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

   https://doi.org/10.1051/0004-6361/201935156  

   Reggiani, Henrique ; Amarsi, Anish M. ; Lind, Karin ; Barklem, Paul S. ; Zatsarinny, Oleg ; Bartschat, Klaus ; Fursa, Dmitry V. ; Bray, Igor ; Spina, Lorenzo ; Meléndez, Jorge ( July 2019 , Astronomy & Astrophysics)

   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 < T eff ∕K < 8000, 0.50 < log g < 5.00, − 5.00 < [Fe/H] < + 0.50, and applied the corrections to potassium abundances extracted from the literature. Results. 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. 

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2. Benchmark PhotoIonization Cross-Sections of Neutral Scandium from the Ground and Excited States

   https://doi.org/10.3390/atoms9040083  

   Tayal, Swaraj S. ; Zatsarinny, Oleg ( December 2021 , Atoms)

   The B-spline R-matrix method has been used to investigate cross-sections for photoionization of neutral scandium from the ground and excited states in the energy region from the 3d and 4s valence electron ionization thresholds to 25 eV. The initial bound states of Sc and the final residual Sc+ ionic states have been accurately calculated by combining the multiconfiguration Hartree-Fock method with the frozen-core close-coupling approach. The lowest 20 bound states of Sc I belonging to the ground 3d4s2 and excited 3d24s, 3d24p, 3d4s4p, 4s24p, and 3d3 configurations have been considered as initial states. The 81 LS final ionic states of Sc+ belonging to the terms of 3p63d2, 3p63d4l (l = 0–3), 3p63d5l (l = 0–3), 3p63d6s, 3p64s2, 3p64s4l (l = 0–3), 3p64s5l (l = 0–1), and 3p64p2 configurations have been included in the final-state close-coupling expansion. The cross-sections are dominated by complicated resonance structures in the low energy region converging to several Sc+ ionic thresholds. The inclusion of all these final ionic states has been noted to significantly impact the near-threshold resonance structures and background cross-sections. The important scattering channels for leaving the residual ion in various final states have been identified, and the 3d electron ionization channels have been noted to dominate the cross-sections at higher photon energies. 

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3. Electron-impact Excitation of Pt i–iii: The Importance of Metastables and Collision Processes in Neutron Star Merger and Laboratory Plasmas

   https://doi.org/10.3847/1538-4365/ace5a1  

   Bromley, S. J. ; McCann, M. ; Loch, S. D. ; Ballance, C. P. ( September 2023 , The Astrophysical Journal Supplement Series)

   The detection of a gravitational-wave signal and subsequent electromagnetic transient from a neutron star merger in 2017 is consistent with expectations of neutron star mergers as anr-process element production site. Within the first few days post-merger, the kilonova spectra are consistent with a blackbody illuminating a mix of heavy,r-process elements. With increasing time, the kilonova transitions to the non-LTE regime where the level populations and ionization balance are determined by both collisional and photoprocesses. Detailed cross section data for electron-impact processes involving the relevant species are often not available. In such circumstances, it is reasonable to use approximate methods as baseline data for use in spectral modeling, and it is useful to evaluate the accuracy of such methods against more sophisticated collision calculations when possible. We describe new calculations of the electron-impact excitation cross sections of Pti–iIiusing the DARCR-matrix codes. Using collisional-radiative models, we show that, at plasma conditions expected in kilonovae, the expressions of van Regemorter and Axelrod are insufficient for producing electron-impact excitation data for complex, heavy species such as the low charge states of Pt. Through comparisons with data generated with the relativistic distorted wave approach, as implemented in the Flexible Atomic Code, we show the distorted wave method produces cross section data that, when incorporated into spectral models, predicts strong spectral feature distributions similar in intensity to those from models built on data computed with theR-matrix approach for the considered ions and plasma conditions.

    

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4. Partial Photoionization Cross Sections of Chromium from the Ground and Excited States

   https://doi.org/10.3390/atoms8030051  

   Zatsarinny, Oleg ; Tayal, Swaraj ( September 2020 , Atoms)

   null (Ed.)

   Partial and total photoionization cross sections of iron-peak elements are important for the determination of abundances in late-type stars and nebular objects. We have investigated photoionization of neutral chromium from the ground and excited states in the low energy region from the first ionization threshold at 6.77 eV to 30 eV. Accurate descriptions of the initial bound states of Cr I and the final residual Cr II ionic states have been obtained in the multiconfiguration Hartree-Fock method together with adjustable configuration expansions and term-dependent non-orthogonal orbitals. The B-spline R-matrix method has been used for the calculation of photoionization cross sections. The 194 LS final ionic states of Cr II 3d44s, 3d34s2, 3d5, 3d44p, and 3d34s4p principal configurations have been included in the close-coupling expansion. The inclusion of all terms of these configurations has significant impact on the near-threshold resonance structures as well as on the nonresonant background cross sections. Total photoionization cross sections from the ground 3d54sa7S and excited 3d54sa5S, 3d44s2a5D, 3d54pz5P, and 3d44s4py5P states of Cr I have been compared with other available R-matrix calculation to estimate the likely uncertainties in photoionization cross sections. We analyzed the partial photoionization cross sections for leaving the residual ion in various states to identify the important scattering channels, and noted that 3d electron ionization channel becomes dominant at higher energies. 

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5. Projectile Coherence Effects in Twisted Electron Ionization of Helium

   https://doi.org/10.3390/atoms11050079  

   Harris, A. L. ( May 2023 , Atoms)

   Over the last decade, it has become clear that for heavy ion projectiles, the projectile’s transverse coherence length must be considered in theoretical models. While traditional scattering theory often assumes that the projectile has an infinite coherence length, many studies have demonstrated that the effect of projectile coherence cannot be ignored, even when the projectile-target interaction is within the perturbative regime. This has led to a surge in studies that examine the effects of the projectile’s coherence length. Heavy-ion collisions are particularly well-suited to this because the projectile’s momentum can be large, leading to a small deBroglie wavelength. In contrast, electron projectiles that have larger deBroglie wavelengths and coherence effects can usually be safely ignored. However, the recent demonstration of sculpted electron wave packets opens the door to studying projectile coherence effects in electron-impact collisions. We report here theoretical triple differential cross-sections (TDCSs) for the electron-impact ionization of helium using Bessel and Laguerre-Gauss projectiles. We show that the projectile’s transverse coherence length affects the shape and magnitude of the TDCSs and that the atomic target’s position within the projectile beam plays a significant role in the probability of ionization. We also demonstrate that projectiles with large coherence lengths result in cross-sections that more closely resemble their fully coherent counterparts.

    

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