LAMOST YSOs. I. Spectroscopically identifying and characterizing M-type young stellar objects

This study utilized LAMOST low-resolution spectra to identify M-type YSOs and characterize their accretion signatures. We measured characteristic features, including hydrogen Balmer, Li {\sc i}, He~{\sc i}, Na {\sc i}, and Ca {\sc ii} lines, as well as molecular absorption bands such as CaH. These features were evaluated for their potential to distinguish between different classes of M-type stars. In addition to the commonly used H$\alpha$ emission and {Li \sc i} $\lambda6708$~\AA~absorption, other features such as He~{\sc i}, Na {\sc i}, Ca~{\sc ii} HK and IRT lines, and CN, CaH, and VO bands also show potential for identifying YSOs from M-type dwarfs and giants. Based on key red-band features, we identified over 8,500 M-type YSO candidates from the LAMOST DR8 archive using the random forest technique, with over 2,300 of them likely being classical T Tauri stars (CTTSs). By incorporating $Gaia$ astrometry, 2MASS photometry, and extinction from 3D dust maps, we estimated the basic properties (such as age and mass) of the YSO candidates based on {\sc parsec} models. We also estimated the mass accretion rates of the CTTS candidates using H$\alpha$ emission. The derived accretion rates show a dependence on age and mass similar to that described in previous studies. Significant scatter in mass accretion rates exists even among stars with similar age and mass, which may be partly attributed to variations in $\rm H\alpha$ emission and the veiling.