# The K2-HERMES Survey: Age and Metallicity of the Thick Disc

## TL;DR

This study uses asteroseismology and spectroscopic data from the K2 and GALAH surveys to refine the age and metallicity estimates of the Galactic thick disc, resolving previous model-data discrepancies.

## Contribution

It demonstrates that revised metallicity measurements align seismic mass predictions with observations, confirming the old age of the thick disc and validating asteroseismic mass scaling.

## Key findings

- Revised metallicity of the thick disc is $ ext{log}(Z/Z_{igodot})=-0.16$.
- Seismic mass distribution matches predictions with updated metallicities.
- Estimated thick disc age is approximately 10 Gyr.

## Abstract

Asteroseismology is a promising tool to study Galactic structure and evolution because it can probe the ages of stars. Earlier attempts comparing seismic data from the {\it Kepler} satellite with predictions from Galaxy models found that the models predicted more low-mass stars compared to the observed distribution of masses. It was unclear if the mismatch was due to inaccuracies in the Galactic models, or the unknown aspects of the selection function of the stars. Using new data from the K2 mission, which has a well-defined selection function, we find that an old metal-poor thick disc, as used in previous Galactic models, is incompatible with the asteroseismic information. We show that spectroscopic measurements of [Fe/H] and [$\alpha$/Fe] elemental abundances from the GALAH survey indicate a mean metallicity of $\log (Z/Z_{\odot})=-0.16$ for the thick disc. Here $Z$ is the effective solar-scaled metallicity, which is a function of [Fe/H] and [$\alpha$/Fe]. With the revised disc metallicities, for the first time, the theoretically predicted distribution of seismic masses show excellent agreement with the observed distribution of masses. This provides an indirect verification of the asteroseismic mass scaling relation is good to within five percent. Using an importance-sampling framework that takes the selection function into account, we fit a population synthesis model of the Galaxy to the observed seismic and spectroscopic data. Assuming the asteroseismic scaling relations are correct, we estimate the mean age of the thick disc to be about 10 Gyr, in agreement with the traditional idea of an old $\alpha$-enhanced thick disc.