2000A&A...359..113G

We have obtained two deep exposures of the ρ Oph cloud core region with the ROSAT High Resolution Imager. The improved position accuracy (1"-6") with respect to previous recent X-ray observations (ROSAT PSPC, and ASCA) allows us to remove positional ambiguities for the detected sources. We also cross-correlate the X-ray positions with IR sources found in the ISOCAM survey of the same region at 6.7 and 14.3µm, in addition to sources (optical and IR) known from ground-based observations, which are young stars (T Tauri stars, with and without circumstellar disks, and protostars). We thus obtain the best-studied sample of X-ray emitting stars in a star-forming region (63 X-ray sources detected, and 55 identified). We find that there is no statistically significant difference between the X-ray luminosity functions of HRI-detected Class II and Class III sources, i.e., T Tauri stars with and without disks, confirming that the contribution of these disks to X-ray emission (for instance by magnetic reconnection between the star and the disk), or to X-ray absorption, must be small. X-ray variability of T Tauri stars can be studied by comparing the HRI data with the previously obtained PSPC data, but also using the fact that some HRI observations were done at different epochs. The resulting statistics show that most of the sources are variable, and that their variability is consistent with a solar-like (hence magnetic) flare origin. We use the information given both by the ISOCAM survey and by our HRI deep exposure to study the T Tauri star population of the ρ Oph dense cores. We confirm that essentially all Class II and Class III sources (embedded T Tauri stars) are X-ray emitters, and that a strong correlation exists between their X-ray luminosity, L_X, and their stellar luminosity, L_*, with L_X/L_*∼10^–4. Most of the new ISOCAM Class II sources are not detected, however, which we explain by the fact that their X-ray luminosities ``predicted'' on the basis of this correlation are too faint to be detected by the HRI. We predict that ∼40 unknown faint or embedded Class III sources remain to be discovered in X-rays in the HRI/ISOCAM overlapping area, down to a limit of L_X∼3x10²⁸erg/s. We show that the bulk of these unknown Class III sources should be made of low- to very low-mass stars (M_*<0.1-0.6M_☉). Prospects for future detections with XMM-Newton and Chandra are discussed.