An Intermediate-age Alpha-rich Galactic Population in K2

TL;DR

This study uses asteroseismic and spectroscopic data to analyze the ages and chemical compositions of stars in the Galaxy, revealing an intermediate-age, alpha-rich population and highlighting the importance of alpha-enrichment effects on age estimates.

Contribution

It provides new age estimates for K2 stars considering alpha-enrichment effects and maps age trends across different Galactic regions, extending previous Kepler-based findings.

Findings

Alpha-rich stars are older than low-alpha stars in the Kepler field.

Including alpha-enrichment effects increases age estimates by up to 10%.

Age distributions of stars converge with distance from the Galactic plane.

Abstract

We explore the relationships between the chemistry, ages, and locations of stars in the Galaxy using asteroseismic data from the K2 mission and spectroscopic data from the Apache Point Galactic Evolution Experiment survey. Previous studies have used giant stars in the Kepler field to map the relationship between the chemical composition and the ages of stars at the solar circle. Consistent with prior work, we find that stars with high [Alpha/Fe] have distinct, older ages in comparison to stars with low [Alpha/Fe]. We provide age estimates for red giant branch (RGB) stars in the Kepler field, which support and build upon previous age estimates by taking into account the effect of alpha-enrichment on opacity. Including this effect for [Alpha/Fe]-rich stars results in up to 10% older ages for low-mass stars relative to corrected solar mixture calculations. This is a significant effect that Galactic archaeology studies should take into account. Looking beyond the Kepler field, we estimate ages for 735 red giant branch stars from the K2 mission, mapping age trends as a function of the line of sight. We find that the age distributions for low- and high-[Alpha/Fe] stars converge with increasing distance from the Galactic plane, in agreement with suggestions from earlier work. We find that K2 stars with high [Alpha/Fe] appear to be younger than their counterparts in the Kepler field, overlapping more significantly with a similarly aged low-[Alpha/Fe] population. This observation may suggest that star formation or radial migration proceeds unevenly in the Galaxy.