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ROSAT X-Ray Study of the Chamaeleon I Dark Cloud. I. The Stellar Population
Feigelson, Eric D.; Casanova, Sophie; Montmerle, Thierry; Guibert, Jean
Astrophysical Journal v.416, p.623 (ApJ Homepage)
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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 Lx/L* = 1.6 × 10-4, within a factor of ±2(1 sigma) and the radius relation by Fx ∝ 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 Halpha 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-mainsequence population (M* > 0.1 Msun) 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%.

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