X-ray emission from T Tauri stars and the role of accretion:: inferences from the XMM-Newton extended survey of the Taurus molecular cloud

Context. T Tau stars display different X-ray properties depending on whether they are accreting (classical T Tau stars; CTTS) or not (weak-line T Tau stars; WTTS). X-ray properties may provide insight into the accretion process between disk and stellar surface. Aims. We use data from the XMM-Newton Extended Survey of the Taurus molecular cloud (XEST) to study differences in X-ray properties between CTTS and WTTS. Methods. XEST data are used to perform correlation and regression analysis between X-ray parameters and stellar properties. Results. We confirm the existence of a X-ray luminosity (L-X) vs. mass (M) relation, L-X proportional to M1.69 +/- 0.11, but this relation is a consequence of X-ray saturation and a mass vs. bolometric luminosity (L-*) relation for the TTS with an average age of 2.4 Myr. X-ray saturation indicates L-X = const. L-*, although the constant is different for the two subsamples: const. = 10(-3.73 +/- 0.05) for CTTS and const. = 10(-3.39 +/- 0.06) for WTTS. Given a similar L-* distribution of both samples, the X-ray luminosity function also reflects a real X-ray deficiency in CTTS, by a factor of approximate to 2 compared to WTTS. The average electron temperatures T-av are correlated with L-X in WTTS but not in CTTS; CTTS sources are on average hotter than WTTS sources. At best marginal dependencies are found between X-ray properties and mass accretion rates or age. Conclusions. The most fundamental properties are the two saturation laws, indicating suppressed L-X for CTTS. We speculate that some of the accreting material in CTTS is cooling active regions to temperatures that may not significantly emit in the X-ray band, and if they do, high-resolution spectroscopy may be required to identify lines formed in such plasma, while CCD cameras do not detect these components. The similarity of the L-X vs. T-av dependencies in WTTS and main-sequence stars as well as their similar X-ray saturation laws suggests similar physical processes for the hot plasma, i.e., heating and radiation of a magnetic corona.