EDP Sciences
Free access
Volume 394, Number 3, November II 2002
Page(s) 911 - 926
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20021146

A&A 394, 911-926 (2002)
DOI: 10.1051/0004-6361:20021146

Coronal density diagnostics with Helium-like triplets: CHANDRA-LETGS observations of Algol, Capella, Procyon, $\epsilon$ Eri, $\alpha$ Cen A&B, UX Ari, AD Leo, YY Gem, and HR 1099

J.-U. Ness1, J. H. M. M. Schmitt1, V. Burwitz2, R. Mewe3, A. J. J. Raassen3, 4, R. L. J. van der Meer3,

P. Predehl2 and A. C. Brinkman3

1  Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
2  Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1603, 85740 Garching, Germany
3  Space Research Organization Netherlands (SRON), Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
4  Astronomical Institute "Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

(Received 24 April 2002 / Accepted 1 August 2002)

We present an analysis of ten cool stars (Algol, Capella, Procyon, $\epsilon$ Eri, $\alpha$ Cen A&B, UX Ari, AD Leo, YY Gem, and HR 1099) observed with the Low Energy Transmission Grating Spectrometer (LETGS) on board the Chandra X-ray Observatory. This sample contains all cools stars observed with the LETGS presently available to us with integration times sufficiently long to warrant a meaningful spectral analysis. Our sample comprises inactive, moderately active, and hyperactive stars and samples the bulk part of activity levels encountered in coronal X-ray sources. We use the LETGS spectra to carry out density and temperature diagnostics with an emphasis on the H-like and the He-like ions. We find a correlation between line flux ratios of the Ly $_\alpha$ and He-like resonance lines with the mean X-ray surface flux. We determine densities using the He-like triplets. For all stars we find no significant deviations from the low-density limit for the ions of Ne, Mg, and Si, while the measured line ratios for the ions of C, N, and O do show evidence for departures from the low-density limit in the active stars, but not in the inactive stars. Best measurements can be made for the O VII triplet where we find significant deviations from the low-density limit for the stars Algol, Procyon, YY Gem, $\epsilon$ Eri, and HR 1099. We discuss the influence of radiation fields on the interpretation of the He-like triplet line ratios in the low- Z ions, which is relevant for Algol, and the influence of dielectronic satellite lines, which is relevant for Procyon. For the active stars YY Gem, $\epsilon$ Eri, and HR 1099 the low f/i ratios can unambiguously be attributed to high densties in the range 1- $3\times10^{10}$ cm -3 at O VII temperatures. We find our LETGS spectra to be an extremely useful tool for plasma diagnostics of stellar coronae.

Key words: atomic data -- techniques: spectroscopic -- stars: coronae -- stars: late-type -- stars: activity -- X-rays: stars

Offprint request: J.-U. Ness, jness@hs.uni-hamburg.de

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