Comment on “Large enhancement in high-energy photoionization of Fe XVII and missing continuum plasma opacity”

C. Blancard (1), J. Colgan (2), Ph. Cossé (1), G. Faussurier (1), C. J. Fontes (2), F. Gilleron (1), I. Golovkin (3), S. B. Hansen (4*), C. A. Iglesias (5), D. P. Kilcrease (2), J. J. MacFarlane (3), R.M. More (6), J.-C. Pain (1), M. Sherrill (2), and B. G. Wilson (5).

(1) Commissariat a l’Energie Atomique et aux Energies Alternatives, F-91297 Arpajon, France. (2) Los Alamos National Laboratories, Bikini Atoll Road, Los Alamos, NM 87545, USA. (3) Prism Computational Sciences, 455 Science Drive, Suite 140, Madison, WI 53711, USA. (4) Sandia National Laboratories, 1515 Eubank SE, Albuquerque, NM 87185, USA. (5) Lawrence Livermore National Laboratories, P.O. Box 808, Livermore, CA 94550, USA. (6) National Institute for Fusion Science, Toki, Gifu, Japan (ret.)

*e-mail: sbhansen@sandia.gov

Recent R-matrix calculations claim to produce a significant enhancement in the opacity of Fe XVII due to atomic core excitations [S. N. Nahar & A.K. Pradhan, Phys. Rev. Letters 116, 235003 (2016), arXiv:1606.02731] and assert that this enhancement is consistent with recent measurements of higher-than-predicted iron opacities [J. E. Bailey et al., Nature 517, 56 (2015)]. This comment shows that the standard opacity models which have already been directly compared with experimental data produce photon absorption cross-sections for Fe XVII that are effectively equivalent to (and in fact larger than) the new R-matrix opacities. Thus, the new R-matrix results cannot be expected to significantly impact the existing discrepancies between theory and experiment because they produce neither a “large enhancement” nor account for “missing continuum plasma opacity” relative to standard models.

Article –> https://arxiv.org/ftp/arxiv/papers/1608/1608.03512.pdf

 

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Electron impact excitation of N IV: calculations with the DARC code and a comparison with ICFT results

K. M. Aggarwal (1), F. P. Keenan (1), K. D. Lawson (2)

(1) Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK. (2) CCFE, Culham Science Centre, Abingdon OX14 3DB, UK.

There have been discussions in the recent literature regarding the accuracy of the available electron impact excitation rates (equivalently effective collision strengths ϒ) for transitions in Be-like ions. In the present paper we demonstrate, once again, that earlier results for ϒ are indeed overestimated (by up to four orders of magnitude), for over 40 per cent of transitions and over a wide range of temperatures. To do this we have performed two sets of calculations for N  IV, with two different model sizes consisting of 166 and 238 fine-structure energy levels. As in our previous work, for the determination of atomic structure the grasp (General-purpose Relativistic Atomic Structure Package) is adopted and for the scattering calculations (the standard and parallelised versions of) the Dirac Atomic R-matrix Code (DARC) are employed. Calculations for collision strengths and effective collision strengths have been performed over a wide range of energy (up to 45 Ryd) and temperature (up to 2.0×10^6 K), useful for applications in a variety of plasmas. Corresponding results for energy levels, lifetimes and A-values for all E1, E2, M1 and M2 transitions among 238 levels of N  IV are also reported.

Article –> http://mnras.oxfordjournals.org/content/461/4/3997.short

 

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The primordial helium abundance and the number of neutrino families

Peimbert, A. (1), Peimbert, M. (1), and Luridiana, V. (2,3)

(1) Instituto de Astronomía, Universidad Nacional Autónoma de México, México.
(2) Instituto de Astrofísica de Canarias, Spain. (3) Departamento de Astrofísica, Universidad de La Laguna, Spain.

Based on observations of H II regions and the new computations of the recombination coefficients of the He I lines by Porter et al. (2013) we obtain a primordial helium abundance by mass of Yp =0.2446±0.0029. We consider thirteen sources of error for the Yp determination, some of them are mainly due to systematic effects, while the rest are mainly due to statistical effects. We compare our results with other determinations of Yp   present in the literature. Combining our Yp value with computations of primordial nucleosynthesis we find a number of neutrino species Neff =2.90±0.22, and a neutron mean life τν = 872 ± 14 (s).

Article –> http://arxiv.org/abs/1608.02062

 

 

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Hyperfine structure constants for singly ionized manganese (Mn II) using Fourier transform spectroscopy

Townley-Smith, Keeley; Nave, Gillian; Pickering, Juliet C.; Blackwell-Whitehead, Richard J.

