Red giant masses and ages derived from carbon and nitrogen abundances

TL;DR

This study develops an empirical method to estimate red giant star masses and ages using carbon and nitrogen abundances combined with spectroscopic data, enabling large-scale age mapping of the Milky Way.

Contribution

It introduces the first empirical relation to determine stellar ages for vast numbers of giants using spectroscopic and abundance data, validated with asteroseismic masses.

Findings

Masses predicted with about 14% fractional error.

Ages derived with approximately 40% error.

Revealed the vertical age structure of the Milky Way disk.

Abstract

We show that the masses of red giant stars can be well predicted from their photospheric carbon and nitrogen abundances, in conjunction with their spectroscopic stellar labels log g, Teff, and [Fe/H]. This is qualitatively expected from mass-dependent post main sequence evolution. We here establish an empirical relation between these quantities by drawing on 1,475 red giants with asteroseismic mass estimates from Kepler that also have spectroscopic labels from APOGEE DR12. We assess the accuracy of our model, and find that it predicts stellar masses with fractional r.m.s. errors of about 14% (typically 0.2 Msun). From these masses, we derive ages with r.m.s errors of 40%. This empirical model allows us for the first time to make age determinations (in the range 1-13 Gyr) for vast numbers of giant stars across the Galaxy. We apply our model to 52,000 stars in APOGEE DR12, for which no direct mass and age information was previously available. We find that these estimates highlight the vertical age structure of the Milky Way disk, and that the relation of age with [alpha/M] and metallicity is broadly consistent with established expectations based on detailed studies of the solar neighbourhood.