eROSITA X-ray scan of the η Chamaeleontis cluster. Member study and search for dispersed low-mass stars
Context. The nearby young open cluster η Chamaeleontis has been observed by eROSITA/SRG during its CalPV phase for 150 ks. The extended ROentgen Survey with an Imaging Telescope Array (eROSITA) data were taken in the field-scan mode, an observing mode of Spectrum-Roentgen-Gamma (SRG) that follows a rectangular grid-like pattern, here covering a 5 × 5 deg field with an exposure depth of about 5 ks.
Aims: The η Cha cluster with an age of about 8 Myr is a key target for investigating the evolution of young stars, and we aim to study the known members in X-rays. Additionally, we search for potential new members of the anticipated dispersed low-mass cluster population in a sensitive wide-field X-ray observation.
Methods: Using eROSITA X-ray data, we studied the η Cha region. Detected sources were identified by cross-matching X-ray sources with Gala and 2MASS, and young stars were identified by their X-ray activity, the position in the color-magnitude diagram, and by their astrometric and kinematic properties. X-ray-luminosities, light curves, and spectra of cluster members were obtained and compared with previous X-ray data. Literature results of other member searches were used to verify our new member candidates in the observed field.
Results: We determine X-ray properties of virtually all known η Cha members and identify five additional stellar systems that show basically identical characteristics, but they are more dispersed. Four of them were previously proposed as potential members; this status is supported by our X-ray study. Based on their spatial distribution, further members are expected beyond the sky region we surveyed. The identified stellar systems very likely belong to the ejected halo population, which brings the total number of η Cha cluster members to at least 23.
Conclusions: Sensitive X-ray surveys are best suited to identifying active stars, and the combination of the ongoing eROSITA all-sky survey with Gala measurements provides an unprecedented opportunity to study the nearby, young stellar population.