Recombination Rate Coefficients for KLL Di-electronic Satellite Lines of Fe XXV and Ni XXVII

The unified method for total electron-ion recombination is extended to study the dielectronic satellite (DES) lines. These lines, formed from radiative decay of autoionizing states, are highly sensitive temperature diagnostics of astrophysical and laboratory plasma sources. The computation of the unified recombination rates is based on the relativistic Breit-Pauli R-matrix method and close coupling approximation. Extending the theoretical formulation developed earlier we present recombination rate coefficients for the 22 satellite lines of KLL complexes of helium-like Fe XXV and Ni XXVII. The isolated resonance approximation, commonly used throughout plasma modeling, treats these resonances essentially as bound features except for dielectronic capture into, and autoionization out of, these levels. A line profile or cross section shape is often assumed. On the other hand, by including the coupling between the autoionizing and continuum channels, the unified method gives the intrinsic spectrum of DES lines which includes not only the energies and strengths, but also the natural line or cross section shapes. A formulation is presented to derive autoionization rates from unified resonance strengths and enable correspondence with the isolated resonance approximation. While the rates compare very well with existing rates for the strong lines to <20%, the differences for weaker DES lines are larger. We also illustrate the application of the present results to the analysis of K ALPHA complexes observed in high-temperature X-ray emission spectra of Fe XXV and Ni XXVII. There are considerable differences with previous results in the total KLL intensity for Fe XXV at temperatures below the temperature of maximum abundance in coronal equilibrium. (Abbreviated Abstract)Comment: 21 pages, 5 figures, to appear in Physica Script

Radiative transfer with scattering for domain-decomposed 3D MHD simulations of cool stellar atmospheres

We present the implementation of a radiative transfer solver with coherent scattering in the new BIFROST code for radiative magneto-hydrodynamical (MHD) simulations of stellar surface convection. The code is fully parallelized using MPI domain decomposition, which allows for large grid sizes and improved resolution of hydrodynamical structures. We apply the code to simulate the surface granulation in a solar-type star, ignoring magnetic fields, and investigate the importance of coherent scattering for the atmospheric structure. A scattering term is added to the radiative transfer equation, requiring an iterative computation of the radiation field. We use a short-characteristics-based Gauss-Seidel acceleration scheme to compute radiative flux divergences for the energy equation. The effects of coherent scattering are tested by comparing the temperature stratification of three 3D time-dependent hydrodynamical atmosphere models of a solar-type star: without scattering, with continuum scattering only, and with both continuum and line scattering. We show that continuum scattering does not have a significant impact on the photospheric temperature structure for a star like the Sun. Including scattering in line-blanketing, however, leads to a decrease of temperatures by about 350\,K below log tau < -4. The effect is opposite to that of 1D hydrostatic models in radiative equilibrium, where scattering reduces the cooling effect of strong LTE lines in the higher layers of the photosphere. Coherent line scattering also changes the temperature distribution in the high atmosphere, where we observe stronger fluctuations compared to a treatment of lines as true absorbers.Comment: A&A, in pres

Relativistic photoionization cross sections for C II

High resolution measurements of photoionization cross sections for atomic ions are now being made on synchrotron radiation sources. The recent measurements by Kjeldsen etal. (1999) showed good agreement between the observed resonance features and the the theoretical calculations in the close coupling approximation (Nahar 1995). However, there were several observed resonances that were missing in the theoretical predictions. The earlier theoretical calculation was carried out in LS coupling where the relativistic effects were not included. Present work reports photoionization cross sections including the relativistic effects in Breit-Pauli R-matrix (BPRM) approximation. The configuration interaction eigenfunction expansion for the core ion C III consists of 20 fine structure levels dominated by the configurations from 1s^22s^2 to 1s^22s3d. Detailed features in the calculated cross sections exhibit the missing resonances due to fine structure. The results benchmark the accuracy of BPRM photoionization cross sections as needed for recent and ongoing experiments.Comment: 13 pages, 3 figure

Atomic data from the Iron Project.XLIII. Transition probabilities for Fe V

An extensive set of dipole-allowed, intercombination, and forbidden transition probabilities for Fe V is presented. The Breit-Pauli R-matrix (BPRM) method is used to calculate 1.46 x 10^6 oscillator strengths for the allowed and intercombination E1 transitions among 3,865 fine-structure levels dominated by configuration complexes with n <= 10 and l <= 9. These data are complemented by an atomic structure configuration interaction (CI) calculation using the SUPERSTRUCTURE program for 362 relativistic quadrupole (E2) and magnetic dipole (M1) transitions among 65 low-lying levels dominated by the 3d^4 and 3d^ 4s configurations. Procedures have been developed for the identification of the large number of fine-structure levels and transitions obtained through the BPRM calculations. The target ion Fe VI is represented by an eigenfunction expansion of 19 fine-structure levels of 3d^3 and a set of correlation configurations. Fe V bound levels are obtained with angular and spin symmetries SL\pi and J\pi of the (e + Fe VI) system such that 2S+1 = 5,3,1, L <= 10, J <= 8 of even and odd parities. The completeness of the calculated dataset is verified in terms of all possible bound levels belonging to relevant LS terms and transitions in correspondence with the LS terms. The fine-structure averaged relativistic values are compared with previous Opacity Project LS coupling data and other works. The 362 forbidden transition probabilities considerably extend the available data for the E2 and M1 transtions, and are in good agreement with those computed by Garstang for the 3d^4 transitions.Comment: 19 pages, 1 figure. This paper marks the beginning of a large-scale effort of ab initio atomic calculations that should eventually lead to re-calculation of accurate iron opacities. Astron. Astrophys. Suppl. Ser. (in press

