The η Chamaeleontis cluster : recent results

The η Chamaeleontis star cluster is one of the most important nearby groupings of pre-main sequence (PMS) stars. With accurately known distance, compact structure and highly-coeval intermediate-aged population, the η Cha cluster is an important laboratory for enhancing our understanding of all PMS star evolutionary issues. In this paper, recent results concerning the stellar population are discussed including the discovery of a rare example of a ∼ 10 Myr-old Classical T Tauri (CTT) star. This and other objects point to an X-ray-faint late-type population residing amongst the ROSAT-detected membership. 1. Characterization of a new star cluster The η Chamaeleontis star cluster is a recently-discovered group of ∼ 10 Myr-old PMS stars at a distance of only 97 pc (Mamajek, Lawson & Feigelson 1999a). The cluster was discovered following a ROSAT HRI pointing at a compact group of 4 ROSAT All-Sky Survey (RASS) sources. The 4 RASS sources were recovered along with 8 additional sources. Spectroscopy of the optical counterparts of these sources (denoted as RECX stars by Mamajek et al. 1999a) showed 10 of these stars had the optical characteristics of late-type Weak-lined T Tauri (WTT) stars, as evidenced by optical activity (Hα emission with equivalent widths EW = 1− 20 Å) and enhanced λ6708 Li I absorption (EW = 0.4 − 0.6 Å). The optical counterparts of the other two X-ray sources were the bright B8 star η Cha and the A7 + A8 binary RS Cha. The PMS nature of the RECX stars was confirmed via Hipparcos and Tycho astrometry. Hipparcos parallaxes and proper motions for η Cha and RS Cha gave a weighted parallax for the two stars of π = 10.3 ± 0.3 mas (distance d = 97 ± 3 pc) and proper motions (μα cos δ, μδ = −30, +28 mas yr ). Half of the late-type RECX stars (RECX 1, 9, 10, 11 and 12) and the X-ray-quiet A5 star HD 75505 can be related to η Cha and RS Cha via common parallax and/or proper motions (Mamajek et al. 1999a, Mamajek, Lawson & Feigelson 2000, Lawson et al. 2001a). Preliminary placement of the RECX stars on the HR diagram suggested a common age of ∼ 10 Myr (Mamajek et al. 1999a). Published work to date on the η Cha cluster includes announcement of its discovery and analysis of spectroscopy of the RECX stars (Mamajek et al. 1999a), a search for microwave radio emission from the RECX stars (Mamajek, Lawson & Feigelson 1999b), consideration of the kinematic origin of the cluster and detailed analysis of the ROSAT X-ray light curves (Mamajek et al. 2000;