The X-ray soft excess in classical T Tauri stars

Aims:We study an anomaly in the X-ray flux (or luminosity) ratio between the O vii λλ21.6-22.1 triplet and the O viii Lyα line seen in classical T Tauri stars (CTTS). This ratio is unusually high when compared with ratios for main-sequence and non-accreting T Tauri stars (Telleschi et al. 2007c, A&A, 468, 443). We compare these samples to identify the source of the excess. A sample of recently discovered X-ray stars with a soft component attributed to jet emission is also considered.
Methods: We discuss data obtained from the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) project, complemented by data from the published literature. We also present data from the CTTS RU Lup.
Results: All CTTS in the sample show an anomalous O vii/O viii flux ratio when compared with WTTS or MS stars. The anomaly is due to an excess of cool, O vii emitting material rather than a deficiency of hotter plasma. The excess plasma must therefore have temperatures of ⪉2 MK. This soft excess does not correlate with UV excesses of CTTS, but seems to be related to the stellar X-ray luminosity. The spectra of the jet-driving TTS do not fit into this context.
Conclusions: The soft excess depends both on the presence of accretion streams in CTTS and on magnetic activity. The gas may be shock-heated near the surface, although it may also be heated in the magnetospheric accretion funnels. The soft component of the jet-driving sources is unlikely to be due to the same process.