The Second APOKASC Catalog: The Empirical Approach

TL;DR

This paper presents an extensive catalog of evolved stars with combined spectroscopic and asteroseismic data, introducing an empirical method to improve parameter accuracy and assess uncertainties across multiple analysis techniques.

Contribution

It introduces a new empirical approach for combining asteroseismic measurements, calibrating stellar parameters, and estimating uncertainties, addressing systematic offsets among different pipelines.

Findings

Asteroseismic parameters show systematic offsets not fixed by solar reference adjustments.

Calibration of large frequency spacing and frequency of maximum power improves parameter consistency.

Measurement dispersion supports using multiple analysis techniques for stellar population studies.

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.