XMM-Newton probes the stellar population in Chamaeleon I South

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 Hα emitters. We detect two bona-fide brown dwarfs (spectral types M 7.5 and M 8) and seven Ha 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 Lx/Lbol ∼ 10-2.5, with a possible decline to Lx/Lbol ∼ 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 Cha I members in the field.