Electron excitation in helium: including the n=4 levels in an R-matrix calculation

Berrington, K A; Kingston, A E

doi:10.1088/0022-3700/20/24/014

Cited by 89 publications

(69 citation statements)

References 22 publications

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“…The detailed descriptions of R-matrix method dealing with the electron-atom collision process have been presented in the previous works [11][12][13][14][15][16][17]. Only a brief outline will be given here.…”

Section: Theoretical Methods and Calculation Resultsmentioning

confidence: 99%

“…Therefore, more complete and precision theoretical results are motivated. In this work, using the R-matrix method [11][12][13][14][15][16][17] we systematically calculated the collision cross sections of sodium from ground state to the first four excited states at incident energy ranging from 0 to 5.4 eV (above the 3s ionization threshold) by four sets of high-quality target states; i.e., 5,9,14, and 19 physical target states, respectively. Our calculated cross sections are in good agreement with the available absolute experimental results [9].…”

Section: Introductionmentioning

confidence: 99%

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Precision calculation of low-energy electron-impact excitation cross sections of sodium

et al. 2010

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The collision cross sections of sodium from the ground state to the first four excited states at the incident energy ranging from 0 to 5.4 eV are calculated using the R-matrix method. The convergences of the cross sections are checked systematically by using four sets of high-quality target states, i.e., 5, 9, 14, and 19 physical target states. The influence of the Rydberg target states on the collision cross sections is also elucidated at higher incident energies; i.e., the amplitude of resonance structures will decrease with respect to the effective quantum number υ of the Rydberg target states. This result is very useful for the calculations of these cross sections at intermediate energy with finite target states by combining the partial-wave-expansion methods valid at low energy with the first Born approximation method valid at high energy, which would be of great importance in obtaining complete cross-section data for related scientific fields.

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Section: Theoretical Methods and Calculation Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Precision calculation of low-energy electron-impact excitation cross sections of sodium

et al. 2010

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“…For the Stark broadening of optical hydrogen lines we used the results from Lemke (1997); for the optical lines they give similar results as are found in Schöning & Butler (1989c). Collision rates for He  were taken from Berrington & Kingston (1987), while photoionization cross-sections are "below-resonance" fits to the calculations of Fernley et al (1987). For He  we used the Stark-broadening calculations of Schöning & Butler (1989c,b,a).…”

Section: Appendix A: Atomic Datamentioning

confidence: 99%

Massive stars at low metallicity

Bouret

Lanz

Martins

et al. 2013

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Aims. We aim to study the properties of massive stars at low metallicity, with an emphasis on their evolution, rotation, and surface abundances. We focus on O-type dwarfs in the Small Magellanic Cloud. These stars are expected to have weak winds that do not remove significant amounts of their initial angular momentum. Methods. We analyzed the UV and optical spectra of twenty-three objects using the NLTE stellar atmosphere code  and derived photospheric and wind properties. Results. The observed binary fraction of the sample is ≈26%, which is consistent with more systematic studies if one considers that the actual binary fraction is potentially larger owing to low-luminosity companions and that the sample was biased because it excluded obvious spectroscopic binaries. The location of the fastest rotators in the Hertzsprung-Russell (H-R) diagram built with fast-rotating evolutionary models and isochrones indicates that these could be several Myr old. The offset in the position of these fast rotators compared with the other stars confirms the predictions of evolutionary models that fast-rotating stars tend to evolve more vertically in the H-R diagram. Only one star of luminosity class Vz, expected to best characterize extreme youth, is located on the zero-age main sequence, the other two stars are more evolved. We found that the distribution of O and B stars in the (N) -v sin i diagram is the same, which suggests that the mechanisms responsible for the chemical enrichment of slowly rotating massive stars depend only weakly on the star's mass. We furthermore confirm that the group of slowly rotating N-rich stars is not reproduced by the evolutionary tracks. Even for more massive stars and faster rotators, our results call for stronger mixing in the models to explain the range of observed N abundances. All stars have an N/C ratio as a function of stellar luminosity that match the predictions of the stellar evolution models well. More massive stars have a higher N/C ratio than the less massive stars. Faster rotators show on average a higher N/C ratio than slower rotators, again consistent with the expected trend of stronger mixing as rotation increases. When comparing the N/O versus N/C ratios with those of stellar evolution models, the same global qualitative agreement is reached. The only discrepant behavior is observed for the youngest two stars of the sample, which both show very strong signs of mixing, which is unexpected for their evolutionary status.

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“…For hydrogen and singly ionized helium these values are computed using polynomials given by Giovanardi et al (1987). For neutral helium the collision strengths are taken from Berrington & Kingston (1987) to levels with n ≤ 4 and from Auer and Mihalas & Stone (1968) to the other ones.…”

Section: Summary Of the Modelmentioning

confidence: 99%