Experimental dielectronic recombination rate coefficients for Na-like S VI and Na-like Ar VIII

Aims. Absolute recombination rate coefficients for two astrophysically relevant Na-like ions are presented. Methods. Recombination rate coefficients of $\ion{S}{vi}$ and $\ion{Ar}{viii}$ are determined from merged-beam type experiments at the CRYRING electron cooler. Calculated rate coefficients are used to account for recombination into states that are field-ionized and therefore not detected in the experiment. Results. Dielectronic recombination rate coefficients were obtained over an energy range covering $\Delta\,n=0$ core excitations. For Na-like Ar a measurement was also performed over the $\Delta\,n=1$ type of resonances. In the low-energy part of the $\ion{Ar}{viii}$ spectrum, enhancements of more than one order of magnitude are observed as compared to the calculated radiative recombination. The plasma recombination rate coefficients of the two Na-like ions are compared with calculated results from the literature. In the $10^3{-}10^4$ K range, large discrepancies are observed between calculated plasma rate coefficients and our data. At higher temperatures, above 105 K, in the case of both ions our data is 30% higher than two calculated plasma rate coefficients, other data from the literature having even lower values. Conclusions. Discrepancies below 104 K show that at such temperatures even state-of-the-art calculations yield plasma rate coefficients that have large uncertainties. The main reason for these uncertainties are the contributions from low-energy resonances, which are difficult to calculate accurately.