Ultracool dwarfs identified using spectra in LAMOST DR7^⋆

¹ CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Beijing 100101, PR China
e-mail: lal@nao.cas.cn
² School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, PR China
³ Graduate Institute of Astronomy, National Central University, 300 Zhongda Road, Zhongli, Taoyuan 32001, Taiwan
e-mail: wpchen@ncu.edu.tw
⁴ Department of Physics, National Central University, 300 Zhongda Road, Zhongli, Taoyuan 32001, Taiwan
⁵ School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁶ Department of Astronomy, School of Physics, Peking University, Beijing 100871, PR China
⁷ Kavli institute of Astronomy and Astrophysics, Peking University, Beijing 100871, PR China

Received: 12 August 2021
Accepted: 15 December 2021

Abstract

In this work, we identify 734 ultracool dwarfs with a spectral type of M6 or later, including one L0. Of this sample, 625 were studied spectroscopically for the first time. All of these ultracool dwarfs are within 360 pc, with a Gaia G magnitude brighter than ∼19.2 mag. By studying the spectra and checking their stellar parameters (T_eff, log g, and [M/H]) derived with the LAMOST pipeline, we found their cool red nature and their metallicity to be consistent with the nature of Galactic thin-disk objects. Furthermore, 77 of them show lithium absorption lines at 6708 Å, further indicating their young ages and substellar nature. Kinematics obtained through LAMOST radial velocities, along with the proper motion and parallax data from Gaia EDR3, also suggest that the majority of our targets are thin-disk objects. Kinematic ages were estimated through the relationship between the velocity dispersion and the average age for a certain population. Moreover, we identified 35 binaries, with 6 of them reported as binaries for the first time.