Experimental Mg IX photorecombination rate coefficient

Abstract

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 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.

Dielectronic recombination data for dynamic finite-density plasmas

“…Overall, we estimate the accuracy of our predicted DR rate coefficients to be ≈±20% for temperatures above a few eV. We note, however, that storage ring results for Fe ions with different charge have demonstrated that all state-of-the-art theoretical methods (e.g., MCBP, MCDF, HULLAC, R-matrix, and FAC) are not able to reproduce reliably the DR resonance strengths and energies of L-shell ions for relative collision energies < ∼ 1 eV due to the limitations of the accuracies of the low-lying resonances theoretical energies (e.g., Savin et al 1999;Schippers et al 2001;Savin et al 2002b;Schippers et al 2004) due to the limitation of the accuracies of the low-lying resonance theoretical energies. The theoretical DR rate coefficients below ≈1 eV should be used with extra caution, and for reliable total DR rate coefficients at plasma temperatures where these resonances play a significant role, these MCBP results need to be supplemented with storage ring measurements of the relevant resonances.…”

“…The published high-temperature DR rate coefficients for fluorine-like ions vary by up to a factor of 4.7. Recent experimental measurements (Savin et al 1997(Savin et al , 1999(Savin et al , 2002a(Savin et al ,b, 2003Schippers et al 2001Schippers et al , 2004 have also shown that many of the earlier computations of low temperature DR rate Present Address: Physics Department, Drake University, Des Moines, IA 50311, USA e-mail: oleg_zoi@yahoo.com coefficients are inaccurate. Taken altogether, it is clear that systematic computations with improved approximations are needed.…”

RECOMBINATION RATE COEFFICIENTS OF BORON-LIKE Ne

“…In this energy range, the discrepancies between measured and calculated results are even larger, which lead to a profound impact on the low-temperature plasma rate coefficients. Even a small variation in low-energy DR resonance positions may induce an uncertainty of a factor 2-3 in the plasma rate coefficients (Schippers et al 2004). The recombination rate coefficients obtained from measurements at storage rings are accurate enough and can be used to benchmark different theoretical descriptions.…”

Experimental rate coefficients of F5+recombining into F4+

“…At these very low energies, the DR plasma rate coefficients are very sensitive to the energy position of the resonances. A few meV uncertainty in the position of DR resonances changes the low-temperature plasma DR rate coefficients by an order of magnitude (Schippers et al 2004). A critical evaluation of the uncertainties in the existing calculated high-temperature DR rate coefficients has been performed by Savin & Laming (2002).…”