121,950 research outputs found
Dielectronic recombination of Fe 3pq ions : a key ingredient for describing X-ray absorption in active galactic nuclei
No full textWe have carried out multiconfiguration Breit-Pauli AUTOSTRUCTURE calculations for the dielectronic recombination (DR) of Fe8+-Fe12+ ions. We obtain total DR rate coefficients for the initial ground level that are an order of magnitude larger than those corresponding to radiative recombination (RR), at temperatures where Fe 3pq () ions are abundant in photoionized plasmas. The resultant total (DR+RR) rate coefficients are then an order of magnitude larger than those currently in use by photoionized plasma modeling codes such as CLOUDY, ION, and XSTAR. These rate coefficients, together with our previous results for and 1, are critical for determining the ionization balance of the M-shell Fe ions that give rise to the prominent unresolved-transition-array X-ray absorption feature found in the spectrum of many active galactic nuclei. This feature is poorly described by CLOUDY and ION, necessitating an ad hoc modification to the low-temperature DR rate coefficients. Such modifications are no longer necessary, and a rigorous approach to such modeling can now take place using these data
Updated opacities from the opacity project
No full textUsing the code autostructure, extensive calculations of inner-shell atomic data have been made for the chemical elements He, C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Cr, Mn, Fe and Ni. The results are used to obtain updated opacities from the Opacity Project (OP). A number of other improvements on earlier work have also been included. Rosseland-mean opacities from the OP are compared with those from OPAL. Differences of 5-10 per cent occur. The OP gives the 'Z-bump', at log(T) 5.2, to be shifted to slightly higher temperatures. The opacities from the OP, as functions of temperature and density, are smoother than those from OPAL. The accuracy of the integrations used to obtain mean opacities can depend on the frequency mesh used. Tests involving variation of the numbers of frequency points show that for typical chemical mixtures the OP integrations are numerically correct to within 0.1 per cent. The accuracy of the interpolations used to obtain mean opacities for any required values of temperature and density depends on the temperature-density meshes used. Extensive tests show that, for all cases of practical interest, the OP interpolations give results correct to better than 1 per cent. Prior to a number of recent investigations which have indicated a need for downward revisions in the solar abundances of oxygen and other elements, there was good agreement between properties of the Sun deduced from helioseismology and from stellar evolution models calculated using OPAL opacities. The revisions destroy that agreement. In a recent paper, Bahcall et al. argue that the agreement would be restored if opacities for the regions of the Sun with 2 × 106T 5 × 106 K (0.7-0.4 R) were larger than those given by OPAL by about 10 per cent. In the region concerned, the present results from the OP do not differ from those of OPAL by more than 2.5 per cent
Collisional ionization equilibrium for optically thin plasmas. I. Updated recombination rate coefficients for bare through sodium-like ions
No full textReliably interpreting spectra from electron-ionized cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and are often highly suspect. This translates directly into the reliability of the collisional ionization equilibrium (CIE) calculations. We make use of state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Na-like ions of all elements from He up to and including Zn. Where measurements exist, these published theoretical DR data agree with recent laboratory work to within typically 35% or better at the temperatures relevant for CIE. We also make use of state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H through to Zn. Here we present improved CIE calculations for temperatures from 104 to 109 K using our data and the recommended electron impact ionization data of Mazzotta et al. for elements up to and including Ni and Mazzotta for Cu and Zn. DR and RR data for ionization stages that have not been updated are also taken from these two additional sources. We compare our calculated fractional ionic abundances using these data with those presented by Mazzotta et al. for all elements from H to Ni. The differences in peak fractional abundance are up to 60%. We also compare with the fractional ionic abundances for Mg, Si, S, Ar, Ca, Fe, and Ni derived from the modern DR calculations of Gu for the H-like through Na-like ions, and the RR calculations of Gu for the bare through F-like ions. These results are in better agreement with our work, with differences in peak fractional abundance of less than 10%
