields" offers a comprehensive analysis of 1916 red giant stars observed in the Kepler fields, highlighting the integration of asteroseismic and spectroscopic data. Authored by an extensive collaboration of experts, this publication represents a significant advance in the field of precision stellar astrophysics, leveraging data from large-scale surveys to refine our understanding of stellar characteristics and evolutionary processes.

Summary of Methodology

The study systematically combines asteroseismic data from the Kepler mission with spectroscopic data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE). Asteroseismic parameters, including surface gravities, masses, radii, and mean densities, were computed primarily using the average large frequency separation ($\Delta \nu$) and the frequency of maximum oscillation power ($\nu_{\rm max}$). The spectroscopic data provided estimates for effective temperatures ($T_{\rm eff}$), metallicities ([M/H]), and surface gravities ($\log g$).

The effective temperatures were calibrated using the Infrared Flux Method (IRFM), with the resulting temperatures approximately 0-200 K cooler than those implied by the Kepler Input Catalog (KIC). The spectroscopic metallicities and effective temperatures employed correction factors based on comparisons with standard star clusters. Asteroseismic gravities, found to be more precise than spectroscopic ones, reveal systematic trends with $T_{\rm eff}$ and $\log g$.

Key Results

• The study reports total uncertainties of 80 K in $T_{\rm eff}$, 0.06 dex in [M/H], 0.014 dex in $\log g$, and 12% in mass and 5% in radius.
• The comparison of asteroseismic and spectroscopic surface gravities showed good agreement, although systematic trends tied to effective temperature and surface gravity were observed.
• Using the combined data, the researchers provided calibrated relations between APOKASC and KIC observable characteristics.
• The catalog also highlights the potential for using asteroseismic constraints to improve distance estimates, surface gravities, and overall stellar parameter accuracy.

Implications and Future Directions

The integration of asteroseismic and spectroscopic data offers a refined approach to determining fundamental stellar properties with applications in understanding stellar evolution and structure. This data set is particularly valuable for constructing stellar populations and mapping Galactic structure. Future work will aim to enhance calibration techniques and explore potential systematic errors further, especially concerning extreme gravity and temperature states. The ongoing development of the APOGEE survey is anticipated to expand the database, offering even richer datasets for analysis.

The APOKASC catalog stands as a critical resource for theorists and observational astronomers alike, providing a wealth of data to test stellar models and improve techniques for analyzing stellar populations within the Milky Way. The detailed analysis and methodology adopted in this paper serve as a cornerstone for future research aimed at uncovering the complexities of stellar astrophysics.