Astronomy and Astrophysics – Astrophysics
Scientific paper
2002-01-31
Astronomy and Astrophysics
Astrophysics
16 pages, 8 figures (3 in color), submitted to Journal of Physics B
Scientific paper
10.1088/0953-4075/35/16/301
The primary X-ray diagnostic lines in He-like ions are mainly excited by electron impact from the ground level to the n = 2 levels, but at high temperatures n > 2 levels are also excited. In order to describe the atomic processes more completely collision strengths are computed for OVII including for the first time all of the following: (i) relativistic fine structure, (ii) levels up to the n = 4, and (iii) radiation damping of autoionizing resonances. The calculations are carried out using the Breit-Pauli R-matrix (BPRM) method with a 31-level eigenfunction expansion. Resonance structures in collision strengths are delineated in detail up to the n = 4 thresholds. For highly charged He-like ions radiation damping of autoionizing resonances is known to be significant. We investigate this effect in detail and find that while resonances are discernibly damped radiatively as the series limit n --> infty is approached from below, the overall effect on effective cross sections and rate coefficients is found to be very small. Collision strengths for the principal lines important in X-ray plasma diagnostics, w,x,y and z, corresponding to the 4 transitions to the ground level 1s^2 (^1S_0) <-- 1s2p (^1P^o_1), 1s2p (^3P^o_2), 1s2p (^3P^o_1), 1s2s (^3S_1), are explicitly shown. It is found that the effective collision strength of the forbidden z-line is up to a factor of 4 higher at T < 10^6 K than previous values. This is likely to be of considerable importance in the diagnostics of photoionized astrophysical plasmas. Significant differences are also found with previous works for several other transitions. This work is carried out as part of the Iron Project-RmaX Network.
Delahaye Franck
Pradhan Anil Kumar
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