Astronomy Astrophysics manuscript no. aa040202 April 8, 2004

We report on a 30 ksec 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 M7.5 and M8) 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 truely 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 , with a possible decline to Lx=Lbol 10 for the lowest mass stars. A group of strongly absorbed stars with unusual 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 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 ..