Dielectronic recombination data for dynamic finite-density plasmas. VIII. The nitrogen isoelectronic sequence
No full textDielectronic recombination data for nitrogen-like ions forming oxygen-like ions has been calculated as part of the assembly of a level-resolved dielectronic recombination database necessary for the modelling of dynamic finite-density plasmas (Badnell et al. 2003). Dielectronic recombination rate coefficients for a selection of ions from O+ to Xe47+ are presented and the results discussed.Fil: Mitnik, Dario Marcelo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Badnell, N. R.. University Of Strathclyde; Reino Unid
A comparison of rosseland-mean opacities from op and opal
No full textMonochromatic opacities from the Opacity Project (OP) have been augmented by hitherto missing inner-shell contributions. OP Rosseland-mean opacities, κR, are compared with results from OPAL for the six elements H, He, C, O, S and Fe. The OPAL data are obtained from the project's website. Agreement for H is close everywhere except for the region of log(T) 6 and log(R) −1 (R=ρ/T36 where ρ is mass density in g cm3 and T6= 106×T with T in K). In that region κR(OPAL) is larger than κR(OP) by up to 13 per cent. The differences are caused by different equations of state (EOS). In the region concerned, OP has the H ground state undergoing dissolution, leading to a small H-neutral ionization fraction, while OPAL has larger values for that fraction. A similar difference occurs for He at log(R) −1 and log(T) 6.4, where OP has the He+ ground state undergoing dissolution. The OPAL website does not provide single-element Rosseland means for elements other than H and He. Comparisons between OP and OPAL are made for mixtures with X= 0.9, Z= 0.1 and Z containing pure C, O or S. There are some differences: at the lower temperatures, say log(T) ≤ 5.5, owing to differences in atomic data, with the OP R-matrix data probably being the more accurate; and at higher temperatures mainly owing to differences in level populations resulting from the use of different EOS theories. In the original OP work, R-matrix data for iron were supplemented by data obtained using the configuration-interaction (CI) code superstructure. The experiment is made of replacing much of the original iron data with new data from the CI code autostructure. Inclusion of intercombination lines gives an increase in κR of up to 18 per cent. The OPAL website does not allow for Z containing pure iron. Comparisons are made for an iron-rich mixture, X= 0.9, Z= 0.1 and Z containing C and Fe with C:Fe = 2:1 by number fraction. There are some differences between OP and OPAL for that case: the OP 'Z-bump' in κR is shifted to slightly higher temperatures, compared with OPAL. Overall, there is good agreement between OP and OPAL Rosseland-mean opacities for the six elements, but there are some differences. Recent work has shown that helioseismology measurements give a very accurate value for the depth of the solar convective zone, RCZ, and that, taking account of recent revisions in abundances, solar models give agreement with that value only if opacities at RCZ are about 20 per cent larger than OPAL values. For the six-element mix at RCZ we obtain κR(OP) to be larger than κR(OPAL) by 5 per cent
Dielectric recombination data for dynamic finite-density plasmas I. Goals and methodology
No full textA programme is outlined for the assembly of a comprehensive dielectronic recombination database within the generalized collisional-radiative (GCR) framework. It is valid for modelling ions of elements in dynamic finite-density plasmas such as occur in transient astrophysical plasmas such as solar flares and in the divertors and high transport regions of magnetic fusion devices. The resolution and precision of the data are tuned to spectral analysis and so are sufficient for prediction of the dielectronic recombination contributions to individual spectral line emissivities. The fundamental data are structured according to the format prescriptions of the Atomic Data and Analysis Structure (ADAS) and the production of relevant GCR derived data for application is described and implemented following ADAS. The requirements on the dielectronic recombination database are reviewed and the new data are placed in context and evaluated with respect to older and more approximate treatments. Illustrative results validate the new high-resolution zero-density dielectronic recombination data in comparison with measurements made in heavy-ion storage rings utilizing an electron cooler. We also exemplify the role of the dielectronic data on GCR coefficient behaviour for some representative light and medium weight elements