Formation of ground and excited states of antihydrogen

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlDifferential and integrated cross sections for the formation of antihydrogen by the impact of intermediate-energy (20-500 keV) antiprotons on positronium are calculated using the first Born approximation. The calculations are carried out for the formation of antihydrogen in ground and various excited electronic states (n = 1-3) when positronium, the target atom, is in the ground state, and for the formation of antihydrogen in the ground state when the positronium is in various excited electronic states (n = 1-2). The 1/n^3 behavior for the capture cross sections is used to calculate the total (that is, all states added together) integrated cross sections. The cross sections for the formation of antihydrogen presented here are obtained from those for the formation of positronium by the impact of positrons on hydrogen atoms by using charge invariance and the principle of detailed balance.We gratefuJly acknowledge the support of the U.S. Air Force Office of Scientific Research through Grant No. AFOSR-87-0342

Elastic scattering of positrons and electrons by argon

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlDifferential and integrated cross sections for the elastic scattering of low- and intermediate-energy (3-300 eV) positrons and electrons by argon atoms are calculated. Higher transport cross sections, representing moments of 1-(cosΘ)^n, for these systems are also obtained for n = 1-4. Model potentials are used to represent the interactions between positrons or electrons and argon atoms. For each impact energy, the phase shifts of the lower partial waves are obtained exactly by numerical integration of the radial equation. The Born approximation is used to obtain the contribution of the higher partial waves to the scattering amplitude. The phase shifts of the seven lowest partial waves are tabulated for various impact energies of positrons and electrons.Support of the National Science Foundation (Grant No. PHY 83-11705) and the Air Force Office of Scientific Research (Grant No. AFOSR-84-0143) is gratefully acknowledged

Contributions of higher partial waves to the elastic scattering amplitude for various long-range interactions

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlThe contributions of higher partial waves to the elastic scattering amplitude are dominated by long-range interactions which fall off as r^-n as r → ∞. Closed-form expressions for the contributions of higher partial waves (2l > n - 3) to the scattering amplitude for various long-range interactions (n ranging from 3 to 8) are presented.This research has been supported, in part, by the Air Force Office of Scientific Research under Grant No. AFOSR-84-0143

Positronium formation from Li and Na atoms by use of pseudopotentials

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlThe differential and total cross sections for the formation of positronium in its ground state from Li and Na atoms by the impact of intermediate-energy positrons are calculated in the first Born and distorted-wave Born approximations. Hellmann-type pseudopotentials are used to represent the alkali-metal ion cores. The difference in the use of pseudopotentials and the static potential for the core representation for evaluating various rearrangement cross sections is discussed.Support of the National Science Foundation (Grant No. PHY-83-1170S) and the Air Force Office of Scientific Research (Grant No. AFOSR-84-0143) is gratefully acknowledged

Relativistic approach for e^± scattering from argon

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlDifferential and integrated elastic, integrated total cross sections as well as various polarization parameters-the spin polarization P and the parameters T and U describing the change in the polarization vector during scattering-for the scattering of electrons and positrons from argon in the energy range of 3-300 eV are calculated using the relativistic Dirac equation. The real part of the projectile-target interaction is represented by a sum of model potentials. The phase shifts for large angular momenta ħl are calculated using the Born approximation. The relativistic calculations for the differential and integrated elastic cross sections obtained using the pure real potential show almost no improvement over those obtained nonrelativistically for positron scattering from argon while similar calculations show some effects, except at low energies (≤ 5 eV) where relativistic terms are sensitive to the form of potentials used, on the values of the differential cross sections for electron scattering from argon. The polarization parameter P for electron scattering is found to be in good agreement with various calculated and measured values. A few different models of the absorption potential for the inelastic processes are used to calculate the integrated inelastic and the integrated total cross sections for positron and electron scattering from argon. It is noticed that even though the integrated elastic and the integrated total cross sections for the scattering of positrons and electrons calculated using some complex model potential agree well with the corresponding measured values, the differential cross section curves using the same model potentials can differ considerably from each other as well as from the experimental values.Support of Air Force Office of Scientific Research (Grant No. AFOSR-87-0342) is gratefully acknowledged