Asteroseismology of 36 Kepler subgiants -- II: Determining ages from detailed modelling

TL;DR

This study uses detailed asteroseismic modelling of 36 Kepler subgiants to derive stellar ages with high precision, demonstrating that these age estimates are relatively robust and less model-dependent than those of other stellar types.

Contribution

The paper provides a comprehensive analysis of the dependencies of seismic age estimates on model parameters and demonstrates the reliability of age determination for subgiants using detailed frequency modelling.

Findings

Ages are insensitive to helium fraction and mixing-length parameter.

Ages strongly depend on metallicity, but this dependence is manageable.

Seismic ages of subgiants can be estimated with about 15% precision.

Abstract

Detailed modelling of stellar oscillations is able to give precise estimates for stellar ages, but the inferred results typically depend on the adopted model parameters used for the age inference. High-quality asteroseismic data with precise measurements of mixed modes are available for 36 Kepler subgiants. To obtain a handle on the robustness of the ages for these stars, we first study the dependencies of seismic ages on three model input parameters. We find that inferred ages do not change systematically with the helium fraction (Y) or the mixing-length parameter but depend strongly on the metallicity ([M/H]) of the model. The results indicate that age estimates of subgiants have less model dependence and hence are more reliable than those of main-sequence stars or red giants. We then model individual oscillation frequencies of the same 36 Kepler subgiants, using observed metallicities, and obtain their ages with an average precision of ~15%. The comparison with previous age estimates with different stellar codes or input physics shows good agreement (mostly within 2#). We hence suggest that seismology-determined ages of subgiants are not greatly model-dependent.