2023MNRAS.523.1199S

Main-sequence turn-off (MSTO) stars and subgiant stars are good tracers of Galactic populations. We present a study of 41 034 MSTO and subgiant stars from the GALAH survey. Using a grid of stellar models that accounts for the variation of O abundances, we determine their ages with a median age uncertainty of ∼9.4 per cent. Our analysis reveals that the ages of high-O stars based on O-enhanced models are smaller than those determined with α-enhanced models, resulting in a mean fractional age difference of -5.3 per cent at [O/α] = 0.2 and -11.0 per cent at [O/α] = 0.4. This age difference significantly impacts the age distribution of thick disc and halo stars, leading to a steeper downward trend in the [Fe/H]-age plane from 8 to 14 Gyr, indicating a shorter formation time-scale and a faster chemical-enhanced history for these populations. We confirm the V-shape of the normalized age-metallicity distribution p(τ∣[Fe/H]) of thin disc stars, which is presumably a consequence of the second gas infall. Additionally, we find that the halo stars in our sample can be divided into two sequences, a metal-rich sequence (Splash stars) and a metal-poor sequence (accreted stars), with the Splash stars predominantly older than 9 Gyr and the accreted halo stars older than 10 Gyr. Finally, we observe two distinct sequences in the relations between various chemical abundances and ages for disc stars, namely a young sequence with ages < ∼8 Gyr and an old sequence with ages > ∼8 Gyr.