The double population of Chamaeleon I detected by Gaia DR2

TL;DR

This study uses Gaia DR2 data to reveal that Chamaeleon I consists of two spatially overlapping but distinct sub-clusters with different distances, ages, and kinematic properties, challenging previous assumptions of a single cluster.

Contribution

The paper provides new insights into the structural, distance, and age properties of Chamaeleon I by identifying and characterizing its two sub-clusters using Gaia DR2 astrometry.

Findings

The median distance of Chamaeleon I is approximately 190 pc.

The northern and southern sub-clusters are at different distances and do not share the same parent population.

Both sub-clusters are coeval and mostly younger than 3 Myr.

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 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.