The Ages and Masses of a Million Galactic-disk Main-sequence Turnoff and Subgiant Stars from the LAMOST Galactic Spectroscopic Surveys

We present estimates of stellar age and mass for 0.93 million Galactic-disk main-sequence turnoff and subgiant stars from the LAMOST Galactic Spectroscopic Surveys. The ages and masses are determined by matching with stellar isochrones using a Bayesian algorithm, utilizing effective temperature $T_{eff}$, absolute magnitude $M_V$, metallicity [Fe/H], and α-element to iron abundance ratio [α/Fe] deduced from the LAMOST spectra. Extensive examinations suggest the age and mass estimates are robust. Overall, the sample stars have a median error of 34% for the age estimates, and half of the stars older than 2 Gyr have age uncertainties of only 20%–30%. The median error for the mass estimates of the whole sample of stars is ∼8%. The huge data set demonstrates good correlations among stellar age, [Fe/H] ([α/H]), and [α/Fe]. Particularly, double-sequence features are revealed in both the age–[α/Fe] and age–[Fe/H]([α/H]) spaces. In the [Fe/H]–[α/Fe] space, stars of 8–10 Gyr exhibit both the thin and thick disk sequences, while younger (older) stars show only the thin (thick) disk sequence, indicating that the thin disk became prominent 8–10 Gyr ago, while the thick disk formed earlier and was almost quenched 8 Gyr ago. Stellar ages exhibit positive vertical and negative radial gradients across the disk, and the outer disk of R ≳ 9 kpc exhibits a strong flare in stellar age distribution.