arXiv subjects

S. A. Yakovleva

Publications and source records attributed to S. A. Yakovleva.

6 recordsLinked to original sources

arXiv2026

Non-LTE corrections for determinations of europium abundances in F-G-K stars in a broad metallicity range

Europium plays a key role in studies of nucleosynthesis in the rapid (r-) process of neutron capture nuclear reactions and the evolution of the r-process element abundances in galaxies. We refine the method for analyzing the Eu~II lines in stellar spectra by updating the Eu~II model atom with recent data on the rate coefficients for inelastic processes in the Eu II + H I collisions. The method was tested by deriving abundances from the Eu~II 4129 A and 6645 A lines in the Sun and five reference stars with well determined atmospheric parameters and high-resolution spectra available. It was shown that abandoning the local thermodynamic equilibrium (LTE) assumption allows the abundances from the two lines to be matched within the error bars, while the difference in the LTE abundances amounts to -0.09 dex for the Sun and -0.07 dex to -0.18 dex for the metal-poor stars. Accounting for the departures from LTE (non-LTE effects) for Eu~II leads to reducing the statistical errors of the derived stellar Eu abundances. The non-LTE abundance corrections for 11 lines of Eu~II were calculated for a grid of the plane-parallel MARCS model atmospheres with effective temperatures, surface gravities, and metallicities relevant to F-G-K type stars. They are publicly available and can be applied for improving stellar Eu abundances in studies of the galactic chemical evolution.

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arXiv2024

3D NLTE modelling of Y and Eu. Centre-to-limb variation and solar abundances

Context. Abundances of s- and r-process elements in Sun-like stars constrain nucleosynthesis in extreme astrophysical events, such as compact binary mergers and explosions of highly magnetised rapidly rotating massive stars. Aims. We measure solar abundances of yttrium (Y) and europium (Eu) using 3D non-local thermal equilibrium (NLTE) models. We use the model to determine the abundance of Y, and also explore the model's ability to reproduce the solar centre-to-limb variation of its lines. In addition, we determine the Eu abundance using solar disc-centre and integrated flux spectra. Methods. We developed an NLTE model of Eu and updated our model of Y with collisional data from detailed quantum-mechanical calculations. We used the IAG spatially resolved high-resolution solar spectra to derive the solar abundances of Y across the solar disc and of Eu for integrated flux and at disc centre using a set of carefully selected lines and a 3D radiation-hydrodynamics model of the solar atmosphere. Results. We find 3D NLTE solar abundances of A(Y)$_{\textrm{3D NLTE}}$=$2.30 \pm 0.03_{\textrm{stat}} \pm 0.07_{\textrm{syst}}$ dex based on observations at all angles and A(Eu)$_{\textrm{3D NLTE}}$=$0.57 \pm 0.01_{\textrm{stat}} \pm 0.06_{\textrm{syst}}$ dex based on the integrated flux and disc-centre intensity. 3D NLTE modelling offers the most consistent abundances across the solar disc, and resolves the problem of severe systematic bias in Y and Eu abundances inherent to 1D LTE, 1D NLTE, and 3D LTE modelling.

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arXiv2022

Non-LTE abundances of zinc in different spectral type stars and the Galactic [Zn/Fe] trend based on quantum-mechanical data on inelastic processes in zinc-hydrogen collisions

We present a new model atom of Zn I-II based on the most up-to-date photoionisation cross-sections, electron-impact excitation rates, and rate coefficients for the Zn I + H I and Zn II + H- collisions. The latter were calculated using the multi-channel quantum asymptotic treatment based on the Born-Oppenheimer approach. Non-LTE analysis was performed for the first time for lines of Zn I and Zn II in the ultraviolet (UV) spectra of two very metal-poor reference stars, HD 84937 and HD 140283. We found consistent non-LTE abundance from the resonance Zn I 2138 A line, the subordinate lines, and the lines of Zn II. In both stars, non-LTE leads to 0.17 dex higher average abundance from Zn I, while, for Zn II lines, non-LTE corrections are minor and do not exceed 0.06 dex. Using lines of Zn I in the high-resolution spectra, we determined the non-LTE abundances for a sample of 80 stars in the -2.5 < [Fe/H] < 0.2 metallicity range. The [Zn/Fe] versus [Fe/H] diagram reveals a dip, with [Zn/Fe] = 0.3 in the most metal-poor stars, a close-to-solar value for [Fe/H] = -1.2, and increasing [Zn/Fe] up to 0.3 in the thick disk stars. The close-to-solar metallicity stars have subsolar [Zn/H] = -0.1, on average. Non-LTE abundances of zinc were derived for the first time for seven reference F to B-type stars. We provide a grid of the non-LTE abundance corrections.

