The X-ray soft excess in classical T Tauri stars

Abstract

We study an anomaly in the X-ray flux (or luminosity) ratio between the OVII 21.6-22.1A triplet and the OVIII Lya 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. 2007). 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. 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. All CTTS in the sample show an anomalous OVII/OVIII flux ratio when compared with WTTS or MS stars. The anomaly is due to an excess of cool, OVII 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 with the stellar X-ray luminosity. The spectra of the jet-driving TTS do not fit into this picture. 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.

5 pages, 5 figures, accepted for Astronomy & Astrophysics Letters

References in corpus (8)

• The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)
• X-ray Emission from T Tauri Stars and the Role of Accretion: Inferences from the XMM-Newton Extended Survey of the Taurus Molecular Cloud
• X-ray emission from MP Muscae: an old classical T Tauri star
• X-rays from T Tau: A test case for accreting T Tauri stars
• X-rays from jet-driving protostars and T Tauri stars
• The first high-resolution X-ray spectrum of a Herbig Star: The case of AB Aurigae
• X-ray accretion signatures in the close CTTS binary V4046 Sgr
• High Resolution X-ray Spectroscopy of T Tauri Stars in the Taurus-Auriga Complex

Cited by in corpus (8)

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• Jets, accretion, coronae and all that: The enigmatic X-rays from the Herbig star HD 163296
• Further X-ray detections of Herbig stars
• Where are the hot ion lines in classical T Tauri stars formed?
• Physical Conditions of Accreting Gas in T Tauri Star Systems
• No Transition Disk? Infrared Excess, PAH, H2, and X-rays from the Weak-Lined T Tauri Star DoAr 21
• X-rays from young stars - a summary of highlights from the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)