Astronomy & Astrophysics

Free access article

A&A 468, 425-442 (2007)
DOI: 10.1051/0004-6361:20066565

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

A. Telleschi¹, M. Güdel¹, K. R. Briggs¹, M. Audard², and F. Palla³

¹ Paul Scherrer Institut, Würenlingen and Villigen, 5232 Villigen PSI, Switzerland
e-mail: atellesc@astro.phys.ethz.ch
² Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, Mail code 5247, New York, NY 10027, USA
³ INAF-Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi, 5, 50125 Firenze, Italy

(Received 13 October 2006 / Accepted 29 November 2006)

Abstract
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 $(L_{\rm X})$ vs. mass (M) relation, $L_{\rm X}\propto M^{1.69~\pm\,\,0.11}$ , but this relation is a consequence of X-ray saturation and a mass vs. bolometric luminosity ( $L_{\rm *}$ ) relation for the TTS with an average age of 2.4 Myr. X-ray saturation indicates $L_{\rm X}$ = const. $L_{\rm *}$ , although the constant is different for the two subsamples: const. = $10^{-3.73~\pm\,\,0.05}$ for CTTS and const. = $10^{-3.39~\pm\,\,0.06}$ for WTTS. Given a similar $L_{\rm *}$ distribution of both samples, the X-ray luminosity function also reflects a real X-ray deficiency in CTTS, by a factor of $\approx$ 2 compared to WTTS. The average electron temperatures $T_{\rm av}$ are correlated with $L_{\rm X}$ 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 $L_{\rm X}$ 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 $L_{\rm X}$ vs. $T_{\rm av}$ 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.

Key words: stars: coronae -- stars: formation -- stars: pre-main sequence -- X-rays: stars

What is OpenURL?