标题：M-subdwarf Research. I. Identification, Modified Classification System, and Sample Construction
作者：Zhang, Shuo; Luo, A-Li; Comte, Georges; Gizis, John E.; Wang, Rui; Li, Yinbi; Qin, Li; Kong, Xiao; Bai, Yu; Yi, Zhenping
作者机构：[Zhang, Shuo; Luo, A-Li; Wang, Rui; Li, Yinbi; Kong, Xiao; Bai, Yu] Chinese Acad Sci, Key Lab Opt Astron, Natl Astron Observ, Beijing 100012, Peoples 更多
通讯作者：Luo, AL;Luo, AL
通讯作者地址：[Luo, AL]Chinese Acad Sci, Key Lab Opt Astron, Natl Astron Observ, Beijing 100012, Peoples R China;[Luo, AL]Univ Chinese Acad Sci, Beijing 100049, Peo 更多
来源：ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES
关键词：stars: late-type; stars: low-mass; stars: statistics; subdwarfs
摘要：We propose a revision of the system developed by Lepine et al. for spectroscopic M-subdwarf classification. Based on an analysis of subdwarf spectra and templates from Savcheva et al., we show that the CaH1 feature originally proposed by Gizis is important in selecting reliable cool subdwarf spectra. This index should be used in combination with the [TiO5, CaH2+CaH3] relation provided by Lepine et al. to avoid misclassification results. In the new system, the dwarf-subdwarf separators are first derived from a sample of more than 80,000 M dwarfs and a "labeled" subdwarf subsample, and these objects are all visually identified from their optical spectra. Based on these two samples, we refit the initial [TiO5, CaH1] relation and propose a new [CaOH, CaH1] relation supplementing the [TiO5, CaH1] relation to reduce the impact of uncertainty in flux calibration on classification accuracy. In addition, we recalibrate the zeta(TiO/CaH) parameter defined in Lepine et al. to enable its successful application to Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectra. Using this new system, we select candidates from LAMOST Data Release 4 and finally identify a set of 2791 new M-subdwarf stars, covering the spectral sequence from type M0 to M7. This sample contains a large number of objects located at low Galactic latitudes, especially in the Galactic anti-center direction, expanding beyond previously published halo- and thick disk-dominated samples. Besides, we detect magnetic activity in 141 objects. We present a catalog for this M-subdwarf sample, including radial velocities, spectral indices and errors, and activity flags, with a compilation of external data (photometric and Gaia Data Release 2 astrometric parameters). The catalog is provided online, and the spectra can be retrieved from the LAMOST Data Release web portal.