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In the calculation of effective collision strengths for the Na-like ion
Fe XVI we have examined in detail several effects that must be
taken into account in order to ensure a reasonable level of accuracy.
In particular the contribution of the high partial waves for both allowed
and forbidden transitions has been found difficult to manage, as the
top-up procedures can only be implemented at high values of the collisional J
symmetry. In the present work we have adopted the values of
, which are adequate for most
transitions although some of the allowed ones within the
complex still presented inacccurate high-energy tails that required truncation.
We conclude from detailed comparisons with previous work that, in spite of
the accuracy that is easily reached in target representation, the level
of accuracy of the resulting effective collision strengths is probably not
better than 10% for
and 20% for the smaller ones.
This is due to effects that come into play
at high degrees of ionization. The present dataset is arguably
the most reliable to date since we have made an attempt to identify the
sources of error, and then take care of them so as to maintain accuracy.
We have thus gained sufficient experience to tackle some of the more difficult
iron ions that are being considered as part of the IRON Project.
Acknowledgements
Part of the present work was carried out during visits
by MEG and CM to the Observatoire de Paris, Meudon, France.
The hospitality received is gratefully acknowledged. The visits were
funded by the CNRS, IVIC, CONICIT, Fundación Polar, the Observatoire de Paris
and the Ministère des Affaires Étrangères. Computations were carried out
at the Ohio Supercomputer Center, Columbus, Ohio, U.S.A., and at CeCalCULA,
Universidad de Los Andes, Mérida, Venezuela. This research has been
supported by CONICIT under contract No. S1-95000521.
The authors are grateful to Dr. H.E. Saraph for a critical reading of the
manuscript.
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