Dielectronic recombination in photoionized gas. II. Laboratory measurements for Fe XVIII and Fe XIX

In photoionized gases with cosmic abundances, dielectronic recombination (D R) proceeds primarily via nlj --> nl'j' core excitations (delta n = 0 DR). We have measured the resonance strengths and energies for Fe XVIII to Fe XV II and Fe XIX to Fe XVIII delta n = 0 DR. Using our measurements, we have c alculated the Fe XVIII and Fe XIX An = 0 DR rate coefficients. Significant discrepancies exist between our inferred rates and those of published calcu lations. These calculations overestimate the DR rates by factors of similar to 2 or underestimate it by factors of similar to 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 t echniques as the above-mentioned calculations. Hence, our measurements call into question all theoretical delta n = 0 DR rates used for ionization bal ance calculations of cosmic plasmas. At temperatures where the Fe XVIII and Fe XIX fractional abundances are predicted to peak in photoionized gases o f cosmic abundances, the theoretical rates underestimate the Fe XVIII DR ra te by a factor of similar to 2 and overestimate the Fe rat DR rate by a fac tor of similar to 1.6. We have carried out new multiconfiguration DiracFock and multiconfiguration Breit-Pauli calculations which agree with our measu red resonance strengths and rate coefficients to within typically better th an less than or similar to 30%. We provide a fit to our inferred rate coeff icients for use in plasma modeling. Using our DR measurements, we infer a f actor of similar to 2 error in the Fe XX through Fe XXIV delta n = 0 DR rat es. We investigate the effects of this estimated error for the well-known t hermal 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.