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

¹ 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

Corresponding author: D. Porquet, dporquet@cea.fr

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 n_e and temperature T_e 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 (T_rad). 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.