LASPM: the LAMOST stellar parameter pipeline for M-type stars and application to the sixth and seventh data release (DR6 and DR7)

The molecular-rich atmospheres of M type stars complicate our understanding to their atmospheric properties. Recently, great progress has been made in atmospheric modeling of M-type stars, and we take advantage of the updated BT-Settl model grid to develop a pipeline LASPM to measure atmospheric parameters (T (eff), log g, [M/H]) of M-type stars from low-resolution spectra. The pipeline was applied to the sixth and seventh data release (DR6 & DR7) of Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), which released atmospheric parameters for 610 419 and 680 185 M-type spectra, respectively. The key algorithm is to find the best-matching for templates in the synthetic spectral library for an observed spectrum, and then minimizing chi (2) through a linear combination of five best-matching templates. The intrinsic precisions of the parameters were estimated by using the multiple epoch observations for the same stars, which are 118 K, 0.20 dex, 0.29 dex for T (eff), log g, and [M/H] respectively. The T (eff) and log g are consistent with the spectral and luminosity classifications by LAMOST 1D pipeline, and the loci of giants and dwarfs both on spectral index and color-magnitude diagrams show the validity. The metallicities of LASPM are also checked with the selected members of four open clusters (NGC 2632, Melotte 22, ASCC_16, and ASCC_19), which are consistent without any bias. Comparing the results between LASPM and the APOGEE Stellar Parameter and Chemical Abundance Pipeline (ASPCAP), there is a scatter of 73 K, 0.22 dex, 0.21 dex for T (eff), log g, and [M/H], respectively.