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Issue A&A
Volume 368, Number 3, March IV 2001
Page(s) 888 - 900
Section Formation, structure and evolution of stars
DOI 10.1051/0004-6361:20010026



A&A 368, 888-900 (2001)
DOI: 10.1051/0004-6361:20010026

CHANDRA-LETGS X-ray observations of Capella

Temperature, density and abundance diagnostics
R. Mewe1, A. J. J. Raassen1, 2, J. J. Drake3, J. S. Kaastra1, R. L. J. van der Meer1 and D. Porquet4

1  Space Research Organization Netherlands (SRON), Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
2  Astronomical Institute "Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3  Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA
4  CEA/DSM/DAPNIA, Service d'Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France

(Received 12 October 2000 / Accepted 15 December 2000)

Abstract
We report an analysis of the X-ray spectrum of Capella from 6 to 175 Åobtained with the Low Energy Transmission Grating Spectrometer (LETGS) on board of the X-ray space observatory CHANDRA. Many emission line features appear that can be resolved much better as compared to former instruments (EUVE and ASCA). Coronal electron densities ($n_{\rm e}$) and temperatures (T) of brightly emitting regions are constrained by an analysis of ratios of density- and temperature-sensitive lines of helium-like ions and highly ionized iron atoms. Lines emitted by e.g., O VII & VIII, Mg X-XII, Si XII-XIV, Fe IX, X & XV-XXIII are used to derive T. Line ratios in the helium-like triplets of C V, N VI, O VII, Mg XI, and Si XIII yield T in the range 0.5-10 MK, and $n_{\rm e}$ in the range 109-1013 cm-3. The Fe IX/X ratio yields $T \simeq$ 0.9 MK, while lines from Fe XVIII to XXII give $T \sim$ 6-10 MK. Flux ratios of Fe XX-XXII lines indicate for the electron densities an upper limit in the range $n_{\rm e} \la$ (2-5) 1012 cm-3. From line ratios of Fe XVII and Fe XVIII we derive constraints on the optical depth $\tau$ of ~1-1.5. An emission measure distribution is derived from Fe line intensities. Results for element abundances (relative to Fe) from a 4-T model are: O and Ne/Fe about solar, N, Mg and Si/Fe ~2$\times$ solar. The results for T and $n_{\rm e}$ are described in terms of quasi-static coronal loop models and it is shown that the X-ray emission originates from compact structures much smaller than the stellar radii.


Key words: stars: individual: Capella -- stars: coronae -- stars: late-type -- stars: activity -- X-rays: stars -- atomic processes vspace-1mm

Offprint request: R. Mewe, R.Mewe@sron.nl

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