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A&A 444, L41-L44 (2005)
DOI: 10.1051/0004-6361:200500208
Letter
Anomalous X-ray line ratios in the cTTS TW Hydrae
J.-U. Ness1 and J. H. M. M. Schmitt21 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 f/i
triplets strongly suggested the detected X-ray emission to be dominated by an accretion
shock. Because of their radiation field dependence He-like f/i 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.
Key words: X-rays: stars -- stars: individual: TW Hya -- stars: pre-main sequence -- stars: coronae -- stars: activity -- accretion
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