The double population of Chamaeleon I detected by Gaia DR2
  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 from Gaia data-release 2 (DR2). Methods. We
identified 140 sources with a reliable counterpart in the Gaia DR2 archive. We
determined the median distance of the cluster using Gaia parallaxes 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 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
distance dN = 192.7+/-0.4 pc, while the southern one dS = 186.5+/-0.7 pc. 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.
