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 − 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 $2s^22p^4 \ ^3P_{2,0,1}$of Fe~XIX. Several sets of calculations including relativistic effects are carried out: (i) Breit-Pauli R-matrix (BPRM), (ii) Relativistic Distorted Wave (RDW), and (iii) a semi-relativistic calculation. Non-relativistic LS coupling calculations are also done for comparison. The BPRM calculations employ a configuration interaction target representation for Fe~XX consisting of 12 LS terms (23-fine structure levels), as in the recently reported BPRM calculations by Donnelly et al (MNRAS, 307, 595, 1999). The background cross sections in all three sets of present calculations agree with one another, but differ considerably from those of Donnelly et al. Owing to much more extensive resonance structures in the present BPRM calculations, the sum of the corresponding recombination rate coefficients for the$^3P_{2,0,1}$ 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 $^3P_{0,1,2}, ,^1D_2, ^1S_0$ 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