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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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