Title: A Stellar Census of the Tucana-Horologium Moving Group Show Chinese title

Title: 南门二-时钟星移动星群的恒星普查

Abstract: We report the selection and spectroscopic confirmation of 129 new late-type (K3-M6) members of the Tuc-Hor moving group, a nearby (~40 pc), young (~40 Myr) population of comoving stars. We also report observations for 13/17 known Tuc-Hor members in this spectral type range, and that 62 additional candidates are likely to be unassociated field stars; the confirmation frequency for new candidates is therefore 129/191 = 67%. We have used RVs, Halpha emission, and Li6708 absorption to distinguish contaminants and bona fide members. Our expanded census of Tuc-Hor increases the known population by a factor of ~3 in total and by a factor of ~8 for members with SpT>K3, but even so, the K-M dwarf population of Tuc-Hor is still markedly incomplete. The spatial distribution of members appears to trace a 2D sheet, with a broad distribution in X and Y, but a very narrow distribution (+/-5 pc) in Z. The corresponding velocity distribution is very small, with a scatter of +/-1.1 km/s about the mean UVW velocity. We also show that the isochronal age (20--30 Myr) and the lithium depletion age (40 Myr) disagree, following a trend seen in other PMS populations. The Halpha emission follows a trend of increasing EW with later SpT, as seen for young clusters. We find that members have been depleted of lithium for spectral types of K7.0-M4.5. Finally, our purely kinematic and color-magnitude selection procedure allows us to test the efficiency and completeness for activity-based selection of young stars. We find that 60% of K-M dwarfs in Tuc-Hor do not have ROSAT counterparts and would be omitted in Xray selected samples. GALEX UV-selected samples using a previously suggested criterion for youth achieve completeness of 77% and purity of 78%. We suggest new selection criteria that yield >95% completeness for ~40 Myr populations.(Abridged) Show Chinese abstract

Abstract: 我们报告了对129颗晚型（K3-M6）塔霍星族新成员的选择及其光谱确认，该星族是一个附近的（~40 pc）、年轻的（~40 Myr）共动星群。我们还报告了在此光谱类型范围内13/17个已知塔霍成员的观测结果，并发现另外62个候选者可能是未关联的场星；因此，新候选者的确认频率为129/191=67%。我们利用视向速度（RVs）、Hα发射线和Li6708吸收线来区分污染物和真正的成员。我们的扩展普查使塔霍星族的已知成员总数增加了约3倍，而对于光谱型> K3的成员则增加了约8倍，但即便如此，塔霍星族的K-M矮星成员仍然明显不完整。成员的空间分布似乎追踪一个二维平面，在X和Y方向上分布较广，但在Z方向上的分布非常狭窄（±5 pc）。相应的速度分布非常小，相对于平均UVW速度的标准差仅为±1.1 km/s。我们还表明，等时年龄（20-30 Myr）和锂丰度年龄（40 Myr）存在分歧，这一趋势在其他PMS群体中也可见。Hα发射线随光谱型后期增加而表现出增强的趋势，这与年轻星团中的观察一致。我们发现，对于光谱型为K7.0-M4.5的成员已经耗尽了锂元素。最后，我们纯动力学和颜色-亮度选择程序允许我们测试基于活动性选择年轻恒星的效率和完整性。我们发现，塔霍星族中的60% K-M矮星没有ROSAT对应物，会在X射线选样样本中被遗漏。使用先前建议的用于GALEX紫外选样的年轻标准可以达到77%的完备性和78%的纯度。我们建议新的选择标准，可为约40 Myr的群体提供>95%的完备性。（摘要）