Ages, masses & kinematics of GALAH MS turnoff stars

Based on the GALAH survey, we use the Yale rotation evolution code to determine the ages of 2926 MSTO stars with a mean age uncertainty of ~10% considering the variation of C and O abundances. The ages of CO-poor stars are systematically affected by ~10% due to the C and O abundances, globally shifting to ~0.5Gyr older compared to the results using solar metal mixture.

Of the stars with [Fe/H]~0.3-0.5 or [O/Fe]<=-0.25, many have fractional age differences of >=20%, and even reach up to 36%. The age-metallicity relation appears to possibly indicate the existence of two distinct sequences: a young sequence of stars with ages mostly <7Gyr, and a relatively older sequence of stars with ages mostly >7Gyr, overlapping at 5Gyr<=age<=7Gyr. Moreover, the trends of abundances-to-age ratios show two corresponding sequences, especially in the [O/Fe]-age plane.