AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 35. Star, Cluster, Galaxy Formation and Evolution
Oral, Monday, May 31, 1999, 2:00-3:30pm, Marquette

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[35.01] The \eta Cha cluster: A New Pre-Main Sequence Open Cluster at d = 97 pc

E.D. Feigelson (Penn State), E.E. Mamajek, W.A. Lawson (UNSW/ADFA)

We report the discovery of an unusual compact cluster of very young stars in the solar neighborhood. Motivated by ROSAT All-Sky Survey findings of young stars in the Chamaeleon region, we obtained a deep ROSAT HRI pointing around \eta Cha (B8). Twelve X-ray sources were found with prominent (R=5-14) stellar counterparts including two early-type stars and 10 Li-rich H\alpha emission-line late-type stars (K3-M5). Hipparchos distances to the brighter stars are d ~q 97 pc. The late-type stars are all the properties of pre-main sequence (PMS) 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 inferred ages 2-18 Myr (8 ± 5 Myr).

We believe this is is the 4th closest open cluster known and the nearest discovered this century; the first open cluster primarily discovered by X-ray astronomy; and (after the TW Hya association) the second closest closest group of PMS stars and second PMS group isolated from any molecular cloud. It will be an important laboratory for understanding several astrophysical issues: the origin of dispersed PMS stars; the history of recent star formation in the solar neighborhood; the loss of molecular material and consequence dynamical evolution of open clusters; the magnetic activity, circumstellar disks, binarity and other properties of older PMS stars; and the properties of a coeval population of PMS brown dwarfs likely to be present in the cluster.

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