Non-LTE line formation of Fe in late-type stars – III. 3D non-LTE analysis of metal-poor stars

Amarsi, A. M.; Lind, K.; Asplund, M.; Barklem, P. S.; Collet, R.

AA(Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia anish.amarsi@anu.edu.au), AB(Max Planck Institute für Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany; Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden), AC(Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia), AD(Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden), AE(Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark)

As one of the most important elements in astronomy, iron abundance determinations need to be as accurate as possible. We investigate the accuracy of spectroscopic iron abundance analyses using archetypal metal-poor stars. We perform detailed 3D non-LTE radiative transfer calculations based on 3D hydrodynamic STAGGER model atmospheres, and employ a new model atom that includes new quantum-mechanical neutral hydrogen collisional rate coefficients. With the exception of the red giant HD122563, we find that the 3D non-LTE models achieve Fe I/Fe II excitation and ionization balance as well as not having any trends with equivalent width to within modelling uncertainties of 0.05 dex, all without having to invoke any microturbulent broadening; for HD122563 we predict that the current best parallax-based surface gravity is overestimated by 0.5 dex. Using a 3D non-LTE analysis, we infer iron abundances from the 3D model atmospheres that are roughly 0.1 dex higher than corresponding abundances from 1D MARCS model atmospheres; these differences go in the same direction as the non-LTE effects themselves.We make available grids of departure coefficients, equivalent widths and abundance corrections, calculated on 1D MARCS model atmospheres and horizontally- and temporally-averaged 3D STAGGER model atmospheres.

Article -> http://esoads.eso.org/doi/10.1093/mnras/stw2077

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General model of depolarization and transfer of polarization of singly ionized atoms by collisions with hydrogen atoms

Derouich, M. (Astronomy Department, Faculty of Science, King Abdulaziz University, 21589 Jeddah, Saudi Arabia)

Simulations of the generation of the atomic polarization is necessary for interpreting the second solar spectrum. For this purpose, it is important to rigorously determine the effects of the isotropic collisions with neutral hydrogen on the atomic polarization of the neutral atoms, ionized atoms and molecules. Our aim is to treat in generality the problem of depolarizing isotropic collisions between singly ionized atoms and neutral hydrogen in its ground state. Using our numerical code, we computed the collisional depolarization rates of the p-levels of ions for large number of values of the effective principal quantum number n* and the Unsöld energy Ep. Then, genetic programming has been utilized to fit the available depolarization rates. As a result, strongly non-linear relationships between the collisional depolarization rates, n* and Ep are obtained, and are shown to reproduce the original data with accuracy clearly better than 10%. These relationships allow quick calculations of the depolarizing collisional rates of any simple ion which is very useful for the solar physics community. In addition, the depolarization rates associated to the complex ions and to the hyperfine levels can be easily derived from our results. In this work we have shown that by using powerful numerical approach and our collisional method, general model giving the depolarization of the ions can be obtained to be exploited for solar applications.

Article -> http://esoads.eso.org/abs/2017NewA…51…32D

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Experimental radiative lifetimes for highly excited states and calculated oscillator strengths for lines of astrophysical interest in singly ionized cobalt (Co II)

P. Quinet (1,2) V. Fivet (1), P. Palmeri (1), L. Engstrom (3), H. Hartman (4;5), H. Lundberg (3) and H. Nilsson (5)

(1) Physique Atomique et Astrophysique, Universite de Mons, Belgium
(2) IPNAS, Universite de Liege, Sart Tilman, Belgium
(3) Department of Physics, Lund University, Sweden
(5) Materials Science and Applied Mathematics, Malmö University, Sweden
(4) Lund Observatory, Lund University, Sweden

This work reports new experimental radiative lifetimes and calculated oscillator strengths for transitions of astrophysical interest in singly ionized cobalt. More precisely, nineteen radiative lifetimes in Co+ have been measured with the time-resolved laser-induced

fluorescence technique using one- and two-step excitations. Out of these, seven belonging to the high lying 3d7(4F)4d configuration in the energy range 90697 – 93738 cm-1 are new, and the other twelve from the 3d7(4F)4p configuration with energies between 45972 and 49328 cm-1 are compared with previous measurements.

In addition, a relativistic Hartree-Fock model including core-polarization effects has been employed to compute transition rates. Supported by the good agreement between theory and experiment for the lifetimes, new reliable transition probabilities and oscillator strengths have been deduced for 5080 Co II transitions in the spectral range 114-8744 nm.

Article –> http://mnras.oxfordjournals.org/content/462/4/3912.abstract

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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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