| Abstract: | 我們使用兩微米巡天觀測（2MASS）、UKIRT深紅外巡天觀測（UKIDSS）、美國海軍天文台動力學觀測（URAT1）、和蓋雅太空巡天觀測（{\it Gaia} DR\,2）的光度和動力學資料來篩選最完整的后髮星團（Coma Berenices）成員。那些擁有{\it Gaia} DR\,2 視差測量資料的成員星提供了最可靠的星團距離資訊，后髮星團距離我們約 86.7~pc（彌散程度約有 7.1~pc），估測其年齡為八億年（$800$~Myr），大於前人所給出的四億到六億年。我們的研究在離星團中心半徑五度的空間中辨認了 192 顆成員星，在基於統計上估算出的場星污染數目的調整後，我們認為后髮星團約有 146 顆成員星。這些成員的亮度分佈為 $J\sim3$~mag 到 $\sim17.5$~mag，相當於 2.3~$M_\sun$ 到 0.06~$M_\sun$（太陽質量）。此星團包括了九顆棕矮星，其中有三顆為首次發現，光譜型態分別是 M8、L1 以及 L4，這項發現拓展了我們對這個星團最暗成員星的認知。
另外我們運用 \textsc{StarGO} 這個群聚分析程式研究 {\it Gaia} DR\,2 的視差與自行運動資料，發現了后髮星團的兩條潮汐尾，這兩條各自延伸約 $50$~pc 的尾巴乃前人所沒發現。后髮星團的成員總質量為 $\sim112^{+5}_{-3}\,\rm{M_\sun}$，其成員星的質量分佈（mass function）在質量區間 $0.25$~$M_\sun$ 到 $2.51~{\rm M_\sun}$ 裡的 $\alpha$ 值約為 $0.79\pm0.16$（$dN/dm = m^{-\alpha}$）。此星團在 $\sim$6.9~pc 的星團潮汐半徑內有 77 顆成員星，這些星的平均座標為 R.A.= 186.8110~deg 和 decl.= 25.8112~deg，另外則有 120 顆成員星在潮汐半徑以外，潮汐半徑外的星數遠大於潮汐半徑內星數，表示此星團正在瓦解。
;We have identified stellar and substellar members in the nearby star cluster Coma Berenices,
using photometry, proper motions, and distances of a combination of 2MASS, UKIDSS, URAT1,
and {\it Gaia} DR\,2 data. Those with {\it Gaia} DR\,2 parallax measurements provide the most
reliable sample to constrain the distance, averaging 86.7~pc with a dispersion 7.1~pc, and the
age $\sim800$~Myr, of the cluster. This age is older than the 400--600~Myr commonly adopted
in the literature. Our analysis, complete within $5\degr$ of the cluster radius, leads to identification
of 192 candidates, among which, after field contamination is considered, about 148 are true members.
The members have $J\sim3$~mag to $\sim17.5$~mag, corresponding to stellar masses 2.3--0.06~M$_\sun$.
Our list contains nine substellar members, including three new discoveries of an M8, an L1 and an L4
brown dwarfs, extending from the previously known coolest members of late-M types to even cooler types. \\
We also report the discovery of tidal structures around the Coma Berenices by only using the {\it Gaia} DR\,2
parallax and proper motion data on searching the spatial and kinematic grouping signature with a clustering
analysis tool, \textsc{StarGO}. A leading and a trailing tails, each with an extension of $\sim50$~pc are revealed
for the first time around this disrupting star cluster. The cluster members,
totaling to $\sim112^{+5}_{-3}\,\rm{M_\sun}$, are clearly mass segregated, and exhibit a flat mass function with
$\alpha \sim 0.79\pm0.16$, in the sense of $dN/dm = m^{-\alpha}$, where $N$ is the number of member stars
and $m$ is stellar mass, in the mass range of $m=0.25$--$2.51~{\rm M_\sun}$. Within the tidal radius of
$\sim$6.9~pc, there are 77 member candidates with an average position, i.e., as the cluster center, of
R.A.= 186.8110~deg, and decl.= 25.8112~deg, and an average distance of 85.8~pc. Additional 120 member
candidates reside in the tidal structures, i.e., outnumbering those in the cluster core. |
