AN IMPROVED HR DIAGRAM FOR CHAMELEON-I PRE-MAIN-SEQUENCE STARS

A new Hertzsprung-Russell diagram for the Chamaeleon I cloud is constr ucted following the optical characterization of ROSAT X-ray sources di scovered within or near the cloud boundary. Of the 117 confirmed or pr obable members within 1 degrees of the cloud cores, we place 80 on the HR diagram. The star formation rate implied by current evolutionary m odels suggests either a recent powerful burst of star formation, or th at older cloud members are absent from the sample. The latter explanat ion is consistent with the expected diffusion of older T Tauri stars a way from the cloud. The cloud initial mass function (IMF) reveals no s tars with mass M > 2 M(circle dot) or stars with M < 0.3 M(circle dot) ; the latter is most likely due to population selection effects. The I MF agrees with the Miller-Scale model. Altogether, there are strong in dications that the current census of pre-main-sequence stars produced by the Chamaeleon I cloud is several times greater than the currently defined population. Classical T Tauri (CTT) and weak-lined T Tauri (WT T) stars in Chamaeleon I have nearly identical age, mass, spatial, bol ometric luminosity, and X-ray luminosity distributions. This result co ntradicts the widely held belief that the WTT phase emerges only after stars have descended significant portions of the Hayashi tracks. We f ind that WTT stars outnumber CTT stars by at least 2:1 from the birthl ine to ages exceeding 10(7) yr. This implies that circumstellar disc d issipation occurs throughout pre-main-sequence evolution.