Radiative recombination data for modeling dynamic finite-density plasmas
No full textWe have calculated partial final-state resolved radiative recombination (RR) rate coefficients from the initial ground and metastable levels of all elements up to and including Zn, plus Kr, Mo, and Xe, for all isoelectronic sequences up to Na-like forming Mg-like. The data are archived according to the Atomic Data and Analysis Structure (ADAS) data class adf48, which spans a temperature range of z2(101-107) K, where z is the initial ion charge. Fits to total rate coefficients have been determined, for both the ground and metastable levels, and those for the ground are presented here. Comparison is made both with previous RR rate coefficients and with (background) R-matrix photoionization cross sections. This RR database complements a dielectronic recombination (DR) database already produced, and both are being used to produce updated ionization balances for both (electron) collisionally ionized and photoionized plasmas
Dielectronic recombination data for dynamic finite-density plasmas - v. the lithium isoelectronic sequence
No full textDielectronic recombination data for the lithium isoelectronic sequence has been calculated as part of the assembly of a dielectronic recombination database necessary for modelling of dynamic finite-density plasmas (Badnell et al. 2003). Dielectronic recombination coefficients for a selection of ions from this sequence are presented and the results discussed
Atomic data from the Iron project LXIV: Radiative transition rates and collision strengths for Ca II
No full textAims.This work reports radiative transition rates and electron impact excitation rate coefficients for levels of the n = 3, 4, 5, 6, 7, 8 configurations of . Methods.The radiative data were computed using the Thomas-Fermi-Dirac central potential method in the frozen core approximation and includes the polarization interaction between the valence electron and the core using a model potential. This method allows for configuration interactions (CI) and relativistic effects in the Breit-Pauli formalism. Collision strengths in LS-coupling were calculated in the close coupling approximation with the R-matrix method. Then, fine structure collision strengths were obtained by means of the intermediate-coupling frame transformation (ICFT) method which accounts for spin-orbit coupling effects. Results. We present extensive comparisons with the most recent calculations and measurements for as well as a comparison between the core polarization results and the "unpolarized" values. We find that core polarization affects the computed lifetimes by up to 20%. Our results are in very close agreement with recent measurements for the lifetimes of metastable levels. The present collision strengths were integrated over a Maxwellian distribution of electron energies and the resulting effective collision strengths are given for a wide range of temperatures. Our effective collision strengths for the resonance transitions are within ~11% from previous values derived from experimental measurements, but disagree with later computations using the distorted wave approximation
Steps toward dielectronic recombination of argon-like ions : A revisited theoretical investigation Of SC3+ and Ti4+
No full textAs an improvement to an earlier study [T.W. Gorczyca, M.S. Pindzola, F. Robicheaux, N.R. Badnell, Phys. Rev. A 56 (1997) 47421, we have calculated dielectronic recombination rate coefficient spectra for Sc3+ and Ti4+ ions as a test case toward the assembly of a database [http://homepages.wmich.edu/similar to gorczyca/drdata/] for the Ar-like isoelectronic sequence required for modeling of dynamic finite-density plasmas [N.R. Badnell et al., A&A 406 (2003) 115 1]. Our theoretical spectra contain dominant Delta N = 0 and Delta N = 1 core excitations channels and exhibit nearly all features found in a recent ion storage ring experiments [S. Schippers, T. Bartsch, C. Brandau, G. Gwinner, J. Linkemann, A. Muller, A.A. Saghiri, A. Wolf, J. Phys. B 31 (1998) 4873; S. Schippers et al., Phys. Rev. A 65 (2002) 042723]. In order to compare Maxwellian-averaged rate coefficients, which are of main interest to the astrophysics community, we have developed an iterative deconvolution procedure that enables us to extract the cross-section from storage ring data. After folding the resultant cross-section with a Maxwellian electron velocity distribution, theoretical and experimental rate coefficient spectra agree better than similar to 18% subject to field reionization effects via high Rydberg states in Sc2+ and Ti3+ ions. (c) 2007 Elsevier B.V. All rights reserved
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