M. H. Chen

Diagnostic Utility of the Relative Intensity of 3C to 3D in Fe XVII

The relative intensity R of the resonance and intercombination line in neon-like Fe XVII, located at 15.01 and 15.26 A, respectively, has been measured at the Lawrence Livermore National Laboratory electron beam ion trap EBIT-II as a function of the relative abundance of sodium-like Fe XVI. Our measurements identify several Fe XVI lines and one Fe XV line in this region. We show that an Fe XVI inner shell satellite line coincides with the intercombination line and can significantly reduce the apparent R. We measure R = 1.90 ± 0.11 when the relative abundance of Fe XVI to Fe XVII is ~1. This explains the anomalously low ratios observed in the solar and stellar coronae. The fact that the apparent relative intensity of the resonance and intercombination line in Fe XVII is sensitive to the strength of an Fe XVI inner shell satellite, and therefore, the relative abundance of Fe XVI to Fe XVII, makes the line ratio a diagnostic of temperature.

Dielectronic recombination in photoionized gas. II. Laboratory measurements for Fe xviii and Fe xix

In photoionized gases with cosmic abundances, dielectronic recombination (DR) proceeds primarily via nlj ) nl@j@ core excitations (*n \ 0 DR). We have measured the resonance strengths and energies for Fe XVIII to Fe XVII and Fe XIX to Fe XVIII *n \ 0 DR. Using our measurements, we have calculated the Fe XVIII and Fe XIX *n \ 0 DR rate coefficients. Signi—cant discrepancies exist between our inferred rates and those of published calculations. These calculations overestimate the DR rates by factors of D 2o r underestimate it by factors of D2 to orders of magnitude, but none are in good agreement with our results. Almost all published DR rates for modeling cosmic plasmas are computed using the same theo- retical techniques as the above-mentioned calculations. Hence, our measurements call into question all theoretical *n \ 0 DR rates used for ionization balance calculations of cosmic plasmas. At temperatures where the Fe XVIII and Fe XIX fractional abundances are predicted to peak in photoionized gases of cosmic abundances, the theoretical rates underestimate the Fe XVIII DR rate by a factor of D2 and over- estimate the Fe XIX DR rate by a factor of D1.6. We have carried out new multicon—guration Dirac- Fock and multicon—guration Breit-Pauli calculations which agree with our measured resonance strengths and rate coefficients to within typically better than We provide a —t to our inferred rate coeffi- (30%. cients for use in plasma modeling. Using our DR measurements, we infer a factor of D2 error in the Fe XX through Fe XXIV *n \ 0 DR rates. We investigate the eUects of this estimated error for the well- known thermal instability of photoionized gas. We —nd that errors in these rates cannot remove the instability, but they do dramatically aUect the range in parameter space over which it forms. Subject headings: atomic dataatomic processesgalaxies: activeinstabilitiesX-rays: general

Dielectronic Recombination in Photoionized Gas: The Importance of Fine-structure Core Excitations

At the low electron temperatures existing in photoionized gases with cosmic abundances, dielectronic recom- bination (DR) proceeds primarily via excitations of core electrons ( DR). At these temperatures, 0 nl r nl Dn 5 0 0 jj the dominant DR channel often involves fine-structure core excitations, which are not included in 2 p r 2 p 1/2 3/2 LS-coupling calculations or the Burgess formula. Using the heavy-ion storage ring at the Max-Planck-Institut fur Kernphysik in Heidelberg, Germany, we have verified experimentally for Fexviii that DR proceeding via this channel can be significant in relation to other recombination rates, especially at the low temperatures characteristic of photoionized gases. At temperatures in photoionized gases near where Fe xviii peaks in fractional abundance, our measured Fe xviii to Fe xvii DR rate coefficient is a factor of »2 larger than predicted by existing Dn 5 0 theoretical calculations. We provide a fit to our measured rate coefficient for ionization equilibrium models. We have carried out new fully relativistic calculations using intermediate coupling, which include the channel and agree to within »30% with our measurements. DR via the channel may 2 p r 2 p

High-resolution measurements of the K-alpha spectra of low-ionizationm species of iron: A new spectral signature of nonequilibrium ionization conditions in young supernova remnants

We present the first systematic laboratory measurements of high-resolution K-alpha spectra of intermediate ions of iron, Fe X-XVII. These lines are not produced in collisional equilibrium plasmas because of the relevant charge states cannot exist at the high electron temperatures required for appreciable excitation of the K-alpha transitions. However, they can provide excellent spectral diagnostics for nonequilibrium ionization conditions, such the ionizing plasmas of young supernova remnants. To facilitate the line identifications, we compare our spectra with theoretical atomic calculations performed using multiconfiguration parametric potential and Dirac-Fock atomic codes. Our measurements also allow direct comparison with time-dependent ionization balance calculations for ionizing plasmas, and good agreement is found.

