Ages and masses of 0.64 million red giant branch stars from the LAMOST Galactic Spectroscopic Survey

Abstract

We present a catalog of stellar age and mass estimates for a sample of\n640\\,986 red giant branch (RGB) stars of the Galactic disk from the LAMOST\nGalactic Spectroscopic Survey (DR4). The RGB stars are distinguished from the\nred clump stars utilizing period spacing derived from the spectra with a\nmachine learning method based on kernel principal component analysis (KPCA).\nCross-validation suggests our method is capable of distinguishing RC from RGB\nstars with only 2 per cent contamination rate for stars with signal-to-noise\nratio (SNR) higher than 50. The age and mass of these RGB stars are determined\nfrom their LAMOST spectra with KPCA method by taking the LAMOST - $Kepler$\ngiant stars having asteroseismic parameters and the LAMOST-TGAS sub-giant stars\nbased on isochrones as training sets. Examinations suggest that the age and\nmass estimates of our RGB sample stars with SNR $>$ 30 have a median error of\n30 per cent and 10 per cent, respectively. Stellar ages are found to exhibit\npositive vertical and negative radial gradients across the disk, and the age\nstructure of the disk is strongly flared across the whole disk of\n$6<R<13$\\,kpc. The data set demonstrates good correlations among stellar age,\n[Fe/H] and [$\\alpha$/Fe]. There are two separate sequences in the [Fe/H] --\n[$\\alpha$/Fe] plane: a high--$\\alpha$ sequence with stars older than\n$\\sim$\\,8\\,Gyr and a low--$\\alpha$ sequence composed of stars with ages\ncovering the whole range of possible ages of stars. We also examine relations\nbetween age and kinematic parameters derived from the Gaia DR2 parallax and\nproper motions. Both the median value and dispersion of the orbital\neccentricity are found to increase with age. The vertical angular momentum is\nfound to fairly smoothly decrease with age from 2 to 12\\,Gyr, with a rate of\nabout $-$50\\,kpc\\,km\\,s$^{-1}$\\,Gyr$^{-1}$. A full table of the catalog is\npublic available online.\n