EDP Sciences
Free access
Volume 418, Number 2, May I 2004
Page(s) 687 - 697
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20040041

A&A 418, 687-697 (2004)
DOI: 10.1051/0004-6361:20040041

X-ray emission from a metal depleted accretion shock onto the classical T Tauri star TW Hya

B. Stelzer1, 2 and J. H. M. M. Schmitt3

1  INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
2  Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
3  Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

(Received 12 February 2003 / Accepted 2 February 2004 )

We present the X-ray spectrum of TW Hya observed at high and intermediate spectral resolution with the Reflection Grating Spectrometer (RGS) and the European Photon Imaging Camera (EPIC) onboard the XMM-Newton satellite. TW Hya is the first classical T Tauri star for which simultaneous X-ray data with both high spectral resolution and high sensitivity were obtained, thus allowing to probe the X-ray emission properties of stars in the early pre-main sequence phase. Despite TW Hya's high X-ray luminosity in excess of  1030 erg/s its X-ray spectrum is dominated by emission lines from rather cool plasma ( $T \approx 3$ MK), and only little emission measure is present at high temperatures ( $T \approx 10$ MK). We determine photon fluxes for the emission lines in the high resolution spectrum, confirming the earlier result from Chandra that the predominant emission is from neon and oxygen, with comparatively weak iron lines. Further, the line ratios of He-like triplets of nitrogen, oxygen and neon require densities of  $n_{\rm e} \sim 10^{13}\,{\rm cm^{-3}}$, about two orders of magnitude higher than for any other star observed so far at high spectral resolution. Finally, we find that nearly all metals are underabundant with respect to solar abundances, while the abundances of nitrogen and neon are enhanced. The high plasma density, the (comparatively) low temperature, and peculiar chemical abundances in the X-ray emitting region on TW Hya are untypical for stellar coronae. An alternative X-ray production mechanism is therefore called for and a natural explanation is an accretion column depleted of grain forming elements. The metal depletion could be either due to the original molecular cloud that formed TW Hya or due to a settling of dust in the circumstellar disk of TW Hya.

Key words: X-rays: stars -- stars: individual: TW Hya -- stars: pre-main sequence -- stars: coronae -- stars: activity -- accretion

Offprint request: B. Stelzer, stelzer@astropa.unipa.it

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