The η Chamaeleontis Cluster: A Remarkable New Nearby Young Open Cluster

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Abstract
A cluster of young, X-ray-emitting stars is found in the vicinity of η Chamaeleontis from a deep ROSAT high-resolution imager observation. The 12 X-ray sources have prominent (R=5-14) stellar counterparts, including two early-type stars (η Cha and RS Cha) and 10 Li-rich, Hα emission-line, late-type stars (K3-M5). Hipparcos astrometry reveals that η Cha, RS Cha, and nearby HD 75505 are comoving at d97 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 η Cha cluster is far from any significant molecular cloud and thus has mysterious origins. The cluster appears to share proper motions with other young stars in the Chamaeleon region and may be a compact extension of the Sco-Cen OB association.