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

Abstract

A young, nearby compact aggregate of X-ray emitting pre-main sequence stars was recently discovered in the vicinity of η Chamaeleontis (Mamajek, Lawson & Feigelson 1999, ApJ, 516, L77). In this paper, we further investigate this cluster: its membership, its environs and 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 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 ǫ 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.

The interstellar medium surrounding the Scorpius-Centaurus association revisited

“…Thus, neither D 3 nor C t should belong to the Sco-Cen association. Moreover, the small branch D t , which shows no interaction with D 2 , points toward a small X-ray emitting compact cluster of PMSs of about 8 Myr near η Cha at l,b ∼ 292.4 • , −21.7 • , and a distance of about 97 pc (Mamajek et al 2000). The cluster was related to the passage of the bubble of the Sco-Cen association.…”

“…P&G's fit for the GB gas is consistent with these mean distances in the range l ∼ 315 • −360 • , withd ∼ 150 pc. In The open triangles indicate the six sight lines sampled by Sembach et al (2003), and the open square indicates the small X-ray emitting compact aggregate of PMSs discovered by Mamajek et al (2000) near to η Cha (see Sect. 6.2).…”

DeepXMM-Newtonobservation of theη Chamaleontis cluster

“…We failed any attempt of fitting a plasma model. ( * * ) X-ray luminosities determined by Mamajek et al (2000) in the 0.1−2.0 ROSAT energy band.…”

“…They later investigated the main properties of the sources (Mamajek et al 2000) and established a connection between the η Chamaleontis cluster and the Chamaleontis stellar association (Frink et al 1998). Both are part of a number of nearly coeval stellar groups that include the TW Hya association and probably originated in the molecular complex ScorpiusCentaurus (see Mamajek et al 2000). Its proximity and low stellar density ensures that the η Chamaleontis cluster is an unique scenario in the solar vicinity.…”

Coronal X-Ray Emission from an Intermediate-Age Brown Dwarf

“…TWA 5B is t Ӎ 12 a faint red object ( mag) located 2Љ north of and co-H p 12 moving with TWA 5A, which itself is a spectroscopic binary (Lowrance et al 1999;Webb et al 1999;Neuhäuser et al 2000). This system is part of the dispersed TW Hydra Association (TWA) of T Tauri stars, all of which are likely outliers of the giant Sco-Cen OB association (Kastner et al 1997;Mamajek, Lawson, & Feigelson 2000). TWA 5 is the first identified pre-main-sequence (PMS) star-BD binary system with an estimated age of Myr (Weintraub et al 2000).…”