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 10 ^ { 30 } 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 _ { e } \sim 10 ^ { 13 } { 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 .