The eta Chamaeleontis cluster: Origin in the Sco-Cen OB association

A young, nearby compact aggregate of X-ray-emitting pre-main-sequence stars was recently discovered in the vicinity of eta Chamaeleontis. In this pape r, we further investigate this cluster: its membership, its environs, and i ts origins. ROSAT High Resolution Imager X-ray data for the cluster's T Tau ri stars show high levels of magnetic activity and variability. The cluster has an anomalous X-ray luminosity function compared to other young cluster s, deficient in stars with low but detectable X-ray luminosities. This sugg ests that many low-mass members have escaped the surveyed core region. Phot ographic 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 c oeval pre-MS stars. We compute the Galactic motion of the cluster from Hipp arcos data and compare it to other young stars and associations in the four th Galactic quadrant. The kinematic study shows that the eta Cha cluster, a s well as members of the TW Hya association and a new group near epsilon Ch a, probably originated near the giant molecular cloud complex that formed t he 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 eta 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 sta r formation in the eta Cha cluster and dispersed its natal molecular gas.