Importance of level mixing on accurate [Fe II] transition rates

Abstract

Context. In a very recent measurement Gurell et al. (2009, A&A, 508, 525) commented that while the theoretical lifetime of a 4 G 5.5 is approximately one tenth of the lifetime of b 2 H 5.5 the corresponding measurement shows this to be close to one fifth. This large discrepancy is attributed to the effect of inadequate level mixing in the theoretical calculations. Aims. The aim of this work is to make a detailed analysis of these level mixing effects on transitions from various lower levels to the a 4 G 5.5 and b 2 H 5.5 levels given in three previous calculations and in the present more extensive CI calculation. Methods. The CIV3 structure codes of Hibbert (1975, Comput. Phys. Commun., 9, 141) and Hibbert et al. (1991, Comput. Phys. Commun., 64, 455) are used in the present work, combined with our "fine-tuning" extrapolation process.Results. The calculated mixing between the upper levels, obtained in previous calculations, is shown to be too weak. The stronger mixing determined in our work gives rise to a calculated lifetime for b 2 H 5.5 within 3% of the measured value. On the other hand our calculated lifetime for a 4 G 5.5 is around 20% lower than the measured value, which has fairly wide error bars. Conclusions. Our enhanced calculations explain the difference between previous calculations of the b 2 H 5.5 lifetime and the recent measured value and confirm the latter. We also suggest a somewhat higher value than experiment for the lifetime of a 4 G 5.5 .

Atomic Data Assessment with PyNeb: Radiative and Electron Impact Excitation Rates for [Fe ii] and [Fe iii]

“…This comparison also brings out observed lines with questionable identifications (e.g., λλ3968.27, 4889.71) and unreliable line-intensity ratios-e.g., I(λ7637.52)/I(λ9051.95), I(λ7686.93)/ I(λ8891.93), and I(λ7733.13)/I(λ9033.49)-due to A-values with magnitude log A −3. Theoretical A-value ratios agree to within 20-25% and, with respect to the observed line-intensity ratios, to within the error bars except for some outliers subject to strong level mixing, as pointed out in [40].…”

“…Excluding atom_SRKFB19_52, the A-value ratios in Tables 8 and 9 agree to ∼ 20−25% except for two cases in HH 202S, I(λ7637.52)/I(λ9051.95) and I(λ7686.93)/I(λ8891.93), and one in HH 204, I(λ7733.13)/I(λ9033.49), which are caused by lines with A-values of order 10 −3 subject to large numerical uncertainties. Most theoretical ratios are within the estimated observational error bars except for those involving A-values strongly affected by level mixing effects that are frequent in Fe II [40].…”

“…Extensive multi-configuration Breit-Pauli (MCBP) calculations of g f -values (weighted oscillator strengths) and A-values for allowed and forbidden transitions in Fe II and Fe III [37][38][39][40][41][42][43] with the code CIV3 have shown that fine-tuning corrections to the Hamiltonian matrix by fitting to the experimental level energies lead to more accurate configuration expansions. However, significant A-value differences (greater than 20-30%) are found for some transitions computed with comparable multi-configuration methods that also implement fine-tuning corrections (e.g., [23,44,45]), which are caused by strong configuration mixing, double-excitation representations, and subtle core-valence correlation involving the closed 3p 6 and open 3d n subshells.…”

Radiative transition rates for the forbidden lines in Fe II

“…In calculations of M1 transitions within a multiplet, particularly in the lower-lying ones, most calculations agree, as shown in Deb & Hibbert (2010a). This is expected because the states involved are spectroscopically almost pure and the dipole matrix elements do not involve the radial functions.…”

Successes and Difficulties in Calculating Atomic Oscillator Strengths and Transition Rates

“…For example, in their studies of E1 Fe II transitions, Corrégé and Hibbert (2005) [29], as well as Deb and Hibbert (2014) [30] found it better to choose a 3d function optimised on the energy of the ground 3d 6 4s 6 D state, whereas Deb and Hibbert (2010a,b,2011) [31][32][33], in studying forbidden transitions involving 3d 7 levels, found that much better results could be obtained by optimising the 3d function on the 3d 7 4 F state. This is possible when a calculation is limited to certain types of transition, or even to a very small number of transitions.…”