The Opacity Project: computational methods

James Colgan

Los Alamos National Laboratory, Los Alamos, NM 87545, USA

This viewpoint relates to an article by K A Berrington et al (1987 J. Phys. B: At. Mol. Opt. Phys. 20 6379–97) and was published as part of a series of viewpoints celebrating 50 of the most influential papers published in the Journal of Physics series, which is celebrating its 50th anniversary. ==> Complete viewpoint


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Systematic non-LTE study of the -2.6 < [Fe/H] < 0.2 F and G dwarfs in the solar neighbourhood. II. Abundance patterns from Li to Eu

Zhao, G.; Mashonkina, L.; Yan, H. L.; Alexeeva, S.; Kobayashi, C.; Pakhomov, Yu.; Shi, J. R.; Sitnova, T.; Tan, K. F.; Zhang, H. W.; Zhang, J. B.; Zhou, Z. M.; Bolte, M.; Chen, Y. Q.; Li, X.; Liu, F.; Zhai, M.

For the first time, we present an extensive study of stars with individual non-local thermodynamic equilibrium (NLTE) abundances for 17 chemical elements from Li to Eu in a sample of stars uniformly distributed over the -2.62 < [Fe/H] < +0.24 metallicity range that is suitable for the Galactic chemical evolution research. The star sample has been kinematically selected to trace the Galactic thin and thick disks and halo. We find new and improve earlier results as follows. (i) The element-to-iron ratios for Mg, Si, Ca, and Ti form a MP plateau at a similar height of 0.3~dex, and the knee occurs at common [Fe/H] ~ -0.8. The knee at the same metallicity is observed for [O/Fe], and the MP plateau is formed at [O/Fe] = 0.61. (ii) The upward trend of [C/O] with decreasing metallicity exists at [Fe/H] < -1.2, supporting the earlier finding of Akerman et al. (iii) An underabundance of Na relative to Mg in the [Fe/H] < -1 stars is nearly constant, with the mean [Na/Mg] ~ -0.5. (iv) The K/Sc, Ca/Sc, and Ti/Sc ratios form well-defined trends, suggesting a common site of the K-Ti production. (v) Sr follows the Fe abundance down to [Fe/H] ~ -2.5, while Zr is enhanced in MP stars. (vi) The comparisons of our results with some widely used Galactic evolution models are given. The use of the NLTE element abundances raises credit to the interpretation of the data in the context of the chemical evolution of the Galaxy.

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3D NLTE analysis of the most iron-deficient star, SMSS0313-6708

Nordlander, Thomas; Amarsi, Anish M.; Lind, Karin; Asplund, Martin; Barklem, Paul S.; Casey, Andy R.; Collet, Remo; Leenaarts, Jorrit

Models of star formation in the early universe depend on the details of accretion, fragmentation and radiative feedback. Different simulations predict different initial mass functions of the first stars, ranging from predominantly low mass (0.1-10 Msol), to massive (10-100 Msol), or even supermassive (100-1000 Msol). The mass distribution of the first stars should lead to unique chemical imprints on the low-mass second and later generation metal-poor stars still in existence. The chemical composition of SMSS0313-6708, which has the lowest abundances of Ca and Fe of any star known, indicates it was enriched by a single massive supernova. However, even weak spectral lines may be affected by strong 3D and NLTE effects in metal-poor stars. If these effects are ignored or treated incorrectly, errors in the inferred abundances may significantly bias the inferred properties of the polluting supernovae. We redetermine the chemical composition of SMSS0313-6708 using 3D NLTE radiative transfer to obtain accurate abundances for Li, Na, Mg, Al, Ca and Fe. The model atoms employ realistic collisional rates, with no calibrated free parameters. We find significantly higher abundances in 3D NLTE than 1D LTE by 0.8 dex for Fe, and 0.5 dex for Mg, Al and Ca, while Li and Na are unaffected to within 0.03 dex. In particular, our upper limit for [Fe/H] is now a factor ten larger, at [Fe/H] < -6.53 (3 sigma), than previous estimates based on <3D> NLTE (i.e. using averaged 3D models). This higher estimate is due to a conservative upper limit estimation, updated NLTE data, and 3D-<3D> NLTE differences, all of which lead to a higher abundance determination. We find that the revised chemical composition of SMSS0313-6708 matches supernova yields for massive progenitors of 20-60 Msol exploding with low energies (1-2 x 10^51 erg), as well as progenitors of 10 Msol with very low explosion energies (< 10^51 erg).

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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, 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.

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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.

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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.

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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.)


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.

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