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A&A 376, 1113-1122 (2001)
DOI: 10.1051/0004-6361:20010959

Line ratios for helium-like ions: Applications to collision-dominated plasmas

D. Porquet^{1, 2}, R. Mewe³, J. Dubau^{4, 5}, A. J. J. Raassen^{3, 6} and J. S. Kaastra<sup>3

1</sup>  CEA/DSM/DAPNIA, Service d'Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
²  DAEC, Observatoire de Paris, Section Meudon, 92195 Meudon Cedex, France
³  Space Research Organization Netherlands (SRON), Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁴  LSAI, U.M.R. 8624, CNRS, Université de Paris Sud, 91405 Orsay Cedex, France
⁵  DARC, Observatoire de Paris, Section Meudon, 92195 Meudon Cedex, France
⁶  Astronomical Institute "Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

(Received 4 May 2001 / Accepted 21 June 2001 )

Abstract
The line ratios R and G of the three main lines of He-like ions (triplet: resonance, intercombination, forbidden lines) are calculated for CV, NVI, OVII, NeIX, MgXI, and SiXIII. These ratios can be used to derive electron density and temperature of hot late-type stellar coronae and O, B stars from high-resolution spectra obtained with Chandra (LETGS, HETGS) and XMM-Newton (RGS). All excitation and radiative processes between the levels and the effect of upper-level cascades from collisional electronic excitation and from dielectronic and radiative recombination have been considered. When possible the best experimental values for radiative transition probabilities are used. For the higher-Z ions (i.e. NeIX, MgXI, SiXIII) possible contributions from blended dielectronic satellite lines to each line of the triplets were included in the calculations of the line ratios R and G for four specific spectral resolutions: RGS, LETGS, HETGS-MEG, HETGS-HEG. The influence of an external stellar radiation field on the coupling of the 2³S (upper level of the forbidden line) and 2³P levels (upper levels of the intercombination lines) is taken into account. This process is mainly important for the lower-Z ions (i.e. CV, NVI, OVII) at moderate radiation temperature ( ). These improved calculations were done for plasmas in collisional ionization equilibrium, but will be later extended to photo-ionized plasmas and to transient ionization plasmas. The values for R and G are given in extensive tables, for a large range of parameters, which could be used directly to compare to the observations.

Key words: X-rays: stars -- atomic processes -- stars: coronae -- stars: late-type -- stars: activity -- early-type stars: activity

Offprint request: D. Porquet, dporquet@cea.fr

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