XMM-Newtonprobes the stellar population in Chamaeleon I South

Abstract

We report on a 30ksec XMM-Newton observation of the central region of the Cha I star forming cloud. Virtually all known T Tauri star members and most of the very-low mass objects including two bona-fide brown dwarfs (spectral types M7.5 and M8) are detected. We identify several new candidate members in the X-ray image. 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. No dramatic changes in the correlations of X-ray luminosity with age, effective temperature and mass are seen at the substellar boundary, and the lowest-mass objects behave similar to the higher-mass T Tauri stars in terms of variability. Thus the same dynamo mechanism seems to operate in both low-mass stars and brown dwarfs, at least at young ages. On the other hand, the only two A-type stars in the field show very different X-ray properties, suggesting that various emission mechanisms may be at work in intermediate-mass stars.