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DIELECTRONIC RECOMBINATION DATA for ASTROPHYSICAL APPLICATIONS: PLASMA RATE-COEFFICIENTS for Feq+ (Q = 7–10, 13–22) and Ni25+ IONS from STORAGE-RING EXPERIMENTS

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DIELECTRONIC RECOMBINATION DATA for ASTROPHYSICAL APPLICATIONS: PLASMA RATE-COEFFICIENTS for Feq+ (Q = 7–10, 13–22) and Ni25+ IONS from STORAGE-RING EXPERIMENTS INTERNATIONAL REVIEW OF ATOMIC AND MOLECULAR PHYSICS (IRAMP) Volume 1, No. 2, July-December 2010, pp. 109-121, © International Science Press, ISSN: 2229-3159 REVIEW ARTICLE DIELECTRONIC RECOMBINATION DATA FOR ASTROPHYSICAL APPLICATIONS: PLASMA RATE-COEFFICIENTS FOR Feq+ (q = 7–10, 13–22) AND Ni25+ IONS FROM STORAGE-RING EXPERIMENTS .. S. SCHIPPERS1, M. LESTINSKY2,3, A. MULLER1, D.W. SAVIN3, E.W. SCHMIDT1 AND A. WOLF2 1Institut für Atom- und Molekülphysik, Justus-Liebig-Universität Giessen, Leihgesterner Weg 217, 35392 Giessen, Germany. 2Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany. 3Columbia Astrophysics Laboratory, Columbia University, 550 West 120th, New York, NY 10027, USA. Abstract: This review summarizes the present status of an ongoing experimental effort to provide reliable rate coefficients for dielectronic recombination of highly charged iron ions for the modeling of astrophysical and other plasmas. The experimental work has been carried out over more than a decade at the heavy-ion storage-ring TSR of the Max-Planck- Institute for Nuclear Physics in Heidelberg, Germany. The experimental and data reduction procedures are outlined. The role of previously disregarded processes such as fine-structure core excitations and trielectronic recombination is highlighted. Plasma rate coefficients for dielectronic recombination of Feq+ ions (q = 7–10, 13–22) and Ni25+ are presented graphically and in a simple parameterized form allowing for easy use in plasma modeling codes. It is concluded that storage-ring experiments are presently the only source for reliable low-temperature dielectronic recombination rate-coefficients of complex ions. PACS Numbers: 34.80.Lx, 52.20.Fs. 1. INTRODUCTION for Nuclear Physics in Heidelberg, Germany. Status reports Dielectronic recombination (DR) is an important electron- on these activities have been presented repeatedly [5–14] ion collision process governing the charge balance in atomic and a comprehensive bibliography of storage-ring DR plasmas [1, 2]. Accurate DR rate coefficients are therefore measurements with astrophysically relevant ions has been required—as well as many other atomic data—for the published recently [15]. interpretation of observations of such plasmas be they man- In particular, we have concentrated on iron ions made or astrophysical. Because of the vast atomic data because of their prominent role in X-ray astronomy. Iron needs most of the data that are presently used in plasma is the most abundant heavy element [16] and still modeling codes have been generated by theoretical contributes to line emission from astrophysical plasmas calculations. In order to assess the reliability of these when lighter elements are already fully stripped. Line calculations and to point out directions for their improvements emission from iron ions is prominent in many spectra benchmarking experiments are vitally needed [3, 4]. taken with X-ray observatories such as XMM-Newton For more than a decade, our collaboration has and Chandra [17]. performed measurements of absolute DR rate coefficients In DR an initially free electron excites another employing the electron-ion merged-beams method at the electron, which is initially bound on the primary ion, and heavy-ion storage ring TSR of the Max-Planck-Institute thereby looses enough energy such that it becomes bound, S. Schippers, A. Müller, E.W. Schmidt, M. Lestinsky, A. Wolf and D.W. Savin too. The DR process is completed if in a second step the theoretical treatment that was geared towards CP and intermediate doubly excited state decays radiatively to a more or less disregarded low-energy DR in order to keep state below the ionization threshold of the recombined the calculations tractable. Modern computers allow more ion. The initially bound core electron may be excited from sophisticated approaches, and recent theoretical work has a state with principal quantum number N to a state with aimed at providing a more reliable recombination data- principal quantum number N¢ . There are an infinite number base by using state-of-the-art atomic codes [2]. Badnell of excitation channels. In practice, however, only the and coworkers [22] have calculated DR rate coefficients smallest excitation steps with N¢ = N (D N = 0 DR), N¢ = for finite-density plasmas. Results have been published N + 1 (D N = 1 DR) and sometimes also N¢ = N + 2 (D N = for the isoelectronic sequences from H-like to Mg-like 2 DR) contribute significantly to the total DR rate [23, and references therein]. Rate coefficients for non- coefficient. In the measurements reported here only D N resonant radiative recombination (RR) are also available = 0 DR and D N = 1 DR have been considered. [24, 25]. Independently, Gu calculated DR and RR rate Energy conservation dictates that the DR resonance coefficients for selected ions of astrophysical interest [26–28]. Although