EDP Sciences
Free access
Volume 498, Number 3, May II 2009
Page(s) 909 - 914
Section Atomic, molecular, and nuclear data
DOI http://dx.doi.org/10.1051/0004-6361/200911799
Published online 27 March 2009
A&A 498, 909-914 (2009)
DOI: 10.1051/0004-6361/200911799

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

I. Orban1, Z. Altun2, A. Källberg3, A. Simonsson3, G. Andler3, A. Paál3, M. Blom3, P. Löfgren3, S. Trotsenko4, 5, S. Böhm1, and R. Schuch1

1  Department of Atomic Physics, Stockholm University, 10691 Stockholm, Sweden
    e-mail: istvan.orban@physto.se
2  Department of Physics, Marmara University, 81040 Istanbul, Turkey
3  Manne Siegbahn Laboratory, Stockholm University, 11814 Stockholm, Sweden
4  Gesellschaft für Schwerionenforschung, 64291 Darmstadt, Germany
5  Institut für Kernphysik, Universität Frankfurt, 60486 Frankfurt, Germany

Received 5 February 2009 / Accepted 22 February 2009

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 103-104 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.

Key words: atomic data -- plasmas -- Sun: corona -- Sun: atoms

© ESO 2009