Highly excited core resonances in photoionization of Fe xvii: Implications for plasma opacities

Abstract

A comprehensive study of high-accuracy photoionization cross sections is carried out using the relativistic Breit-Pauli R-matrix (BPRM) method for (hν + Fe XVII → Fe XVIII + e). Owing to its importance in hightemperature plasmas, the calculations cover a large energy range, particularly the myriad photoexcitation-of-core (PEC) resonances including the n = 3 levels not heretofore considered. The calculations employ a close-coupling wave-function expansion of 60 levels of the core ion Fe XVIII ranging over a wide energy range of nearly 900 eV between the n = 2 and n = 3 levels. Strong-coupling effects due to dipole transition arrays 2p 5 → 2p 4 (3s,3d) manifest themselves as large PEC resonances throughout this range and enhance the effective photoionization cross sections orders of magnitude above the background. Comparisons with the erstwhile Opacity Project (OP) and other previous calculations show that the currently available cross sections considerably underestimate the bound-free cross sections. A level-identification scheme is used for spectroscopic designation of the 454 bound fine structure levels of Fe XVII, with n 10, l 9, and 0 J 8 of even and odd parities, obtained using the ab initio BPRM method (compared to 181 LS bound states in the OP work). The calculated energies are compared with those available from the National Institute for Standards and Technology database, which lists 63 levels with very good agreement. Level-specific photoionization cross sections are computed for all levels. In addition, partial cross sections for leaving the core ion Fe XVII in the ground state are also obtained. These results should be relevant to modeling of astrophysical and laboratory plasma sources requiring (i) photoionization rates, (ii) extensive nonlocal-thermodynamic-equilibrium models, (iii) total unified electron-ion recombination rates including radiative and dielectronic recombination, and (iv) plasma opacities. We particularly examine PEC and non-PEC resonance strengths and emphasize their expanded role to incorporate inner-shell excitations for improved opacities, as shown by the computed monochromatic opacity of Fe XVII.

Seismic models of O and B stars with Kurucz’s opacity data

“…The splitting of spectroscopic LS terms into individual levels may be necessary for final calculations of photoionization cross-sections with resonance structures. This is suggested by Kurucz's treatment of autoionizing transitions (Castelli & Kurucz 2004) and by recent results published by Nahar et al (2011), which include the fine structure explicitly for Fe XVII. Similarly, the new effective cross-sections with photoexcitation-of-core (PEC) resonances for O III and Fe XXI are much higher, up to two orders of magnitude, than in the OP data ( Fig.…”

Detailed Opacity Calculations for Astrophysical Applications

“…The point here is that the latest contribution is included in all opacity codes relying on a distorted-wave modelling of photo-ionization, but was not included in the previous R-matrix computations. Comparison between the present SCO-RCG (distorted-wave) in blue and R-matrix [76] photo-ionization cross-sections (black curve) obtained from [44]. The experimental spectrum is also represented (red curve).…”

“…Recently [75], Nahar et al reported extensive R-matrix calculations [76] of unprecedented complexity for iron ion Fe XVII, with a wavefunction expansion of 99 Fe XVIII LS core states from n ≤ 4 complexes (equivalent to 218 fine-structure levels) and found a large enhancement in background photo-ionization cross-sections (up to orders of magnitude) in addition to strongly peaked photo-excitation-of-core resonances (see the black curve of Figure 17). Figure 18 displays total and photo-ionization opacities of ground 1s 2 2s 2 2p 5 and excited 1s 2 2s 2 2p 5 3p configurations.…”

“…OP [42,43] 0.59 R-matrix [75,76] 0.51 ATOMIC [39,40] 0.60 OPAS [82][83][84] 0.70 SCO-RCG [17] 0.64 SCRAM [85] 0.77 TOPAZ [86,87] 0.62 Cold [72] 0.75…”

IPOPv2: Photoionization of Ni XIV – a test case

“…One of the consequences was to upset seismology findings so far considered reliable (depth of the convection zone, He content at the surface, sound speed profile) and one suggested way out of this difficulty was to modify the stellar opacities. Even though the revisited abundances have increased again since 2004, the debate is going on about the reliability of the opacities used by the astrophysical community and about the relevance of the "new solar abundances" (see for example Pinsonneault and Delahaye 2009;Delahaye et al 2010;Nahar et al 2011;Delahaye and Pinsonneault 2006, DP06 hereafter).…”