The η Chamaeleontis Cluster: Origin in the Sco‐Cen OB Association

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Abstract
A young, nearby compact aggregate of X-ray-emitting pre-main-sequence stars was recently discovered in the vicinity of η Chamaeleontis. In this paper, we further investigate this cluster: its membership, its environs, and its origins. ROSAT High Resolution Imager X-ray data for the cluster's T Tauri stars show high levels of magnetic activity and variability. The cluster has an anomalous X-ray luminosity function compared to other young clusters, deficient in stars with low but detectable X-ray luminosities. This suggests that many low-mass members have escaped the surveyed core region. Photographic photometry from the USNO-A2.0 catalog indicates that additional, X-ray-quiet members exist in the cluster core region. The components of the eclipsing binary RS Cha, previously modeled in the literature as post-main sequence (MS) with discordant ages, are shown to be consistent with being coeval pre-MS stars. We compute the Galactic motion of the cluster from Hipparcos data and compare it to other young stars and associations in the fourth Galactic quadrant. The kinematic study shows that the η Cha cluster, as well as members of the TW Hya association and a new group near Cha, probably originated near the giant molecular cloud complex that formed the two oldest subgroups of the Sco-Cen OB association roughly 10-15 Myr ago. Their dispersal is consistent with the velocity dispersions seen in giant molecular clouds. A large H I filament and dust lane located near η Cha has been identified as part of a superbubble formed by Sco-Cen OB winds and supernova remnants. The passage of the superbubble may have terminated star formation in the η Cha cluster and dispersed its natal molecular gas.
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