7 Effective collision strengths

Thermal averaging of the collision strengths is done using the linear interpolation method described by Burgess & Tully (1992). The resulting effective collision strengths $\sl \Upsilon$ are given in Table 9 for the temperature range $6.3 \leq {\rm log}\,T \leq 8.1$ which is centred on the temperature where Fe⁺²² is abundant under conditions of coronal ionization equilibrium (see Arnaud & Rothenflug 1985). For temperatures below five million degrees the abundance of Fe⁺²² will be negligeable. Astrophysical situations may exist where Fe⁺²² is abundant at temperatures lower than this; in these cases one would need to extend the temperature range below $10^{6.3}\,$K. Anyone interested in obtaining copies of our energy dependent collision strengths for the transitions dealt with here should send a request to tully@obs-nice.fr. We plan to install them eventually in the TIPbase databank that is being set up at the CDS (Centre de données astronomiques de Strasbourg). Files containing the thermally averaged collision strengths are available now from the A&A databank at CDS.

Acknowledgements

This work was supported by the PPARC grant GR/H93576, by the EC network contract ERB CHRX CT920013 and by the NSERC (Natural Sciences and Engineering Research Council of Canada). The figures were drawn using the graphics package TVB developed by Georges Gonczi at the Observatoire de la Côte d'Azur. An English version of this is now available at http://www.obs-nice.fr/tvb/tvb.html. We wish to thank Alan Burgess of Cambridge University for providing us with the latest version of his interactive program OmeUps. We are grateful to Guo-xin Chen, D.G. Hummer and Hannelore E. Saraph for reading and commenting on an earlier version of the paper. Some convenient merging of tables and the inclusion of a discussion on the quality of results are the important modifications which we incorporated after studying the helpful report supplied by the referee, Professor Harry Nussbaumer of Zurich.