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

SIMBAD Objects

© ESO 2002

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access.

An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account.
    In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.