Ages of Main-Sequence Turn-Off Stars from the GALAH Survey

TL;DR

This study estimates ages of main-sequence turn-off stars from the GALAH survey using stellar models, revealing two distinct age sequences in the Milky Way's disk and highlighting the importance of chemical abundances and stellar age in Galactic studies.

Contribution

It provides a new age estimation for MSTO stars considering C and O abundances, and identifies two distinct age sequences in the Galactic disk based on chemical and age data.

Findings

Ages of 2926 MSTO stars estimated with ~10% uncertainty.

Identification of two distinct age sequences in the Galactic disk.

[Y/Mg] as an effective chemical clock.

Abstract

Main sequence turn-off (MSTO) stars are good tracers of Galactic populations since their ages can be reliably estimated from atmospheric parameters. Based on the GALAH survey, we use the Yale Rotation Evolution Code to determine ages of 2926 MSTO stars with mean age uncertainty $\sim$10\% considering the variation of C and O abundances. Ages of CO-poor stars are systematically affected by $\sim$10\% due to the C and O abundances, globally shifting to $\sim$0.5 Gyr older compared to the results using solar metal-mixture. Of the stars with \mbox{[Fe/H] $\sim$0.3-0.5} or \mbox{[O/Fe]~$\leq$~-0.25}, many have fractional age differences~$\geq$~20\%, and even reach up to 36\%. The age-metallicity relation appears to possibly exist two distinct sequences: a young sequence of stars with age mostly $<$ 7 Gyr, and a relatively older sequence of stars with age mostly $>$ 7 Gyr, overlapping at 5 Gyr $\leq$~age~$\leq$ 7 Gyr. Moreover, the trends of abundances to age ratios show two corresponding sequences, especially in [O/Fe]-age plane. We also find that [Y/Mg] is a good chemical clock in disk populations. The young sequence and the old sequence can not be separated based on chemistry or kinematics, therefore stellar age is an important parameter to distinguish these two sequences in our sample.