Assessment of the fluorescence and auger database used in plasma modeling

We have investigated the accuracy of the 1s vacancy fluorescence database of Kaastra & Mewe resulting from the initial atomic physics calculations and the subsequent scaling along isoelectronic sequences. In particular, we have focused on the relatively simple Be- and F-like 1s vacancy sequences. We find that the earlier atomic physics calculations for the oscillator strengths and autoionization rates of singly charged B II and Ne II are in sufficient agreement with our present calculations. However, the substantial charge dependence of these quantities along each isoelectronic sequence, the incorrect configuration averaging used for B II, and the neglect of spin-orbit effects (which become important at high Z) all cast doubt on the reliability of the Kaastra & Mewe data for application to plasma modeling.

Measurement of the 3s{sub 1/2}-3p{sub 3/2} resonance line in Na-like U{sup 81+}

The wavelength of the 3s{sub 1/2}-3p{sub 3/2} transition of Na-like U{sup 81+} was determined to be 9.499 85{+-}0.000 15 Aa (1305.12{+-}0.02 eV), using the EBIT-I electron beam ion trap. The measurement is many times more sensitive to the radiative contributions from quantum electrodynamics than earlier measurements for Pt{sup 67+} and Pb{sup 71+}. Our result strongly deviates from various predictions employing scaled hydrogenic quantum electrodynamic corrections and establishes a benchmark for multielectron QED calculations that agrees well with the trend established by ab initio calculations.

Dielectronic Recombination of Fe XXI and Fe XXII via N = 2→N' = 2 Core Excitations: Data

We have measured dielectronic recombination (DR) resonance strengths and energies for carbon-like Fe xxi forming Fe xx and for boron-like Fe xxii forming Fe xxi via N ¼ 2 ! N 0 ¼ 2 core excitations. All measurements were carried out using the heavy-ion Test Storage Ring at the Max-Planck-Institute for Nuclear Physics in Heidelberg, Germany. We have also calculated these resonance strengths and energies using three independent, state-of-the-art perturbative techniques: a multiconfiguration Breit-Pauli (MCBP) method using the code AUTOSTRUCTURE, a multiconfiguration Dirac-Fock (MCDF) method, and a relativistic configuration interaction method using the Flexible Atomic Code (FAC). Overall, reasonable agreement is found between our experimental results and our theoretical calculations. The most notable discrepancies tend to occur for relative collision energies d3 eV. We have used our measured 2 ! 2r esults to produce Maxwellianaveraged rate coefficients for Fe xxi and Fe xxii. Our experimentally derived rate coefficients are estimated to be accurate to better than � 20% both for Fexxi at kBTe > 0: 5e V and for Fexxii at kBTe > 0:001 eV. For these !

Atomic physics measurements in an electron Beam Ion Trap

An electron Beam Ion Trap at Lawrence Livermore National Laboratory is being used to produce and trap very‐highly‐charged ions (q≥70+) for x‐ray spectroscopy measurements. Recent measurements of transition energies and electron excitation cross sections for x‐ray line emission are summarized.

Measurements of low temperature dielectronic recombination in L-shell iron for modeling X-ray photoionized cosmic plasmas

The iron L-shell ions (Fe17+ to Fe23+) play an important role in determin- ing the thermal and ionization structures and line emission from photoionized plasmas. Current uncertainties in the theoretical low temperature dielectronic recombination (DR) rate coefficients for these ions significantly affect our ability to model and inter- pret the line emission from observations of photoionized plasmas. To help resolve this issue, we have initiated a program of laboratory measurements to produce reliable low temperature DR rates for the L-shell iron. Here we present some of our recent results and discuss some of the astrophysical implications.

Hyperfine splitting of the 2s1/2 and 2p1/2 levels in lithium-like Pr56+

Measurements of hyperfine splittings in highly charged ions are sensitive to details of the nuclear structure and the nuclear magnetic field distribution, but the proper interpretation of the measurements requires that the atomic structure is understood in sufficient detail. We discuss the reasoning behind various recent experiments and what advantage is offered by the study of the Li-like ion of a mid-Z element such as praseodymium.