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Abstract: We study an anomaly in the X-ray flux or luminosity ratio between the OVII21.6-22.1A triplet and the OVIII Lya line seen in classical T Tauri starsCTTS. This ratio is unusually high when compared with ratios formain-sequence and non-accreting T Tauri stars Telleschi et al. 2007. Wecompare these samples to identify the source of the excess. A sample ofrecently discovered X-ray stars with a soft component attributed to jetemission is also considered. We discuss data obtained from the XMM-NewtonExtended Survey of the Taurus Molecular Cloud XEST project, complemented bydata from the published literature. We also present data from the CTTS RU Lup.All CTTS in the sample show an anomalous OVII-OVIII flux ratio when comparedwith WTTS or MS stars. The anomaly is due to an excess of cool, OVII emittingmaterial rather than a deficiency of hotter plasma. The excess plasma musttherefore have temperatures of <~2 MK. This soft excess does not correlate withUV excesses of CTTS, but seems to be related with the stellar X-ray luminosity.The spectra of the jet-driving TTS do not fit into this picture. The softexcess depends both on the presence of accretion streams in CTTS and onmagnetic activity. The gas may be shock-heated near the surface, although itmay also be heated in the magnetospheric accretion funnels. The soft componentof the jet-driving sources is unlikely to be due to the same process.

Autor: M. Guedel, A. Telleschi


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