The Second APOKASC Catalog: The Empirical Approach

Abstract: We present a catalog of stellar properties for a large sample of 6676 evolved stars with APOGEE spectroscopic parameters and \textit{Kepler} asteroseismic data analyzed using five independent techniques. Our data includes evolutionary state, surface gravity, mean density, mass, radius, age, and the spectroscopic and asteroseismic measurements used to derive them. We employ a new empirical approach for combining asteroseismic measurements from different methods, calibrating the inferred stellar parameters, and estimating uncertainties. With high statistical significance, we find that asteroseismic parameters inferred from the different pipelines have systematic offsets that are not removed by accounting for differences in their solar reference values. We include theoretically motivated corrections to the large frequency spacing ($\Delta \nu$) scaling relation, and we calibrate the zero point of the frequency of maximum power ($\nu_{\rm max}$) relation to be consistent with masses and radii for members of star clusters. For most targets, the parameters returned by different pipelines are in much better agreement than would be expected from the pipeline-predicted random errors, but 22\% of them had at least one method not return a result and a much larger measurement dispersion. This supports the usage of multiple analysis techniques for asteroseismic stellar population studies. The measured dispersion in mass estimates for fundamental calibrators is consistent with our error model, which yields median random and systematic mass uncertainties for RGB stars of order 4\%. Median random and systematic mass uncertainties are at the 9\% and 8\% level respectively for RC stars.

An Overview of the Second APOKASC Catalog: The Empirical Approach

The paper "The Second APOKASC Catalog: The Empirical Approach" by Pinsonneault et al. presents a comprehensive catalog of stellar properties derived from asteroseismic and spectroscopic data for 6676 evolved stars. Utilizing data from the Kepler mission and the Apache Point Observatory Galactic Evolution Experiment (APOGEE), this research endeavors to enhance the understanding of stellar populations in the Milky Way through empirical calibration of asteroseismic measurements.

Key Features and Methodology

The paper details the assembly of the APOKASC catalog, which offers insights into various stellar parameters, including evolutionary state, surface gravity, mean density, mass, radius, and age. A novel empirical approach is adopted for combining asteroseismic measurements from multiple pipelines, calibrating inferred stellar parameters, and estimating uncertainties. A significant finding is that systematic offsets in the asteroseismic parameters from different pipelines persist, even after accounting for solar reference value differences. Notably, theoretical corrections to the large frequency spacing ($\Delta \nu$) scaling relation are employed, and the zero point of the frequency of maximum oscillation power ($\nu_{\rm max}$) relation is scrutinized.

The empirical calibration against fundamental data, particularly for mass, is a major advancement in this research. The catalog reports calibrated masses and radii consistent with fundamental observations, utilizing theoretically informed $\Delta \nu$ corrections and precise calibration of the $\nu_{\rm max}$ zero point using star cluster data. This approach addresses discrepancies noted with previous scaling relations and refines the precision of significant stellar parameters.

Numerical Results and Implications

The catalog presents median mass uncertainties for RGB stars at approximately 4% and for RC stars at 9%. The systematic differences observed between pipeline methods underscore the strengths of employing a multi-pipeline approach to mitigate biases and enhance measurement reliability. The research highlights the strong correlations between surface [C/N] abundance and mass, suggesting potential avenues for age estimation and calibration across broader stellar populations.

This work significantly impacts several areas:
- Galactic Archeology: By providing more accurate calibers for stellar properties, it improves the reconstruction of the Milky Way's formation history.
- Stellar Physics: The empirical approach serves as a framework for future studies, offering potential calibration pathways for models of stellar evolution and structure.
- Data Utilization: APOKASC-2 sets a precedent for employing large datasets, showcasing the value of combining time domain asteroseismology with rich background spectroscopic data.

Speculation on Future Developments

The methodical calibration approach showcased in this paper offers potential for future development in asteroseismic studies of different stellar populations. The enhancement of techniques for correcting and interpreting asteroseismic data is likely to lead to further refinements in mass and age estimates. Additionally, integration with upcoming releases of precise parallax measurements from Gaia could further refine radius estimates, particularly for red clumps.

In conclusion, the Second APOKASC Catalog represents a meaningful step in advancing empirical methodologies for stellar characterization and provides a foundation for forthcoming research in astrophysics. The incorporation of multiple analysis techniques combined with empirical scaling calibrations establishes a more reliable template for future stellar population studies and theoretical developments in asteroseismology.