An improved HR diagram for Chamaeleon I pre-main-sequence stars

A new Hertzsprung-Russell diagram for the Chamaeleon I cloud is constructed following the optical characterization of ROSAT X-ray sources discovered within or near the cloud boundary. Of the 117 confirmed or probable members within 1 degrees of the cloud cores, we place 80 on the HR diagram. The star formation rate implied by current evolutionary models suggests either a recent powerful burst of star formation, or that older cloud members are absent from the sample. The latter explanation is consistent with the expected diffusion of older T Tauri stars away from the cloud. The cloud initial mass function (IMF) reveals no stars 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 IMF agrees with the Miller-Scale model. Altogether, there are strong indications 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 (WTT) stars in Chamaeleon I have nearly identical age, mass, spatial, bolometric luminosity, and X-ray luminosity distributions. This result contradicts the widely held belief that the WTT phase emerges only after stars have descended significant portions of the Hayashi tracks. We find that WTT stars outnumber CTT stars by at least 2:1 from the birthline to ages exceeding 10(7) yr. This implies that circumstellar disc dissipation occurs throughout pre-main-sequence evolution.