标题： 体积受限样本的 63 颗 M7 至 M9.5 次矮星 I. 空间运动、动力学年龄和锂元素 显示英文标题

标题： A Volume-limited Sample of 63 M7--M9.5 Dwarfs I. Space Motion, Kinematic Age, and Lithium

摘要： 在由 63 颗 M7 至 M9.5 光谱型的超冷矮星组成的体积受限样本中，我们利用 VLT 的 UVES 和 Keck 观测台的 HIRES 获取了高分辨率光谱。在这篇第一篇论文中，我们介绍了来自 DENIS 和 2MASS 目标天体的体积完整样本，并推导出视向速度和空间运动。样本的运动学特征与这些恒星主要为年轻盘成员一致。样本的动力学年龄为 3.1 Gyr。我们发现，有六个目标显示出强烈的锂线，表明它们是几亿年以下的褐矮星。其中五个年轻的褐矮星之前未被识别。通过比较较晚光谱型的锂检测比例，我们在 M9 光谱型处看到该比例有一个意外的局部最大值。目前尚不清楚这个最大值是由于统计不足，还是由于包括年轻褐矮星的光谱外观、锂线形成以及低质量恒星形成率在内的多种物理效应的组合所致。 显示英文摘要

摘要： In a volume-limited sample of 63 ultracool dwarfs of spectral type M7--M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this first paper we introduce our volume-complete sample from DENIS and 2MASS targets, and we derive radial velocities and space motion. Kinematics of our sample are consistent with the stars being predominantly members of the young disk. The kinematic age of the sample is 3.1 Gyr. We find that six of our targets show strong Li lines implying that they are brown dwarfs younger than several hundred million years. Five of the young brown dwarfs were unrecognized before. Comparing the fraction of Li detections to later spectral types, we see a hint of an unexpected local maximum of this fraction at spectral type M9. It is not yet clear whether this maximum is due to insufficient statistics, or to a combination of physical effects including spectral appearance of young brown dwarfs, Li line formation, and the star formation rate at low masses.