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[29.15] Transition Rates for Fe IV

R. Rubin (NASA/Ames), C. Froese Fischer (Vanderbilt)

We extend our earlier Breit-Pauli calculations, using improved codes, to the calculations of the transition rates for [Fe~IV] Froese Fischer & Rubin, 1998, J.Phys.B., 31, 1657). The computations are carried out for the 16 terms in the 3s²3p⁶3d⁵ configuration, which comprise the 37 lowest-lying energy levels (below 1 Ryd). Preliminary results are presented for both E2 and M1 forbidden transitions in the various multiplets. The final results will be published in Froese Fischer & Rubin, in preparation.

These atomic data are important for interpreting astronomical observations over a wide spectral range from the ultraviolet to the infrared. For instance, in their study of the bipolar planetary nebula Mz~3, Zhang & Liu (2002, MNRAS, 337, 499) found evidence for high electron density (N_e) in the Fe⁺⁺ region of log~N_e(cm^-3)~= 6.5. They also suggested that N_e in the central emitting core could be even higher. They did observe five optical [Fe IV] lines, all of which arise from levels covered here. An interpretation of those data would benefit from improved A-values, particularly when dealing with higher density gas where the statistical equilibrium for the energy level populations depends critically on the transition rates.

Recently Rodr\'iguez (2003, ApJ, in press) found for five H~II regions that the Fe⁺³ abundance derived from observations of [Fe IV] lines was systematically lower than expected. This is in the same direction as found earlier by Rubin et al. (1997, ApJ, 474, L131) for the Orion Nebula. While the A-values used are unlikely to be the cause of the problem, improved values will help narrow down where to seek the root cause.

RR acknowledges support from the NASA Long-Term Space Astrophysics program.

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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.