the new theoretical work removes the energies Eres are given by Eres = Ed – Ei with Ed and Ei being the total electron energies of the doubly excited striking low-temperature disagreement that was found resonance state and the initial state, respectively. Both between experimental and early theoretical results, significant theoretical uncertainties remain as discussed Ed and Ei can amount to several 100 keV. In contrast, Eres can be less than 1 eV. Calculating such a small difference above. Experimental benchmarks are thus indispensable of two large numbers with sufficient accuracy is a for arriving at a reliable DR data base for the astrophysical considerable challenge even for state-of-the-art atomic modeling, particularly of low-temperature plasmas. structure codes [2]. Unfortunately, small uncertainties The present review is organized as follows. The in low-energy DR resonance positions can translate into experimental procedure for obtaining a DR rate huge uncertainties of the calculated DR rate coefficient coefficient from a storage-ring experiment is outlined in in a plasma [18]. Therefore, experimental DR measure- Section 2. In Section 3 the various steps for deriving a ments are particulary valuable for ions with strong resonances plasma rate coefficient from the measured data are briefly at energies of up to a few eV which decisively determine discussed. In Section 4 illustrative examples are given the low-temperature DR rate coefficients important for near for selected iron ions and, finally, fit parameters for a neutrals in an electron ionized plasma or for complex ions in convenient representation of our experimentally derived photoionized plasmas. Almost all iron ions more complex DR plasma rate coefficients are listed for Ni25+ and Feq+ than helium-like belong to this latter class. with (q = 7– 10, 13– 22). Because of their dependence on subtle details of the particular atomic structure of each Cosmic atomic plasmas can be divided into collisionally ion species reliable DR rate coefficients cannot be ionized plasmas (CP) and photoionized plasmas (PP) [1] obtained by interpolations along isonuclear or isoelec- both covering broad temperature ranges. Historically, most tronic sequences of ions, i.e. a separate measurement has theoretical recombination data were calculated for CP been carried out for each individual ion. Lithiumlike Ni25+ (see e.g. [19]) where highly charged ions exist only at has been included in this compilation since its DR rather large temperatures, e.g. in the solar corona. At resonance structure is relatively simple and therefore these temperatures, recombination rate coefficients are serves as a pedagogical example for the presentation of the largely insensitive to low-energy DR resonances. experimental technique and the subsequent data analysis. Consequently, the theoretical uncertainties are much Throughout this paper we refer exclusively smaller at higher than at lower plasma temperatures to the charge states of the primary ions before recombination. which are typical for PP. If the CP rate coefficients are also used for the astrophysical modeling of PP, 2. EXPERIMENTAL PROCEDURE inconsistencies arise. This has been noted, e.g., in the Heavy-ion storage rings equipped with electron coolers astrophysical modeling of X-ray spectra from active serve as an excellent experimental environment for galactic nuclei by Netzer [20] and Kraemer et al. [21]. electron-ion collision studies [29, 30]. In electron-ion It is clear, that the large discrepancies at low merged-beams experiments at heavy-ion storage-rings a temperatures between the experimental and the early fast-moving ion beam is collinearly merged with a theoretical rate coefficients are due to a simplified magnetically guided electron beam with an overlap length 110 International Review of Atomic and Molecular Physics, 1 (2), July-December 2010 Dielectronic Recombination Data for Astrophysical Applications: Plasma Rate-coefficients for Fe q+ (q = 7–10, 13–22)... L of the order of 1-2 m. Recombined ions are separated with the electron and ion masses me and mi and with from the primary beam in the first bending magnet downbeam of the interaction region and directed onto a G g g - g -2 g - 2 q = i e( i1)( e 1) cos ... (3) single particle detector. Since the reaction products are moving fast and are confined in a narrow cone they can g = - b 2 g =1/ 1 - b 2 easily be detected with an efficiency h of nearly 100%. where the Lorentz factors are i1/ 1 i , e e and q = 0° for a merged-beams arrangement with Storage rings measure the electron-ion (RR + DR) copropagating beams. In a storage-ring experiment b is recombination cross section times the relative velocity i kept fixed and E is varied by changing b via the cathode convolved with the energy spread of the experiment, called cm e voltage at the electron gun. a merged beams recombination rate coefficient (MBRRC). This differs from a plasma recombination rate Further details of the various aspects of the experi- coefficient (PRRC) for a Maxwellian temperature mental and data reduction procedures at the Heidelberg distribution. From the measured count rate R, the stored heavy-ion storage ring TSR have been discussed in depth in Refs.
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