The double population of Chamaeleon I detected byGaiaDR2

Abstract

Context. Chamaeleon I represents an ideal laboratory to study the cluster formation in a low-mass environment. Recently, two sub-clusters spatially located in the northern and southern parts of Chamaeleon I were found with different ages and radial velocities.Aims. In this Letter we report new insights into the structural properties, age, and distance of Chamaeleon I based on the astrometric parameters fromGaiadata release 2 (DR2).Methods. We identified 140 sources with a reliable counterpart in theGaiaDR2 archive. We determined the median distance of the cluster usingGaiaparallaxes and fitted the distribution of parallaxes and proper motions assuming the presence of two clusters. We derived the probability of each single source of belonging to the northern or southern sub-clusters, and compared the HR diagram of the most probable members to pre-main sequences isochrones.Results. The median distance of Chamaeleon I is ~190 pc. This is consistent with the revised estimate using theTycho-GaiaAstrometric Solution, but it is about 20 pc larger than the value commonly adopted in the literature. From a Kolmogorov–Smirnov test of the parallaxes and proper-motion distributions we conclude that the northern and southern clusters do not belong to the same parent population. The northern population has a distancedN= 192.7+0.4−0.4pc, while the southern onedS= 186.5+0.7−0.7pc. The two sub-clusters appear coeval, at variance with literature results, and most of the sources are younger than 3 Myr. The northern cluster is more elongated and extends towards the southern direction partially overlapping with the more compact cluster located in the south. A hint of a relative rotation between the two sub-clusters is also found.