Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements. VII. Relativistic calculations for O VI and O VII for UV and X-ray modeling

Aimed at ionization balance and spectral analysis of UV and X-ray sources, we present self-consistent sets of photoionization cross sections, recombination cross sections, and rate coefficients for Li-like O VI and He-like O VII. Relativistic fine structure is considered through the Breit-Pauli R-matrix (BPRM) method in the close coupling approximation, implementing the unified treatment for total electron-ion recombination subsuming both radiative and di-electronic recombination processes. Self-consistency is ensured by using an identical wavefunction expansion for the inverse processes of photoionization and photo-recombination. Radiation damping of resonances, important for H-like and He-like core ions, is included. Compared to previous LS coupling results without radiative decay of low-n (<= 10) resonances, the presents results show significant reduction in O VI recombination rates at high temperatures. In addition to the total rates, level-specific photoionization cross sections and recombination rates are presented for all fine structure levels n (lSLJ) up to n <= 10, to enable accurate computation of recombination-cascade matrices and spectral formation of prominent UV and X-ray lines such as the 1032,1038 A doublet of O VI, and the `triplet' forbidden, intercombination, and resonance X-ray lines of O VII at 22.1, 21.8, and 21.6 \ang respectively. Altogether, atomic parameters for 98 levels of O VI and 116 fine structure levels of O VII are theoretically computed. These data should provide a reasonably complete set of photoionization and recombination rates in collisional or radiative equilibrium.Comment: 33 pages, 8 figures, submitted to ApJ

Photoionization and recombination of Fe XIX

Photoionization cross sections and recombination rate coefficients are presented for the L-shell ground state fine structure levels $2s^22p^4 \ ^3P_{2,0,1}$of Fe~XIX. Several sets of calculations including relativistic effects are carried out: (i) Breit-Pauli R-matrix (BPRM), (ii) Relativistic Distorted Wave (RDW), and (iii) a semi-relativistic calculation. Non-relativistic LS coupling calculations are also done for comparison. The BPRM calculations employ a configuration interaction target representation for Fe~XX consisting of 12 LS terms (23-fine structure levels), as in the recently reported BPRM calculations by Donnelly et al (MNRAS, 307, 595, 1999). The background cross sections in all three sets of present calculations agree with one another, but differ considerably from those of Donnelly et al. Owing to much more extensive resonance structures in the present BPRM calculations, the sum of the corresponding recombination rate coefficients for the$^3P_{2,0,1}$ levels are up to 50% higher than the LS rates at low temperarures but comparable for higher temperatures; in contrast to the results of Donnelly et al who obtained the LS rates to be higher than their BPRM results by about a factor of 2. Reasons for these discrepancies are discussed.Comment: 7 pages, 3 figures, MNRAS, In Pres

Recombination Rate Coefficients for KLL Di-electronic Satellite Lines of Fe XXV and Ni XXVII

The unified method for total electron-ion recombination is extended to study the dielectronic satellite (DES) lines. These lines, formed from radiative decay of autoionizing states, are highly sensitive temperature diagnostics of astrophysical and laboratory plasma sources. The computation of the unified recombination rates is based on the relativistic Breit-Pauli R-matrix method and close coupling approximation. Extending the theoretical formulation developed earlier we present recombination rate coefficients for the 22 satellite lines of KLL complexes of helium-like Fe XXV and Ni XXVII. The isolated resonance approximation, commonly used throughout plasma modeling, treats these resonances essentially as bound features except for dielectronic capture into, and autoionization out of, these levels. A line profile or cross section shape is often assumed. On the other hand, by including the coupling between the autoionizing and continuum channels, the unified method gives the intrinsic spectrum of DES lines which includes not only the energies and strengths, but also the natural line or cross section shapes. A formulation is presented to derive autoionization rates from unified resonance strengths and enable correspondence with the isolated resonance approximation. While the rates compare very well with existing rates for the strong lines to <20%, the differences for weaker DES lines are larger. We also illustrate the application of the present results to the analysis of K ALPHA complexes observed in high-temperature X-ray emission spectra of Fe XXV and Ni XXVII. There are considerable differences with previous results in the total KLL intensity for Fe XXV at temperatures below the temperature of maximum abundance in coronal equilibrium. (Abbreviated Abstract)Comment: 21 pages, 5 figures, to appear in Physica Script

[O II] line ratios

Based on new calculations we reconfirm the low and high density limits on the forbidden fine structure line ratio [O II] I(3729)/I(3726): lim_{N_ e} --> 0} = 1.5 and lim_{N_ e} --> \infty} = 0.35. Employing [O II] collision strengths calculated using the Breit-Pauli R-matrix method we rule out any significant deviation due to relativistic effects from these canonical values. The present results are in substantial agreement with older calculations by Pradhan (1976) and validate the extensive observational analysis of gaseous nebulae by Copetti and Writzel (2002) and Wang et al (2004) that reach the same conclusions. The present theoretical results and the recent observational analyses differ significantly from the calculations by MacLaughlin and Bell (1998) and Keenan et al (1999). The new maxwellian averaged effective collision strengths are presented for the 10 transitions among the first 5 levels to enable computations of [O II] line ratios.Comment: Submitted to MNRAS (Letters), 4 pages, 2 figures, 1 tabl