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∝ M^{1.69~± 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 ≈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.