This work is an extension of our previous Breit-Pauli R-matrix calculations of effective collision strengths for krypton ions to elements in the fifth and sixth row of the periodic table. We have shown here that unlike the scattering calculations for krypton ions the semi-relativistic TCC approach, essentially an algebraic recoupling technique using term-coupling coefficients, is not applicable to the computation of collisional data for heavy ions such as XeIII, IV, VI and BaII, IV. On the other hand, comparison of our BP results with test calculations using the Dirac R-matrix method reveals that the inclusion of relativistic effects in the electron scattering process based on a full intermediate coupling approach yields electron excitation rates of heavy elements which can be used in the derivation of diagnostics for highly excited nebular spectra. The comparison of our results with those for lighter ions confirms our previous conclusions (cf. Paper I) that atomic physics calculations do not corroborate a systematic enhancement of effective collision strengths for heavy elements by orders of magnitude as was suggested in PB94. Consequently the nebular spectrum of NGC 7027 should be carefully reinvestigated using our data to obtain reliable abundances of the heavy ions. With regard to the anomalous intensities of collisionally excited lines new constraints from overabundances of these species are extremely valuable for understanding the process of heavy element formation in the progenitor of NGC 7027 and the subsequent enrichment of the planetary nebula.
Acknowledgements
We thank Drs. W. Eissner, P.H.
Norrington and F.A. Parpia for making their program
packages available to us. Furthermore TS is
indebted to the Deutsche Forschungsgemeinschaft for
financial support under grant Bu 703/2-1. The calculations were performed on
the Cray Y-MP, T90 computers and the HP cluster of the Bayerische Akademie der
Wissenschaften at the Leibniz
Rechenzentrum, München. We gratefully acknowledge a grant of computer
time by Cray Research Inc. in cooperation with the Leibniz Rechenzentrum.