X-rays from T Tauri: a test case for accreting T Tauri stars
Abstract
Context: The generation of X-rays in accreting T Tauri stars (TTS) is thought to be predominantly due to energy dissipation in magnetic fields, but alternative X-ray generation mechanisms have been proposed, such as heating in accretion shocks near the stellar surface, or in shocks forming in jets.
Aims: We test models and trends discussed in the literature using X-ray data from the classical TTS T Tau.
Methods: High-resolution spectroscopy from the Reflection Grating Spectrometers on XMM-Newton is used to infer electron densities, element abundances and the thermal structure of the X-ray source. We also discuss the ultraviolet light curve obtained by the Optical Monitor, and complementary ground-based photometry. A high-resolution image from Chandra constrains contributions from the two companions of T Tau N.
Results: The X-ray grating spectrum is rich in emission lines, but shows an unusual mixture of features from very hot (≈30 MK) and very cool (1-3 MK) plasma, both emitted by similar amounts of emission measure. The cool plasma confirms the picture of a soft excess in the form of an enhanced O vii/O viii Lyα flux ratio, similar to that previously reported for other accreting TTS. Diagnostics from lines formed by this plasma indicate low electron densities (⪉1010 cm-3). The Ne/Fe abundance ratio is consistent with a trend in pre-main sequence stars in which this ratio depends on spectral type, but not on accretion.
Conclusions: On the basis of line density diagnostics, we conclude that the density of the cool “soft-excess” plasma is orders of magnitude below that predicted for an accretion shock, assuming previously determined accretion rates of (3-6) × 10-8 M⊙ yr-1. We argue that loading of magnetic field lines with infalling material suppresses the heating process in a part of the corona. We thus suggest that the X-ray production of T Tau is influenced by the accretion process although the X-rays may not form in the bulk of the accretion footpoints.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- June 2007
- DOI:
- 10.1051/0004-6361:20066318
- arXiv:
- arXiv:astro-ph/0612589
- Bibcode:
- 2007A&A...468..529G
- Keywords:
-
- stars: coronae;
- stars: formation;
- stars: pre-main sequence;
- X-rays: stars;
- stars: individual: T Tauri;
- Astrophysics
- E-Print:
- 12 pages, 7 figures, A&