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arXiv2021

The formation of the Milky Way halo and its dwarf satellites: A NLTE-1D abundance analysis. V. The Sextans galaxy

We present a homogeneous set of accurate atmospheric parameters for a sample of eleven very metal-poor (-3.32 <= [Fe/H] <= -2.61) stars in the Sextans dwarf spheroidal galaxy (dSph) and the non-local thermodynamic equilibrium (NLTE) abundances of, at least, seven chemical elements based on high-resolution UVES/VLT and HDS/Subaru spectra. For each star, its effective temperature and surface gravity were derived from the spectral energy distribution and the known distance, and the Fe abundance was obtained as the average from lines of Fe I and Fe II. Based on abundances of Mg, Ca, and Ti, we show that all the investigated stars reveal alpha-enhancements of 0.4~dex to 0.2~dex, and there is a hint of a decline in alpha/Fe for [Fe/H] > -2.8. The Sextans stars are deficient in Ba. The new result is an extremely tight relation between Ba and Mg, suggesting their common origin in massive stars and Ba synthesis in the r-process events on the timescales of standard supernovae. The exception is a C-enhanced star S 15-19 which is strongly enhanced in Ba. This star is unlikely to be a CEMP-s star because of low abundances of Sr and Y ([Y/Fe] < -1) that are produced in the s-process as efficiently as Ba and a non-detection of variation in the radial velocity. No distinctions from the Milky Way halo and the Sculptor and Ursa Minor dSphs were found in a history of early enrichment in Na and Ni, namely, the Sextans stars are deficient in Na and have close-to-solar Ni/Fe.

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arXiv2020

Influence of Collisions with Hydrogen on Titanium Abundance Determinations in Cool Stars

We performed the non-local thermodynamic equilibrium (non-LTE) calculations for Ti I-II with the updated model atom that includes quantum-mechanical rate coefficients for inelastic collisions with hydrogen atoms. We have calculated for the first time the rate coefficients for bound-bound transitions in inelastic collisions of titanium atoms and ions with hydrogen atoms and for the charge-exchange processes: Ti I + H <-> Ti II + H- and Ti II + H <-> Ti III + H-. The influence of these data on non-LTE abundance determinations has been tested for the Sun and four metal-poor stars. For Ti I and Ti II, the application of the derived rate coefficients has led to an increase in the departures from LTE and an increase in the titanium abundance compared to that, obtained with approximate formulas for the rate coefficients. In metal-poor stars, we have failed to achieve consistent non-LTE abundances from lines of two ionization stages. The known in the literature discrepancy in the non-LTE abundances from Ti I and Ti II lines in metal-poor stars cannot be solved by improvement of the rates of inelastic processes in collisions with hydrogen atoms in non-LTE calculations with classical model atmospheres.

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arXiv2020

Observational constraints on the origin of the elements II. 3D non-LTE formation of Ba ii lines in the solar atmosphere

Context. The pursuit of more realistic spectroscopic modelling and consistent abundances has led us to begin a new series of papers designed to improve current solar and stellar abundances of various atomic species. To achieve this, we have began updating the three-dimensional (3D) non-local thermodynamic equilibrium (non-LTE) radiative transfer code, Multi3D, and the equivalent one-dimensional (1D) non-LTE radiative transfer code, MULTI. Aims. We examine our improvements to these codes by redetermining the solar barium abundance. Barium was chosen for this test as it is an important diagnostic element of the s-process in the context of galactic chemical evolution. New Ba II + H collisional data for excitation and charge exchange reactions computed from first principles had recently become available and were included in the model atom. The atom also includes the effects of isotopic line shifts and hyperfine splitting. Method. A grid of 1D LTE barium lines were constructed with MULTI and fit to the four Ba II lines available to us in the optical region of the solar spectrum. Abundance corrections were then determined in 1D non-LTE, 3D LTE, and 3D non-LTE. A new 3D non-LTE solar barium abundance was computed from these corrections. Results. We present for the first time the full 3D non-LTE barium abundance of $A({\rm Ba})=2.27\pm0.02\pm0.01$, which was derived from four individual fully consistent barium lines. Errors here represent the systematic and random errors, respectively.

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