ROSAT X-ray study of the Chamaeleon I dark cloud. I. The stellar population

Two soft X-ray images of the Chamaeleon I star-forming cloud obtained with the ROSAT Position Sensitive Proportional Counter are presented. Seventy reliable, and 19 possible additional, X-ray sources are found. Eighty percent of these sources are certainly or probably identified with T Tauri stars formed in the cloud. Nineteen to 39 are proposed new "weak" T Tauri (WTT) stars which, when confirmed by optical spectroscopy, will significantly enlarge the known cloud population. Individual T Tauri X-ray luminosities range from ∼6 × 10²⁸ to ∼2 × 10³¹ ergs s^-1 (0.4-2.5 keV), or ∼10²-10⁴ times solar levels. The ROSAT images are an order of magnitude more sensitive, with 3-4 times more stellar identifications, than earlier Einstein Observatory images of the cloud. A wide range of issues is addressed by these data. The spatial distribution and Hertzsprung-Russell diagram locations of the stars indicate that WTT stars and "classical" T Tauri (CTT) stars are coeval. Their X-ray luminosity functions are also essentially identical, suggesting that CTT stars have the same surface magnetic activity as WTT stars. The X-ray luminosities of well-studied Chamaeleon I cloud members are strongly correlated with a complex of four stellar properties: bolometric luminosity, mass, radius, and effective temperature. The first relation can be expressed by the simple statement that low-mass Chamaeleon I stars have L_X/L_* = 1.6 × 10^-4, within a factor of ±2 (1 σ) and the radius relation by F_X ∝ R_*. There is thus no evidence of magnetic saturation of the stellar surfaces. We find no evidence for the absorption of soft X-rays in CTT winds and/or boundary layers traced by the strength of the Hα emission. The mean X-ray luminosity for an unbiased optically selected T Tauri sample is 1.6 × 10²⁹ ergs s^-1, and we find evidence for temporal evolution of X-ray emission for stars within the pre-main-sequence evolutionary phase. The total pre-main-sequence population (M_* > 0.1 M_⊙) of the cloud is estimated to be ≳200 stars, with X-ray-detected WTT stars outnumbering X-ray-detected CTT stars by at least 2:1. The inferred star formation efficiency for the cloud cores is ≈20%.