Catherine Ramsbottom

Nebular and Auroral Emission Lines of [Ar IV] in the Optical Spectra of Planetary Nebulae

Recent R-matrix calculations of electron impact excitation rates in Ar IV are used to calculate the emission-line ratio: ratio diagrams (R1, R2), (R1, R3), and (R1, R4), where R1 = I(4711 ?)/I(4740 ?), R2 = I(7238 ?)/I(4711 + 4740 ?), R3 = I(7263 ?)/I(4711 + 4740 ?), and R4 = I(7171 ?)/I(4711 + 4740 ?), for a range of electron temperatures (Te = 5000-20,000 K) and electron densities (Ne = 10-106 cm-3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of Te and Ne from measurements of the [Ar IV] lines in a spectrum. Plasma parameters deduced for a sample of planetary nebulae from (R1, R3) and (R1, R4), using observational date obtained with the Hamilton echelle spectrograph on the 3 m Shane Telescope at the Lick Observatory, are found to show excellent internal consistency and to be in generally good agreement with the values of Te and Ne estimated from other line ratios in the echelle spectra. These results provide observational support for the accuracy of the theoretical ratios and, hence, the atomic data adopted in their derivation. In addition, they imply that the 7171 ? line is not as seriously affected by telluric absorption as previously thought. However, the observed values of R2 are mostly larger than the theoretical high-temperature and density limit, which is due to blending of the Ar IV 7237.54 ? line with the strong C II transition at 7236 ?.

Electron-impact excitation rates for transitions involving the n=2 and n=3 levels of beryllium-like N IV

An R-matrix calculation for the electron impact excitation of the berylliumlike ion N IV is described, in which the first 12 eigenstates formed from the 2s2, 2s2p, 2p2 and 2s3l (l = s, p, d) configurations are included in the wavefunction expansion. These 12 ionic states correspond to 20 fine-structure levels, leading to a total of 190 possible independent transitions. Excitation rates, obtained by averaging the electron collision strengths over a Maxwellian distribution of electron velocities, are tabulated for all fine-structure transitions between the lowest n = 2 states together with those between the n = 2 and n = 3 levels, for electron temperatures ranging from 1000 to 1 000 000 K. Comparisons are made with other available theoretical data and significant deviations are found to occur. In particular the present results indicate that the excitation rates, for most of the transitions considered, are significantly affected by resonances converging to the 2s3l thresholds.

Calculation of photoionized plasmas with an average-atom model

We use a simple average-atom model (NIMP) to calculate the distribution of ionization in a photoionization-dominated plasma, for comparison with recent experimental measurements undertaken on the Z-machine at the Sandia National Laboratory. The agreement between theory and experiment is found to be as good for calculations with an average-atom model as for those generated by more detailed models.

N IV emission lines in the ultraviolet spectra of gaseous nebulae

Theoretical electron density sensitive emission-line ratios, determined using electron impact excitation rates calculated with the R-matrix code, are presented for R = I(2s(sup 2) (1)S-2s2p 3P(sub 2))/I(2s(sup 2) (1)S-2s2p 3P(sub 1) = 1(1483 A)/I(1486 A) in N IV. These are found to be up to an order of magnitude different from those deduced by previous authors, principally due to the inclusion of excitation rates for transitions among the 2s2p (3)p fine-structure levels. The observed values of R for several planetary nebulae, symbiotic stars and the Cygnus Loop supernova remnant, measured from spectra obtained with the International Ultraviolet Explorer (IUE) satellite and the Hopkins Ultraviolet Explorer (HUT), lead to electron densities which are in excellent agreement with those deduced from line ratios in other species. This provides observational support for the accuracy of the atomic data adopted in the present calculations.

Electron impact excitation of Fe II : Total LS effective collision strengths

We present total effective collision strengths for electron-impact excitation of Fe II, calculated using the parallel R-matrix program PRMAT, for all sextet to quartet forbidden transitions among the lowest 113 LS states formed from the 3d64s, 3d7, 3d54s2, 3d64p and 3d54s4p basis configurations. After a detailed and systematic study of configuration-interaction effects in both the target and the collision wavefunctions, it was found that an additional 21 configurations needed to be included in the CI expansion to obtain significantly more accurate target state and collision wavefunctions. A total of 1785 individual lines are considered over a wide range of electron temperatures, 30–100 000 K, of particular importance in astrophysical applications. A detailed comparison is made with a previous 3 configuration approximation and significant differences of up to a factor of 2 are reported at the lowest temperatures considered. The inclusion of the additional 3d54s2 and 3d54s4p levels in the present work together with the additional CI effects are found to cause significant changes to the total effective collision strengths for the low-lying transitions from the ground state, particularly at very low temperatures. The disparities are found to be considerably less at the higher temperatures considered.

