Showing papers by "Igor Bray published in 2019"

Non-LTE analysis of K I in late-type stars

Abstract: Context. Older models of Galactic chemical evolution (GCE) predict [K/Fe] ratios as much as 1 dex lower than those inferred from stellar observations. Abundances of potassium are mainly based on analyses of the 7698 Å resonance line, and the discrepancy between GCE models and observations is in part caused by the assumption of local thermodynamic equilibrium (LTE) in spectroscopic analyses.Aims. We study the statistical equilibrium of K I, focusing on the non-LTE effects on the 7698 Å line. We aim to determine how non-LTE abundances of potassium can improve the analysis of its chemical evolution, and help to constrain the yields of GCE models.Methods. We construct a new model K I atom that employs the most up-to-date atomic data. In particular, we calculate and present inelastic e+K collisional excitation cross-sections from the convergent close-coupling (CCC) and the B -Spline R -matrix (BSR) methods, and H+K collisions from the two-electron model (LCAO). We constructed a fine, extended grid of non-LTE abundance corrections based on 1D MARCS models that span 4000 eff ∕K In concordance with previous studies, we find severe non-LTE effects in the 7698 Å line. The line is stronger in non-LTE and the abundance corrections can reach approximately − 0.7 dex for solar-metallicity stars such as Procyon. We determine potassium abundances in six benchmark stars, and obtain consistent results from different optical lines. We explore the effects of atmospheric inhomogeneity by computing for the first time a full 3D non-LTE stellar spectrum of K I lines for a test star. We find that 3D modeling is necessary to predict a correct shape of the resonance 7698 Å line, but the line strength is similar to that found in 1D non-LTE.Conclusions. Our non-LTE abundance corrections reduce the scatter and change the cosmic trends of literature potassium abundances. In the regime [Fe/H] ≲−1.0 the non-LTE abundances show a good agreement with the GCE model with yields from rotating massive stars. The reduced scatter of the non-LTE corrected abundances of a sample of solar twins shows that line-by-line differential analysis techniques cannot fully compensate for systematic LTE modelling errors; the scatter introduced by such errors introduces a spurious dispersion to K evolution.

Roadmap on photonic, electronic and atomic collision physics: II. Electron and antimatter interactions

Abstract: We publish three Roadmaps on photonic, electronic and atomic collision physics in order to celebrate the 60th anniversary of the ICPEAC conference. In Roadmap II we focus on electron and antimatter interactions. Modern theoretical and experimental approaches provide detailed insight into the many body quantum dynamics of leptonic collisions with targets of varying complexity ranging from neutral and charged atoms to large biomolecules and clusters. These developments have been driven by technological progress and by the needs of adjacent areas of science such as astrophysics, plasma physics and radiation biophysics. This Roadmap aims at looking back along the road, explaining the evolution of the field, and looking forward, collecting contributions from eighteen leading groups from the field.

Non-LTE analysis of K I in late-type stars.

Abstract: Older GCE models predict [K/Fe] ratios as much as 1 dex lower than those inferred from stellar observations. Abundances of potassium are mainly based on analyses of the 7698 $\AA$ resonance line, and the discrepancy between models and observations is in part caused by the LTE assumption. We study the statistical equilibrium of KI, focusing on the non-LTE effects on the $7698 \ \AA$ line. We aim to determine how non-LTE abundances of K can improve the analysis of its chemical evolution, and help to constrain the yields of models. We construct a model atom that employs the most up-to-date data. In particular, we calculate and present inelastic e+K collisional excitation cross-sections from the convergent close-coupling and the $B$-Spline $R$-matrix methods, and H+K collisions from the two-electron model. We constructed a fine grid of non-LTE abundance corrections that span $4000<\teff / \rm{K}<8000$, $0.50<\lgg<5.00$, $-5.00<\feh<+0.50$, and applied the corrections to abundances from the literature. In concordance with previous studies, we find severe non-LTE effects in the $7698 \ \AA$ line, which is stronger in non-LTE with abundance corrections that can reach $\sim-0.7\,\dex$. We explore the effects of atmospheric inhomogeneity by computing a full 3D non-LTE stellar spectrum of KI for a test star. We find that 3D is necessary to predict a correct shape of the resonance 7698 $\AA$ line, but the line strength is similar to that found in 1D non-LTE. Our non-LTE abundance corrections reduce the scatter and change the cosmic trends of literature K abundances. In the regime [Fe/H]$\lesssim-1.0$ the non-LTE abundances show a good agreement with the GCE model with yields from rotating massive stars. The reduced scatter of the non-LTE corrected abundances of a sample of solar twins shows that line-by-line differential analysis techniques cannot fully compensate for systematic modelling errors.

Wave-packet continuum-discretization approach to proton collisions with helium

Abstract: We extend the two-center wave-packet convergent close-coupling method to proton collisions with helium. The target is treated as a three-body system, where correlations between the electrons are taken into account. We apply a frozen-core approximation, where one of the electrons is described by the He+1s orbital, and obtain the helium singlet wave functions as well as the energy levels using a numerical approach. The wave-packet approach is used to discretize the continuum of the target and the hydrogen atom formed after electron capture by the projectile. Convergence of the results is studied in terms of the included projectile- and target-centered states. We present electron-capture and single- and double-ionization cross sections for protons incident on He in the ground state in the energy range from 15 keV to 1 MeV. We also provide partial cross sections for electron capture and direct excitation into the n=2 shell states of hydrogen and helium, respectively. Our results are in good agreement with available experimental data and other calculations, where available.

