EDP Sciences
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Volume 367, Number 1, February III 2001
Page(s) 282 - 296
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20000419

A&A 367, 282-296 (2001)
DOI: 10.1051/0004-6361:20000419

Helium-like triplet density diagnostics

Applications to CHANDRA-LETGS X-ray observations of Capella and Procyon
J.-U. Ness1, R. Mewe2, J. H. M. M. Schmitt1, A. J. J. Raassen2, 3, D. Porquet4, J. S. Kaastra2, R. L. J. van der Meer2, V. Burwitz5 and P. Predehl5

1  Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
2  Space Research Organization Netherlands (SRON), Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
3  Astronomical Institute "Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
4  CEA/DSM/DAPNIA, Service d'Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
5  Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1603, 85740 Garching, Germany

(Received 18 September 2000 / Accepted 1 December 2000)

Electron density diagnostics based on the triplets of helium-like C V, N VI, and O VII are applied to the X-ray spectra of Capella and Procyon measured with the Low Energy Transmission Grating Spectrometer (LETGS) on board the Chandra X-ray Observatory. New theoretical models for the calculation of the line ratios between the forbidden (f), intercombination (i), and the resonance (r) lines of the helium-like triplets are used. The (logarithmic) electron densities (in cgs units) derived from the f/i ratios for Capella are $< 9.38 {\rm cm^{-3}}$ for O VII (2$\sigma$ upper limit) ( $f/i=4.0\pm 0.25$), $9.86\pm0.12 {\rm cm^{-3}}$ for N VI ( $f/i=1.78\pm 0.25$), and $9.42\pm 0.21 {\rm cm^{-3}}$ for C V ( $f/i=1.48\pm 0.34$), while for Procyon we obtain $9.28^{+0.4}_{-9.28} {\rm cm^{-3}}$ for O VII ( $f/i=3.28\pm 0.3$), $9.96\pm 0.23 {\rm cm^{-3}}$ for N VI ( $f/i=1.33\pm 0.28$), and < 8.92 ${\rm cm^{-3}}$ for C V ( $f/i=0.48\pm 0.12$). These densities are quite typical of densities found in the solar active regions, and also pressures and temperatures in Procyon's and Capella's corona at a level of $T\sim 10^6$ K are quite similar. We find no evidence for densities as high as measured in solar flares. Comparison of our Capella and Procyon measurements with the Sun shows little difference in the physical properties of the layers producing the C V, N VI, and O VII emission. Assuming the X-ray emitting plasma to be confined in magnetic loops, we obtain typical loop length scales of ${L_{\rm Capella}} \ge 8 {L_{\rm Procyon}}$ from the loop scaling laws, implying that the magnetic structures in Procyon and Capella are quite different. The total mean surface fluxes emitted in the helium- and hydrogen-like ions are quite similar for Capella and Procyon, but exceed typical solar values by one order of magnitude. We thus conclude that Procyon's and Capella's coronal filling factors are larger than corresponding solar values.

Key words: atomic data -- atomic processes -- techniques: spectroscopic -- stars: individual: Capella & Procyon -- stars: coronae -- stars: late-type -- stars: activity -- X-rays: stars

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

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