2004A&A...423.1029S

We report on a 30ks XMM-Newton observation of the central region of the ChaI star forming cloud. The field includes a substantial fraction of the known pre-main-sequence population of ChaI South, including all thirteen known very-low mass Hα emitters. We detect two bona-fide brown dwarfs (spectral types M 7.5 and M 8) and seven Hα emitting objects near the hydrogen burning mass limit, including six of seven earlier detections by ROSAT. Three objects classified as ChaI 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 ChaI 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 ChaI is located near L_x/L_bol∼10^–2.5, with a possible decline to L_x/L_bol∼10^–3 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 ChaI members in the field.