Issue A&A Volume 423, Number 3, September I 2004 Page(s) 1029 - 1044 Section Stellar structure and evolution DOI http://dx.doi.org/10.1051/0004-6361:20040202

A&A 423, 1029-1044 (2004)
DOI: 10.1051/0004-6361:20040202

XMM-Newton probes the stellar population in Chamaeleon I South

B. Stelzer¹, G. Micela¹ and R. Neuhäuser<sup>2

1</sup>  INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
    e-mail: stelzer@astropa.unipa.it
²  Astrophysikalisches Institut und Universitäts-Sternwarte, Schillergässchen 2-3, 07745 Jena, Germany

(Received 5 February 2004 / Accepted 31 March 2004 )

Abstract
We report on a 30 ks XMM-Newton observation of the central region of the Cha I star forming cloud. The field includes a substantial fraction of the known pre-main-sequence population of Cha I South, including all thirteen known very-low mass H emitters. We detect two bona-fide brown dwarfs (spectral types M 7.5 and M 8) and seven H emitting objects near the hydrogen burning mass limit, including six of seven earlier detections by ROSAT. Three objects classified as Cha I candidate members according to their NIR photometry are revealed by XMM-Newton, providing further evidence for them being truly young stars. A total of 11 new X-ray sources without known optical/IR counterpart may comprise further as yet unrecognized faint cloud members. Spectral analysis of the X-ray bright stars shows that previous X-ray studies in Cha I have underestimated the X-ray luminosities, as a result of simplified assumptions on the spectral shape. In particular, the extinction is variable over the field, such that the choice of a uniform value for the column density is inappropriate. We establish that the X-ray saturation level for the late-type stars in Cha I is located near , with a possible decline to for the lowest mass stars. A group of strongly absorbed stars with unusually hard X-ray emission is clustered around HD 97048, a HAeBe star and the only confirmed intermediate-mass star in the field. While the X-ray properties of HD 97048 are indistinguishable from those of its lower-mass neighbors, another presumably A-type star (identified as such based on NIR photometry) stands out as the softest X-ray emitter in the whole sample. This suggests that various X-ray emission mechanisms may be at work in intermediate-mass pre-main-sequence stars. We find that X-ray luminosity follows a tight correlation with age, effective temperature, and mass. No dramatic changes in these correlations are seen at the substellar boundary, suggesting that the same dynamo mechanism operates in both low-mass stars and brown dwarfs, at least at young ages. The variability of the lowest-mass objects is also similar to that of higher-mass T Tauri stars. X-ray flares are seen in about 1/10th of the Cha I members in the field.

Key words: X-rays: stars -- stars: pre-main-sequence -- stars: low-mass, brown dwarfs -- stars: coronae -- stars: activity

SIMBAD Objects

© ESO 2004

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