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E.E. Mamajek, W.A. Lawson (UNSW/ADFA), E.D. Feigelson (Penn State)
We present the properties of stars in the unusual \eta Cha open cluster discovered with the ROSAT X-ray observatory. Optical spectra of the ten R=10-14 X-ray late-type stars show strong Li 6707 absorption (EW ~q 0.5 Å) and H\alpha emission (EW = 1-20 Å) consistent with weak-lined T Tauri stars. Location in the H-R diagram assuming all stars have the same 97 pc distance as the brighter members indicates stellar ages ranging from 2 to 18 Myr. Their X-ray levels are high (\log Lx = 28.4-30.6 erg/s, \log (Lx/Lbol = -4.2 to -2.7) and most show strong variability. Several members are probably binaries. Four lines of evidence indicate that 20-30 non-X-ray members of the cluster are also present within 15\arcmin (0.4 pc) of \eta Cha including: clustering of red stars; radial counts of faint stars; initial mass function calculations; and the anomalous X-ray luminosity function.
The existence of a compact cluster of pre-main sequence stars at (l,b) = (292\circ,-22\circ) so close to the Sun is surprising given its distance from the Galactic Plane, Gould Belt and any molecular or dark cloud. The cluster is probably dynamically unbound and evaporating stars. We discuss Hipparchos proper motions for \eta Cha members and other samples of early type stars and dispersed T Tauri stars in the region. The \eta Cha cluster is probably a condensation within a larger kinematic structure of comoving pre-main sequence stars at least 10 pc in extent, which may in turn represent a ~q15 pc extension of the Sco-Cen OB association which is already >100 pc in extent. We discuss the possible origin and evolution of the \eta Cha cluster within this context; e.g., the possible stripping of \eta Cha gas by Sco-Cen winds or supernova explosions.
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