Fully differential cross sections for single ionization of helium by energetic protons

Abstract: Fully differential cross sections for single ionization of helium by fast proton impact in different kinematical regimes in the scattering plane were recently measured in a high-precision experiment [O. Chuluunbaatar et al., Phys. Rev. A 99, 062711 (2019)] and calculated using the first Born approximation. We use the nonperturbative wave-packet convergent close-coupling approach to calculate this process more accurately in all the kinematical regimes considered in the experiment. The obtained results show that the coupling between channels and multiple-scattering effects, combined with a more accurate treatment of the target structure, significantly improves the agreement between theory and experiment, especially in the apparently most difficult regions away from the so-called Bethe ridge, where the deviation in the positions of the binary peak observed in the experiment and calculated using the first Born approximation is largest. We also present fully differential cross sections in the same kinematical regimes but for incident projectile energies of 500 keV and 2 MeV. Corresponding results for the so-called perpendicular and azimuthal planes are also exhibited.

Proton-beam stopping in hydrogen

Abstract: The stopping cross section for protons passing through hydrogen is calculated for the energy range between 10 keV and 3 MeV. Both the positive and neutral charge states of the projectile are accounted for. The two-center convergent close-coupling method is used to model proton collisions with hydrogen. In this approach, electron-capture channels are explicitly included by expanding the scattering wave function in a basis of both target and projectile pseudostates. Hydrogen collisions with hydrogen are modeled using two methods: the single-center convergent close-coupling approach is used for the calculation of one-electron processes, while two-electron processes are calculated using the Born approximation. The aforementioned approaches are also applied to the calculation of the charge-state fractions. These are then used to combine the proton-hydrogen and hydrogen-hydrogen stopping cross sections to yield the total stopping cross section for protons passing through hydrogen.

Laser-driven production of the antihydrogen molecular ion

Abstract: The feasibility of producing the molecular antihydrogen anion ${{\overline{\mathrm{H}}}_{2}}^{\ensuremath{-}}$ in the laboratory is investigated. Utilizing reaction rates calculated here involving the interaction of laser excited-state antihydrogen atoms held in magnetic minimum traps, key processes are identified that could lead to anion production, as well as competing effects leading to anti-atom loss. These are discussed in the context of present-day and near-future experimental capabilities.

Electron-impact dissociation of vibrationally-excited molecular hydrogen into neutral fragments

Abstract: We present convergent close-coupling (CCC) calculations of electron-impact dissociation of vibrationally-excited molecular hydrogen into neutral fragments. This work follows from our previous results for dissociation of molecular hydrogen in the ground vibrational level [Scarlett et al., Eur. Phys. J. D 72, 34 (2018)], which were obtained from calculations performed in a spherical coordinate system. The present calculations, performed utilizing a spheroidal formulation of the molecular CCC method, reproduce the previous dissociation cross sections for the ground vibrational level, while allowing the extension to scattering on excited levels.

Spin asymmetry in electron-impact ionization

Abstract: We investigate electron-impact single ionization of He-like ions of charge state $q=0,\ensuremath{\cdots},6$ in the initial ground ($1\phantom{\rule{0.16em}{0ex}}^{1}S$) and metastable ($2\phantom{\rule{0.16em}{0ex}}^{3}S$) states. Good agreement between theory and experiment is established for the total cross sections, and then the scaling behavior with $q$ is considered as a function of the ratio $u={E}_{i}/{E}_{I}\ensuremath{\le}10$ of the incident electron energy ${E}_{i}$ and the ionization threshold ${E}_{I}$. While the expected ${E}_{I}^{2}$ scaling of the cross sections is confirmed, we also find that at each scaled energy $u$ the spin asymmetry (for the $2\phantom{\rule{0.16em}{0ex}}^{3}S$ state) converges rapidly to a nonzero constant with increasing $q$. This indicates that, despite ${E}_{I}$ increasing with ${q}^{2}$, exchange effects remain undiminished on the broad scaled-energy range considered. We suggest that this physical behavior is more universal than just for the He-like ions.

Effect of cascade transitions on the polarization of light emitted after electron-impact excitation of Zn by spin-polarized electrons

Abstract: We investigate the possible effect of cascade transitions from the $(4s5p)\phantom{\rule{0.16em}{0ex}}{}^{3}{P}_{0,1,2}$ states to the $(4s5s)\phantom{\rule{0.16em}{0ex}}{}^{3}{S}_{1}$ state of Zn. The polarization of the light emitted in the subsequent decay to the $(4s4p)\phantom{\rule{0.16em}{0ex}}{}^{3}{P}_{0,1,2}$ states has been the subject of recent controversy, with significant disagreement between the experimental data reported by Pravica et al. [Phys. Rev. A 83, 040701 (2011)] and by Clayburn and Gay [Phys. Rev. Lett. 119, 093401 (2017)] in the cascade-free region below $\ensuremath{\approx}7.6$ eV incident energy and relatively good agreement above. The cross sections for excitation of the $(4s5p)\phantom{\rule{0.16em}{0ex}}{}^{3}{P}_{0,1,2}$ states, as well as higher-lying triplet states, and the linear polarization of the cascade radiation seem too small to produce a significant alignment of the $(4s5s)\phantom{\rule{0.16em}{0ex}}{}^{3}{S}_{1}$ state, thereby raising additional questions regarding the origin of the relatively large linear polarizations measured above the cascade threshold.

Convergent close-coupling calculations of positron scattering on H −

Abstract: The convergent close-coupling method has been applied to positron scattering by the hydrogen negative ion. Convergence of the cross sections is achieved and internal consistency of the method is verified using both the single- and two-center approaches. Calculations were performed using accurate target wave functions obtained with the multicore approach. Obtained results for Ps formation and breakup cross sections are compared with previous calculations.