Diagnosing the Stellar Population and Tidal Structure of the Blanco1 Star Cluster

Abstract

We present the stellar population, using {\it Gaia}\,DR2 parallax, kinematics, and photometry, of the young ($\sim 100$~Myr), nearby ($\sim 230$~pc) open cluster, Blanco1. A total of 644 member candidates are identified via the unsupervised machine learning method \textsc{StarGO} to find the clustering in the 5-dimensional position and proper motion parameter ($X$, $Y$, $Z$, $\hat{I}{¼}_{\hat{I}}\pm \cos \hat{I}´$, $\hat{I}{¼}_{\hat{I}}´$) space. Within the tidal radius of $10.0\pm 0.3$~pc, there are 488 member candidates, 3 times more than those outside. A leading tail and a trailing tail, each of 50--60~pc in the Galactic plane, are found for the first time for this cluster, with stars further from the cluster center streaming away faster, manifest stellar stripping. Blanco1 has a total detected mass of $285\pm 32$~M${}_{\odot }$ with a mass function consistent with a slope of $\hat{I}\pm =1.35\pm 0.2$ in the sense of $dN/dm\propto m^{-\hat{I}\pm }$, in the mass range of 0.25--2.51~M${}_{\odot }$, where $N$ is the number of members and $m$ is stellar mass. A Minimum Spanning Tree ($\hat{I}{›}_{\mathrm{MSR}}$) analysis shows the cluster to be moderately mass segregated among the most massive members ($\gtrsim 1.4$~M${}_{\odot }$), suggesting an early stage of dynamical disintegration.

16 pages, 13 figures, accepted by ApJ