317 research outputs found
Atomic data from the Iron Project. LXIV. Radiative transition rates and collision strengths for Ca II
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 Ca
II. 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. We present extensive comparisons with the most
recent calculations and measurements for Ca II 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 latter computations
using the distorted wave approximation.Comment: 8 pages, 4 figures. The atomic data from this work, including energy
levels, A-values, and effective collision strengths, is available in
electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A
Dielectronic recombination of Fe 3p^q ions: a key ingredient for describing X-ray absorption in active galactic nuclei
We have carried-out multi-configuration Breit-Pauli AUTOSTRUCTURE
calculations for the dielectronic recombination (DR) of Fe^{8+} - Fe^{12+}
ions. We obtain total DR rate coefficients for the initial ground-level which
are an order of magnitude larger than those corresponding to radiative
recombination (RR), at temperatures where Fe 3p^q (q=2-6) 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 q=0 and 1, are critical for determining the
ionization balance of the M-shell Fe ions which 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.Comment: Submitted to Ap.
Revised Results for Non-thermal Recombination Flare Hard X-Ray Emission
Brown and Mallik (BM) recently showed that, for hot sources, recombination of
non-thermal electrons (NTR) onto highly ionised heavy ions is not negligible
compared to non-thermal bremsstrahlung (NTB) as a source of flare hard X-rays
(HXRs) and so should be included in modelling non-thermal HXR flare emission.
In view of major discrepancies between BM results for the THERMAL continua and
those of the Chianti code and of RHESSI solar data, we critically re-examine
and correct the BM analysis and modify the conclusions concerning the
importance of NTR. Although the analytic Kramers expression used by BM is
correct for the purely hydrogenic recombination cross section, the heuristic
expressions used by BM to extend the Kramers expression beyond the `bare
nucleus' case to which it applies had serious errors. BM results have therefore
been recalculated using corrected expressions, which have been validated
against the results of detailed calculations. At T ~ 10-30 MK the dominant ions
are Fe 22+, 23+, 24+ for which BM erroneously overestimated NTR emission by
around an order of magnitude. Contrary to the BM claim, NTR in hot flare
plasmas does NOT dominate over NTB, although in some cases it can be comparable
and so still very important in inversions of photon spectra to derive electron
spectra, especially as NTR includes sharp edge features. The BM claim of
dominance of NTR over NTB in deka-keV emission is incorrect due to a serious
error in their analysis. However, the NTR contribution can still be large
enough to demand inclusion in spectral fitting, the spectral edges having
potentially serious effects on inversion of HXR spectra to infer fast electron
spectra.Comment: 6 pages, 8 figures, 1 tabl
Dielectronic recombination of W^20+ (4d^10 4f^8): addressing the half-open f-shell
A recent measurement of the dielectronic recombination (DR) of W^20+
[Schippers et al Phys. Rev. A83, 012711 (2011)] found an exceptionally large
contribution from near threshold resonances (<1eV). This still affected the
Maxwellian rate coefficient at much higher temperatures. The experimental
result was found to be a factor 4 or more than that currently in use in the
100-300eV range which is of relevance for modeling magnetic fusion plasmas. We
have carried-out DR calculations with AUTOSTRUCTURE which include all
significant single electron promotions. Our intermediate coupling (IC) results
are more than a factor of 4 larger than our LS-coupling ones at 1eV but still
lie a factor 3 below experiment here. If we assume complete (chaotic) mixing of
near-threshold autoionizing states then our results come into agreement (to
within 20%) with experiment below about 2eV. Our total IC Maxwellian rate
coefficients are 50-30% smaller than those based-on experiment over 100-300eV.Comment: 10 pages, 8 figures, submitted to Phys.Rev.
Atomic data for astrophysics : Ni XII
We present new large-scale R-matrix (up to n = 4) scattering calculations for the electron collisional excitation of Cl-like Ni xii. We used the intermediate-coupling frame transformation method. We compare predicted and observed line intensities using laboratory and solar spectra, finding good agreement for all the main soft X-ray lines. With the exception of the three strongest transitions, large discrepancies with previous estimates are found, especially for the decays from the lowest 3s2 3p4 3d levels. This includes the forbidden UV lines. The atomic data for the n = 4 levels are the first to be calculated. We revise previous experimental energies, and suggest several new identifications. We point out the uncertainty in the wavelength of the 3s2 3p52P1/2-3s2 3p4 3d 2D3/2 transition, which is important for density diagnostics
Atomic data and density diagnostics for S IV.
We present a new large-scale R-matrix scattering calculation for S iv. We used the intermediate-coupling frame transformation method and applied term energy corrections. Our calculation has a much larger configuration-interaction (CI) and close-coupling (CC) expansion than previous calculations. Despite that, we find good agreement in the predicted intensities of the decays from the three 3s 3p2 4P levels around 1400 Ă
, important for density diagnostics. A discrepancy between the observed and predicted intensity of the 1404.8 Ă
line, which is known to be blended at least with an O iv transition, is still present. Significant differences compared to previous models are found instead for the 1062.7 and 1073.0 Ă
lines, useful for diagnostics in low density plasma such as in nebulae. Several other significant differences were also found, concerning the population of the 3s 3p 3d 4F9/2 metastable level, and the intensities of several transitions
Atomic data from the IRON Project. I. Electron-impact scattering of Fe17+ using <I>R</I>-matrix theory with intermediate coupling
We present results for electron-impact excitation of F-like Fe calculated using R-matrix theory where an intermediate-coupling frame transformation (ICFT) is used to obtain level-resolved collision strengths. Two such calculations are performed, the first expands the target using 2s2 2p5, 2s 2p6, 2s2 2p4 3l, 2s 2p5 3l, and 2p6 3l configurations while the second calculation includes the 2s2 2p4 4l, 2s 2p5 4l, and 2p6 4l configurations as well. The effect of the additional structure in the latter calculation on the n=3 resonances is explored and compared with previous calculations. We find strong resonant enhancement of the effective collision strengths to the 2s2 2p4 3s levels. A comparison with a Chandra X-ray observation of Capella shows that the n=4 R-matrix calculation leads to good agreement with observation</p
Near-threshold dielectronic recombination studies of Si-like ions
Dielectronic recombination rate coefficients are computed for the Si-like isoelectronic sequence, focusing on the near-threshold resonances of S2+
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