Electron-impact excitation rates for transitions involving the n=2 and n=3 levels of beryllium-like Ne VII

Abstract Electron impact excitation rates evaluated using the multichannel R -matrix method are presented for the Be-sequence ion Ne VII. Twenty-six target eigenstates consisting of the 6 n = 2 states with configurations 2 s 2 , 2 s 2 p , and 2p 2 and the 20 n = 3 states with configurations 2 s 3 s , 2 s 3 p , 2 s 3 d , 2 p 3 s , 2 p 3 p , and 2 p 3 d are included in the expansion of the total wave function. These 26 ionic states correspond to 46 fine-structure levels, leading to a total of 1035 possible independent transitions. Effective collision strengths, obtained by averaging the electron collision strengths over a Maxwellian distribution of electron velocities, are tabulated for all fine-structure transitions between the lowest n = 2 levels together with those between the n = 2 and n = 3 levels, for electron temperatures, T , in the range log T (K) = 4 to log T (K) = 6.4. Comparisons are made with other available theoretical data and significant deviations are found to occur. In particular the present results indicate that the excitation rates for many of the forbidden transitions considered are significantly enhanced by resonances converging to the n = 3 thresholds.

Electron-impact excitation of Fe II. Collision strengths and effective collision strengths for low-lying fine-structure forbidden transitions.

Context. Considerable demand exists for electron excitation data for Ni ii, since lines from this abundant ion are observed in a wide variety of laboratory and astrophysical spectra. The accurate theoretical determination of these data can present a significant challenge however, due to complications arising from the presence of an open 3d-shell in the description of the target ion. Aims. In this work we present collision strengths and Maxwellian averaged effective collision strengths for the electron-impact excitation of Ni ii. Attention is concentrated on the 153 forbidden fine-structure transitions between the energetically lowest 18 levels of Ni ii .E ffective collision strengths have been evaluated at 27 individual electron temperatures ranging from 30–100 000 K. To our knowledge this is the most extensive theoretical collisional study carried out on this ion to date. Methods. The parallel R-matrix package RMATRX II has recently been extended to allow for the inclusion of relativistic effects. This suite of codes has been utilised in the present work in conjunction with PSTGF to evaluate collision strengths and effective collision strengths for all of the low-lying forbidden fine-structure transitions. The following basis configurations were included in the target model – 3d 9 ,3 d 8 4s, 3d 8 4p, 3d 7 4s 2 and 3d 7 4s4p – giving rise to a sophisticated 295 jj-level, 1930 coupled channel scattering problem. Results. Comprehensive comparisons are made between the present collisional data and those obtained from earlier theoretical evaluations. While the effective collision strengths agree well for some transitions, significant discrepancies exist for others.

Configuration Interaction Effects in low-energy electron collisions with Fe II

Partial wave collision strengths are presented for low-energy electron-impact transitions in Fe II between the 3d 6 4 sa 6 D e ground state and the 3d 7 a 4 F e , 3d 6 4s a 4 D e and 3d 7 a 4 P e low-lying excited states. The collision strengths are calculated in LS coupling using a new Fortran 95 R-matrix program including all terms of the 3d 6 4s, 3d 7 , 3d 6 4p, 3d 5 4s 2 and 3d 5 4s4p configurations in the close coupling expansion of the collision wavefunction. Special emphasis is given to the inclusion of configuration interaction (CI) effects both in the target and in the collision wavefunctions. In both cases series of calculations are carried out where additional CI terms are included systematically. It is found that in order to obtain close to converged low-energy partial wave collision strengths two-electron excitations from the 3p shell to the 3d shell as well as pseudo s and d orbitals must be included in the CI expansions. Also resonance effects in low-energy partial wave collision strengths are found to depend sensitively on the representation of the d orbitals in the CI expansion of the collision wavefunction. Finally, CI models for both the target and collision wavefunctions are defined which can be used in proposed calculations to obtain accurate total collision strengths and effective collision strengths for transitions between LS-coupled terms and between fine-structure levels of Fe II.

Valence and L-shell photoionization of Cl-like argon using R-matrix techniques

Photoionization cross sections are obtained using the relativistic Dirac Atomic R-matrix Codes (DARC) for all valence and L-shell energy ranges between 27-270eV. A total of 557 levels arising from the dominant configurations 3s

Photodetachment cross sections for the 4So and 2Do bound states of the negative carbon ion

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