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

A young, nearby compact aggregate of X-ray emitting pre-main sequence starswas recently discovered in the vicinity of eta Cha (B8V). In this paper, wefurther investigate this cluster: its membership, its environs and origins.ROSAT HRI X-ray data for the cluster's T Tauri stars show high levels ofmagnetic activity and variability. The cluster has an anomalous X-rayluminosity function compared to other young clusters, deficient in stars withlow, but detectable X-ray luminosities. This suggests that many low-massmembers have escaped the surveyed core region. Photographic photometry from theUSNO-A2.0 catalog indicates that additional, X-ray-quiet members exist in thecluster core region. The components of the eclipsing binary RS Cha, previouslymodeled in the literature as post-MS with discordant ages, are shown to beconsistent with being coeval pre-MS stars. We compute the Galactic motion ofthe cluster from Hipparcos data, and compare it to other young stars andassociations in the fourth Galactic quadrant. The kinematic study shows thatthe eta Cha cluster, the TW Hya association, and a new group near epsilon Cha,probably originated near the giant molecular cloud complex that formed the twooldest subgroups of the Sco-Cen OB association roughly 10-15 Myr ago. Theirdispersal is consistent with the velocity dispersions seen in giant molecularclouds. A large H I filament and dust lane located near eta Cha has beenidentified as part of a superbubble formed by Sco-Cen OB winds and supernovaremnants. The passage of the superbubble may have terminated star-formation inthe eta Cha cluster and dispersed its natal molecular gas.