Experimental Mg IX photorecombination rate coefficient

¹ Institut für Atom- und Molekülphysik, Justus-Liebig-Universität Giessen, Leihgesterner Weg 217, 35392 Giessen, Germany
² Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany

Corresponding author: S. Schippers, Stefan.E.Schippers@strz.uni-giessen.de

Received: 3 March 2004
Accepted: 2 April 2004

Abstract

The rate coefficient for radiative and dielectronic recombination of beryllium-like magnesium ions was measured with high resolution at the Heidelberg heavy-ion storage ring TSR. In the electron-ion collision energy range 0–207 eV resonances due to  () and () core excitations were detected. At low energies below 0.15 eV the recombination rate coefficient is dominated by strong resonances with the strongest one occuring at an energy of only 21 meV. These resonances decisively influence the  recombination rate coefficient in a low temperature plasma. The experimentally derived  dielectronic recombination rate coefficient (± systematical uncertainty) is compared with the recommendation by Mazzotta et al. (1998, A&AS, 133, 403) and the recent calculations by Gu (2003, ApJ, 590, 1131) and by Colgan et al. (2003, A&A, 412, 597). These results deviate from the experimental rate coefficient by 130%, 82% and 25%, respectively, at the temperature where the fractional abundance of  is expected to peak in a photoionized plasma. At this temperature a theoretical uncertainty in the  resonance positions of only 100 meV would translate into an uncertainty of the plasma rate coefficient of almost a factor 3. This finding emphasizes that an accurate theoretical calculation of the  recombination rate coefficient from first principles is challenging.