750 research outputs found
Electron impact on K+: mechanisms for extreme ultraviolet submission
A series of R-matrix calculations on K+ is used to derive electron excitation and ionization cross sections. The excitation cross section to the 4s and 3d levels leading to the K+ 60.1, 60.8 and 61.3nm emission lines shows poor agreement with the cross beam experiment of Zapesochny et al (1986, Zh. Eksp. Teor. Fiz. 90 1972 [Sov. Phys. JETP 63 1155]). Cross sections are also presented for exciting the 4p, 5s and 4d levels, the autoionizing 3s open-shell levels, and for ionization. It is shown how pseudoresonances in the calculated cross section can be eliminated by increasing the target
basis.</p
The high partial wave phenomenon of spin changing atomic transitions
The collisional transition between two highly excited atomic states with different spin is investigated theoretically. Taking helium-like n1S â n3P as an example, it is found that the transition is driven in the highly ion-ized Fe ion purely by exchange, and the cross section becomes increasingly dominated by partial waves of high orbital angular momentum as the scattering energy increases. Whereas for the near-neutral Li ion the transition is dominated by channel coupling in low partial waves. Analytical bench-marks and numerical methods are developed for the accurate calculation of the exchange integral at high angular momentum. It is shown how the partial wave and energy dependence of the collision strength for high n spin changing transitions in the highly ionized ion is related to the overlap of the extended atomic orbitals.</p
Fe VII lines in the spectrum of RR Telescopii
Thirteen transitions within the ground 3d^2 configuration of Fe VII are
identified in ultraviolet and optical spectra of the symbiotic star RR
Telescopii obtained with the STIS instrument of the Hubble Space Telescope. The
line fluxes are compared with theoretical data computed with the recent atomic
data of K.A. Berrington et al., and high resolution optical spectra from
VLT/UVES are used to identify blends. Seven branching ratios are measured, with
three in good agreement with theory and one affected by blending. The
lambda5277/lambda4943 branching ratio is discrepant by > 3 sigma, indicating
errors in the atomic data for the lambda5277 line. A least-squares minimization
scheme is used to simultaneously derive the temperature, T, and density, N_e,
of the RR Tel nebula, and the interstellar extinction, E(B-V), towards RR Tel
from the complete set of emission lines. The derived values are: log T/K = 4.50
+/- 0.23, log N_e/cm^-3=7.25 +/- 0.05, and E(B-V)<0.27. The extinction is not
well-constrained by the Fe VII lines, but is consistent with the more accurate
value E(B-V)=0.109^{+0.052}_{-0.059} derived here from the Ne V
lambda2974/lambda1574 ratio in the STIS spectrum. Large differences between the
K.A. Berrington et al. electron excitation data and the earlier F.P. Keenan &
P.H. Norrington data-set are demonstrated, and the latter is shown to give
worse agreement with observations.Comment: To be published in Astronomy & Astrophysics; 7 pages, 4 figure
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
Photoionization and recombination of Fe XIX
Photoionization cross sections and recombination rate coefficients are
presented for the L-shell ground state fine structure levels $2s^22p^4 \
^3P_{2,0,1}^3P_{2,0,1}$
levels are up to 50% higher than the LS rates at low temperarures but
comparable for higher temperatures; in contrast to the results of Donnelly et
al who obtained the LS rates to be higher than their BPRM results by about a
factor of 2. Reasons for these discrepancies are discussed.Comment: 7 pages, 3 figures, MNRAS, In Pres
The influence of electron collisions on non-LTE Li line formation in stellar atmospheres
The influence of the uncertainties in the rate coefficient data for
electron-impact excitation and ionization on non-LTE Li line formation in cool
stellar atmospheres is investigated. We examine the electron collision data
used in previous non-LTE calculations and compare them to recent calculations
that use convergent close-coupling (CCC) techniques and to our own calculations
using the R-matrix with pseudostates (RMPS) method. We find excellent agreement
between rate coefficients from the CCC and RMPS calculations, and reasonable
agreement between these data and the semi-empirical data used in non-LTE
calculations up to now. The results of non-LTE calculations using the old and
new data sets are compared and only small differences found: about 0.01 dex (~
2%) or less in the abundance corrections. We therefore conclude that the
influence on non-LTE calculations of uncertainties in the electron collision
data is negligible. Indeed, together with the collision data for the charge
exchange process Li(3s) + H Li^+ + H^- now available, and barring the
existence of an unknown important collisional process, the collisional data in
general is not a source of significant uncertainty in non-LTE Li line formation
calculations.Comment: 8 pages, accepted by Astronomy and Astrophysics; Replaced with minor
corrections following proof
Lower entropy bounds and particle number fluctuations in a Fermi sea
We demonstrate, in an elementary manner, that given a partition of the single
particle Hilbert space into orthogonal subspaces, a Fermi sea may be factored
into pairs of entangled modes, similar to a BCS state. We derive expressions
for the entropy and for the particle number fluctuations of a subspace of a
fermi sea, at zero and finite temperatures, and relate these by a lower bound
on the entropy. As an application we investigate analytically and numerically
these quantities for electrons in the lowest Landau level of a quantum Hall
sample.Comment: shorter version, typos fixe
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
Improved collision strengths and line ratios for forbidden [O III] far-infrared and optical lines
Far-infrared and optical [O III] lines are useful temeprature-density
diagnostics of nebular as well as dust obscured astrophysical sources. Fine
structure transitions among the ground state levels 1s^22s^22p^3 \ ^3P_{0,1,2}
give rise to the 52 and 88 micron lines, whereas transitions among the
levels yield the well-known optical lines 4363,
4959 and 5007 Angstroms. These lines are excited primarily by electron impact
excitation. But despite their importance in nebular diagnostics collision
strengths for the associated fine structure transitions have not been computed
taking full account of relativistic effects. We present Breit-Pauli R-matrix
calculations for the collision strengths with highly resolved resonance
structures. We find significant differences of up to 20% in the Maxwellian
averaged rate coefficients from previous works. We also tabulate these to lower
temperatures down to 100 K to enable determination of physical conditions in
cold dusty environments such photo-dissociation regions and ultra-luminous
infrared galaxies observed with the Herschel space observatory. We also examine
the effect of improved collision strengths on temperature and density sensitive
line ratios.Comment: Letter in press, Monthly Notices of Royal Astronomical Society, 5
pages, 6 figure
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