Experimental Mg IX photorecombination rate coefficient

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 reso- nances due to 2s → 2p (∆N = 0) and 2s → 3l (∆N = 1) core excitations were detected. At low energies below 0.15 eV the recombination rate coefficient is dominated by strong 1s 2 (2s 2p 3 P) 7l resonances with the strongest one occuring at an energy of only 21 meV. These resonances decisively influence the Mg  recombination rate coefficient in a low temperature plasma. The experimentally derived Mg  dielectronic recombination rate coefficient (±15% 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 Mg  is expected to peak in a photoionized plasma. At this temperature a theoretical uncertainty in the 1s 2 (2s 2p 3 P) 7l 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 Mg  recombination rate coefficient from first principles is challenging.