Dielectronic Recombination in Photoionized Gas: The Importance of Fine-structure Core Excitations

Abstract

At the low electron temperatures existing in photoionized gases with cosmic abundances, dielectronic recombination (DR) proceeds primarily via excitations of core electrons ( DR). At these temperatures, nl r nl Dn ϭ 0 j j

Dielectronic recombination data for dynamic finite-density plasmas

“…Before carrying out systematic calculations of DR rate coefficients for a whole sequence, we therefore first checked the chosen approximations by comparing to the available experimental data. Figure 1 presents the comparison of our MCBP DR rate coefficients with results of measurements using the heavy-ion TSR in Heidelberg (Savin et al 1997(Savin et al , 1999 for Fe 17+ to Fe 16+ recombination. The cooler electrons employed in this technique have an anisotropic Maxwellian distribution with low perpendicular and parallel temperatures of k B T ⊥ ≈ 15 meV and k B T ≈ 0.15 meV, and allow resolution of separated resonances associated with 2 → 2 core excitation.…”

“…Precise experimental measurements with high resolution have also been performed using TSR for fluorine-like Fe 17+ by Savin et al (1997Savin et al ( , 1999 for DR via N = 2 → N = 2 core excitation, also using the TSR. Significant discrepancies were found between experimentally derived rate coefficients and previously published theoretical results.…”

“…Hence, the measurements call into question all previous theoretical 2 → 2 DR rate coefficients used for ionization balance calculations of photoionized cosmic plasmas. In the work of Savin et al (1997Savin et al ( , 1999, new theoretical calculations were also carried out using MCBP and multiconfiguration Dirac-Fock (MCDF) methods. These new theoretical results yielded good agreement with experiment, mainly due to inclusion of 2p 3/2 -2p 1/2 core fine-structure excitations.…”

“…The published high-temperature DR rate coefficients for fluorine-like ions vary by up to a factor of 4.7. Recent experimental measurements (Savin et al 1997(Savin et al , 1999(Savin et al , 2002a(Savin et al ,b, 2003Schippers et al 2001Schippers et al , 2004 have also shown that many of the earlier computations of low temperature DR rate Present Address: Physics Department, Drake University, Des Moines, IA 50311, USA e-mail: oleg_zoi@yahoo.com coefficients are inaccurate. Taken altogether, it is clear that systematic computations with improved approximations are needed.…”

Ionization and Recombination with Electrons: Laboratory Measurements and Observational Consequences

“…We have carried out measurements of DR via ∆N = 0 core excitations for Al-like Fe 13+ (in preparation), Mg-like Fe 14+ (in preparation), Na like Fe 15+ [20,21], F-like Fe 17+ [16,22], O-like Fe 18+ [16,23], N-like Fe 19+ [24], C-like Fe 20+ [25], B-like Fe 21+ [25], and Be-like Fe 22+ (in preparation). We have also carried out ∆N = 1 measurements for Na-like Fe 15+ [20,21], O-like Fe 18+ [16,23], and Be-like Fe 22+ (in preparation).…”