2021ApJS..253...19Z

We have used the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) low-resolution and medium-resolution spectroscopic surveys to explore the chromospheric statistical properties and variations of M stars. There are 738,477 LAMOST low-resolution spectra of 622,523 M stars. We have calculated equivalent widths of the Ca II H&K, Hα, Hβ, Hγ, Hδ, He I D3, and Ca II infrared triplet lines. Based on the Hα EWs, we found that 33,374 objects showed Hα emissions. We revised the relationship between the active fraction and spectral subtype, and confirmed that the active fractions increased from M0 to M3, and possibly declined from M4 to subsequent types. Among 86,005 objects with repeated spectra, 19,422 objects showed Hα variations. The variation fraction of stars with M4 and subsequent types with a fully convective envelope was higher than those of M0-M3 with a partially convective envelope. By combining the distances from Gaia, we have determined the positions of the M dwarfs in the catalog for M stars. The active fraction decreased rapidly in the height range of 0-400 pc above and below the Galactic plane, and maintained a steady trend in 400-1000 pc. We cross-matched the M catalog of the LAMOST low-resolution survey with LAMOST medium-resolution spectra. We obtained 272,181 spectra of 13,355 M objects and calculated the Hα EWs. Among them, 3813 stars show variations, which might be caused by chromospheric activity evolution with orbital phase or time. The Hα emission for 972 objects shows variations over short and long timescales, especially on short timescales of 20 minutes.