We expand on the comprehensive study of hyperfine structure (HFS) in Mn II conducted by Holt et al. (1999) by verifying hyperfine magnetic dipole constants (A) for 20 levels previously measured by Holt et al. (1999) and deriving A constants for 47 previously unstudied levels. The HFS patterns were measured in archival spectra from Fourier transform (FT) spectrometers at Imperial College London and the National Institute of Standards and Technology. Analysis of the FT spectra was carried out in XGREMLIN. Our A constant for the ground level has a lower uncertainty by a factor of 6 than that of Blackwell-Whitehead et al.

Article -> http://esoads.eso.org/abs/2016MNRAS.461…73T

 

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Young stars and ionized nebulae in M83: comparing chemical abundances at high metallicity

Fabio Bresolin (1), Rolf-Peter Kudritzki (1), Miguel A. Urbaneja (2), Wolfgang Gieren (3), I-Ting Ho (1), Grzegorz Pietrzynski (3).

(1) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA. (2) Institut für Astro- und Teilchenphysik, Universität Innsbruck Technikerstr. 25/8, 6020 Innsbruck, Austria. (3) Departamento de Astronomía, Universidad de Concepción Casilla 160-C, Concepción, Chile.

We present spectra of 14 A-type supergiants in the metal-rich spiral galaxy M83. We derive stellar parameters and metallicities, and measure a spectroscopic distance modulus m-M = 28.47 ±  0.10 (4.9 ± 0.2 Mpc), in agreement with other methods. We use the stellar characteristic metallicity of M83 and other systems to discuss a version of the galaxy mass-metallicity relation that is independent of the analysis of nebular emission lines and the associated systematic uncertainties. We reproduce the radial metallicity gradient of M83, which flattens at large radii, with a chemical evolution model, constraining gas inflow and outflow processes. We carry out a comparative analysis of the metallicities we derive from the stellar spectra and published HII region line fluxes, utilizing both the direct, Te-based method and different strong-line abundance diagnostics. The direct abundances are in relatively good agreement with the stellar metallicities, once we apply a modest correction to the nebular oxygen abundance due to depletion onto dust. Popular empirically calibrated strong-line diagnostics tend to provide nebular abundances that underestimate the stellar metallicities above the solar value by ~0.2 dex. This result could be related to difficulties in selecting calibration samples at high metallicity. The O3N2 method calibrated by Pettini and Pagel gives the best agreement with our stellar metallicities. We confirm that metal recombination lines yield nebular abundances that agree with the stellar abundances for high metallicity systems, but find evidence that in more metal-poor environments they tend to underestimate the stellar metallicities by a significant amount, opposite to the behavior of the direct method.

Article –> http://arxiv.org/abs/1607.06840

 

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Photoionization of Ar VIII

Liang Liang, Wen-xian Jiang, Chao Zhou

Physics Department, Xi’an University of Architecture and Technology, Xi’an 710055, People’s Republic of China

The photoionization cross section, energy levels and widths of 22 Rydberg series (in the autoionization region) for Na-like Ar VIII were investigated by using of R-matrix method. The relativistic distorted-wave method is used to calculate the radial functions, and QB method of Quigly–Berrington [L. Quigley, K. A. Berrington, J. Pelan, Comput. Phys. Commun. 144 (1998) 225] is employed to calculate the resonance levels and widths. We have identified the formant in the figure of the photoionization cross section.

Article –> http://www.sciencedirect.com/science/article/pii/S0969806X16302213

 

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Scaling of collision strengths for highly-excited states of ions of the H- and He-like sequences

L. Fernández-Menchero (1,2), G. Del Zanna (3), N. R. Badnell (1)

(1) Department of Physics, University of Strathclyde. Glasgow G4 0NG, UK. (2) Department of Physics and Astronomy, Drake University, 2507 University Avenue. Des Moines, IA 50311, USA. (3) Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, UK.

Emission lines from highly-excited states (n ≥ 5) of H- and He-like ions have been detected in astrophysical sources and fusion plasmas. For such excited states, R-matrix or distorted wave calculations for electron-impact excitation are very limited, due to the large size of the atomic basis set needed to describe them. Calculations for n ≥ 6 are also not generally available. We study the behavior of the electron-impact excitation collision strengths and effective collision strengths for the most important transitions  used to model electron collision dominated astrophysical plasmas, solar, for example. We investigate the dependence on the relevant parameters: the principal quantum number n or the nuclear charge Z. We also estimate the importance of coupling to highly-excited states and the continuum by comparing the results of different sized calculations. We provide analytic formulae to calculate the electron-impact excitation collision strengths and effective collision strengths to highly-excited states (n ≥ 8) of H- and He-like ions. These extrapolated effective collision strengths can be used to interpret astrophysical and fusion plasma via collisional-radiative  modelling.

Article –>  https://arxiv.org/abs/1607.02487

 

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