Letter to the Editor

Anomalous X-ray line ratios in the cTTS TW Hydrae

¹ Department of Physics, Rudolf Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, UK e-mail: ness@thphys.ox.ac.uk
² Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany

Received: 31 August 2005
Accepted: 26 October 2005

Abstract

The cTTS TW Hya has been observed with high-resolution X-ray spectrometers. Previously found high densities inferred from He-like  triplets strongly suggested the detected X-ray emission to be dominated by an accretion shock. Because of their radiation field dependence He-like  ratios do not provide unambiguous density diagnostics. Here we present additional evidence for high densities from ratios of Fe xvii lines. Key Fe xvii line ratios in TW Hya deviate from theoretical expectations at low densities as well as from the same measurements in a large sample of stellar coronae. However, a quantitative assessment of densities is difficult because of atomic physics uncertainties. In addition, estimates of low optical depth in line ratios sensitive to resonance scattering effects also support a high-density emission scenario in the X-ray emitting regions of cTTS.