Dielectronic Recombination in Photoionized Gas. II. Laboratory Measurements for Fe xviii and Fe xix

Abstract

In photoionized gases with cosmic abundances, dielectronic recombination (DR) proceeds primarily via nlj → nl ′ j ′ core excitations (∆n = 0 DR). We have measured the resonance strengths and energies for Fe XVIII to Fe XVII and Fe XIX to Fe XVIII ∆n = 0 DR. Using our measurements, we have calculated the Fe XVIII and Fe XIX ∆n = 0 DR rate coefficients. Significant discrepancies exist between our inferred rates and those of published calculations. These calculations overestimate the DR rates by factors of ∼ 2 or underestimate it by factors of ∼ 2 to orders of magnitude, but none are in good agreement with our results. Almost all published DR rates for modeling cosmic plasmas are computed using the same theoretical techniques as the above-mentioned calculations. Hence, our measurements call into question all theoretical ∆n = 0 DR rates used for ionization balance calculations of cosmic plasmas. At temperatures where the Fe XVIII and Fe XIX fractional abundances are predicted to peak in photoionized gases of cosmic abundances, the theoretical rates underestimate the Fe -2 -XVIII DR rate by a factor of ∼ 2 and overestimate the Fe XIX DR rate by a factor of ∼ 1.6. We have carried out new multiconfiguration Dirac-Fock and multiconfiguration Breit-Pauli calculations which agree with our measured resonance strengths and rate coefficients to within typically better than ∼ < 30%. We provide a fit to our inferred rate coefficients for use in plasma modeling. Using our DR measurements, we infer a factor of ∼ 2 error in the Fe XX through Fe XXIV ∆n = 0 DR rates. We investigate the effects of this estimated error for the well-known thermal instability of photoionized gas. We find that errors in these rates cannot remove the instability, but they do dramatically affect the range in parameter space over which it forms.

Atomic data for neutron-capture elements

“…Unfortunately, the energies of these states have not been determined for the vast majority of atomic ions beyond the second row of the Periodic Table. Inaccuracies in the calculated energies of these nearthreshold autoionizing resonances are thus the chief source of uncertainty in computed low-temperature DR rate coefficients (see, e.g., Savin et al 1999;Ferland 2003). While Robicheaux et al (2010) showed that thermal broadening of the continuum threshold in finite density plasmas alleviates to some extent the problem of identifying which resonances lie just above the threshold (and can contribute to DR) and which do not, this effect is not likely to be sufficiently large to mitigate the comparatively large uncertainties in resonance positions resulting from theoretical calculations.…”

“…Perhaps the most reliable method of obtaining highly accurate DR rate coefficients is electron-ion beam merging techniques with heavy-ion storage rings such as the TSR (Savin et al 1999(Savin et al , 2006Schippers et al 2010). Unfortunately, it is extremely challenging to use storage ring experiments to measure DR rate coefficients of low charge-to-mass species such as the Se ions investigated in this paper.…”

Dielectronic recombination data for dynamic finite-density plasmas

“…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.…”