X-ray Emission from T Tauri Stars and the Role of Accretion: Inferences from the XMM-Newton Extended Survey of the Taurus Molecular Cloud

摘要

T Tau stars display different X-ray properties depending on whether they areaccreting (classical T Tau stars; CTTS) or not (weak-line T Tau stars; WTTS).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. Weperform correlation and regression analysis between X-ray parameters andstellar properties. We confirm the existence of a X-ray luminosity (Lx) vs.mass (M) relation, Lx ~ M^(1.69 +/- 0.11), but this relation is a consequenceof X-ray saturation and a mass vs. bolometric luminosity (L*) relation for theTTS with an average age of 2.4 Myr. X-ray saturation indicates Lx = 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 areal X-ray deficiency in CTTS, by a factor of ~ 2 compared to WTTS. The averageelectron temperatures Tav are correlated with Lx in WTTS but not in CTTS; CTTSsources are on average hotter than WTTS sources. The most fundamentalproperties are the two saturation laws, indicating suppressed Lx for CTTS. Wespeculate that some of the accreting material in CTTS is cooling active regionsto temperatures that may not significantly emit in the X-ray band, and if theydo, high-resolution spectroscopy may be required to identify lines formed insuch plasma, while CCD cameras do not detect these components. The similarityof the Lx vs. Tav dependencies in WTTS and main-sequence stars as well as theirsimilar X-ray saturation laws suggests similar physical processes for the hotplasma, i.e., heating and radiation of a magnetic corona.