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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. Predehl51 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)
Abstract
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
for O VII (2
upper limit)
(
),
for N VI (
),
and
for C V (
), while for Procyon we obtain
for O VII (
),
for N VI (
),
and < 8.92
for C V (
).
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
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
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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