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Subsections
The effective collision strength
was obtained from the
calculated collision strengths as described in Hummer et al. (1993) and
discussed in IP XI.
![\begin{figure}
\includegraphics[width=8.8cm,clip]{ds1586f5.eps}\end{figure}](/articles/aas/full/1999/02/ds1586/Timg24.gif) |
Figure 5:
Thermally averaged collision strengths for excitation of the ground
state fine-structure line in
P III, S IV and Cl V |
The collisional de-excitation rate coeffficient q(T) is related to
by
|  |
(1) |
where
is the statistical weight of the upper state and T is the
electron temperature in K. Our values for
for the three ions are given in Table 5
and in Fig. 5.
Table 5:
Effective collision strengths for excitation of the 2P
1/2 -
2P
3/2 fine-structure lines in P III, S IV and Cl V
 |
The present results for S IV differ significantly from those of
Johnson et al. (1986), that were also obtained from a
close-coupling calculation. In order to
understand the cause of the discrepancy we simulated their calculation by
cutting off our CC expansion at six target terms. We found that as a result
the resonances were all shifted to higher energies, particularly the broad and
tall feature between .03 and .1 Ryd. Analysis of this feature shows that it
is chiefly made up of resonances with configurations 3s3p2nl.
Johnson et al. had included all parent terms to these resonances
and correlation configurations
in their calculation, but they
omitted the channel coupling effects to higher terms. As a result
the resonance positions were shifted and the overlap between the
Maxwell velocity distribution function at low temperatures and the collision
strength is very much smaller.
![\begin{figure}
\includegraphics[width=8.8cm,clip]{ds1586f6.eps}\end{figure}](/articles/aas/full/1999/02/ds1586/Timg28.gif) |
Figure 6:
Thermally averaged collision strengths for excitation of the ground state
fine-structure line in S IV. Full line: 21 term CC
calculation, dotted line: 6 term CC calculation, triangles: results of
Johnson
et al. (1986) |
Figure 6 shows a comparison of our present
results including 21 target terms with the results obtained when the CC
expansion
is cut short at six terms and with the results of
Johnson
et al. (1986) who
included the same
six terms. Our six term results agree with those of Johnson et al., although
they used different CI and target orbitals. It is clear that the migration of
the resonances to higher energies effected by the omission of channel
coupling effects causes the discrepancy.
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