The eta Chamaeleontis cluster: A remarkable new nearby young open cluster

A cluster of young, X-ray-emitting stars is found in the vicinity of eta Ch amaeleontis from a deep ROSAT high-resolution imager observation. The 12 X- ray sources have prominent (R = 5-14) stellar counterparts, including two e arly-type stars (eta Cha and RS Cha) and 10 Li-rich, H alpha emission-line, late-type stars (K3-M5). Hipparcos astrometry reveals that eta Cha, RS Cha , and nearby HD 75505 are comoving at d similar or equal to 97 pc. The late -type stars have all the properties of pre-main-sequence weak-lined T Tauri stars: high magnetic activity, high Li abundance, and (assuming d = 97 pc) bolometric luminosities 1-2 mag above the main sequence, with ages ranging from 2 to 18 Myr. As with the TW Hya association, the eta Cha cluster is f ar from any significant molecular cloud and thus has mysterious origins. Th e cluster appears to share proper motions with other young stars in the Cha maeleon region and may be a compact extension of the Sco-Cen OB association .