High-temperature behaviour of the helium-like K ALPHA G ratio: the effect of improved recombination rate coefficients for calcium, iron, and nickel

TL;DR

This paper demonstrates that using recent, accurate recombination rate coefficients significantly alters the predicted G ratios of helium-like ions at high temperatures, impacting astrophysical and laboratory plasma diagnostics.

Contribution

It introduces improved recombination rate coefficients for calcium, iron, and nickel, showing their substantial effect on high-temperature G ratio predictions compared to previous approximate methods.

Findings

Recombination rates from recent calculations differ markedly from earlier methods.

New rates lead to G ratios up to six times lower at high temperatures.

Excellent agreement between different modern calculation methods supports their accuracy.

Abstract

It is shown that above the temperature of maximum abundance, recombination rates into the excited states of He-like ions that are calculated using earlier, more approximate methods differ markedly from rates obtained from recent distorted-wave and R-Matrix calculations (unified recombination rate coefficients) for Ca, Fe, and Ni. The present rates lead to G ratios that are greatly lower than those resulting from the more approximate rates in previous works, by up to a factor of six at high electron temperatures. Excellent agreement between the distorted-wave and the R-Matrix rates, as well as excellent agreement in the G ratios calculated from them, provides support for the accuracy of these new values which have a broad applicability to the modelling and interpreting of X-ray spectra from a variety of astrophysical and laboratory sources.