arXiv subjects

L. I. Mashonkina

Publications and source records attributed to L. I. Mashonkina.

At least 19 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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arXiv2026

HD 188101: A Spotted B Star with Abundance Anomalies

Based on spectroscopic and photometric observations, we have determined the fundamental parameters of the poorly studied star HD 188101 with a weak magnetic field. Its effective temperature $T_{\rm eff} = 14200 \pm 990$ K and surface gravity log g = 3.70 $\pm$ 0.16 are typical for main-sequence B9 stars. The He, C, O, Mg, Si, Ti, and Sr abundances have been determined by taking into account the departures from local thermodynamic equilibrium. Overabundances of Si, Ti, and Sr relative to their solar abundances have been revealed. The He abundance is lower than the solar one, but the difference is within the error limits. In addition to the photometric variability known from Kepler data, we have found changes in absorption for He I, Mg II, Si II, Si III, Ti II, and Fe II lines, with different He I and Mg II lines giving different abundances for the same phase of observations. The star HD 188101 is shown to belong to the group of chemically peculiar He-weak SiTiSr stars.

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arXiv2025

Distinct barium isotope ratios in CEMP-s and CEMP-rs stars

We present a spectroscopic analysis of ten carbon enhanced metal-poor (CEMP) stars of type CEMP-s and CEMP-rs and determine their NLTE abundances of Ba and Eu, as well as the fractions of the odd Ba isotopes (F_odd). The Ba abundances inferred from the resonance Ba II 4554 and 4934 A lines depend on the adopted Ba isotope mixture. We perform calculations for different F_odd from 0.1 to 1.0 and determine the corresponding abundances from the Ba II resonance lines in each sample star. In addition, we determine the Ba abundances from the Ba II subordinate lines, which are almost independent of F_odd. We then compare the Ba abundances derived from the subordinate lines with those from the Ba II resonance lines. We found different F_odd values in CEMP-s and CEMP-rs stars. CEMP-s stars exhibit F_odd = 0.05$_{-0.03}^{+0.07}$, 0.17$_{-0.14}^{+0.63}$, 0.19$_{-0.14}^{+0.50}$, and 0.19$_{-0.12}^{+0.33}$. The obtained values agree, within the error bars, with the s-process F_odd = 0.10 and the solar F_odd = 0.18. Although the uncertainties are large, in three of four stars, the possibility of Ba isotopes origin in a pure r-process with F_odd = 0.75 can be excluded. CEMP-rs stars show F_odd = 0.34$_{-0.21}^{+0.55}$, 0.36$_{-0.14}^{+0.23}$, 0.44$_{-0.22}^{+0.43}$, 0.53$_{-0.38}^{+0.47}$, and 0.57$_{-0.31}^{+0.43}$, which are higher compared to those in CEMP-s stars. Although the uncertainties are large, in four of five stars, the possibility of a pure s-process origin for the Ba isotopes can be excluded. The obtained values agree, within the error bars, with the predicted i-process F_odd = 0.6 to 0.8. Our analysis of CEMP-rs stars with [Ba/Eu] > 0 argues that their [Ba/Eu] and F_odd cannot be jointly explained by a mixture of material produced by the r- and s-processes. The obtained results argue that the i-process is responsible for the chemical composition of these CEMP-rs stars.

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arXiv2025

Unlocking the mystery of Sr synthesis in the early Galaxy through analysis of barium isotopes in very metal-poor stars

We determine the contributions of the rapid (r) and slow (s) neutron capture processes to the Ba isotope mixture, along with Ba, Eu, and Sr NLTE abundances, in a sample of very metal-poor stars. The selected stars formed before the contribution from the main s-process in low- and intermediate-mass stars became significant. Some of our sample stars are enhanced in Sr, with [Sr/Ba] reaching up to 0.7. These stars gained their high Sr abundance from a poorly understood process, which may appear to be a weak s-process or a weak r-process. Our aim is to uncover the nature of this additional Sr source. The abundances derived from the resonance Ba II lines are influenced by the adopted Ba isotope mixture. We compute Ba isotope mixtures corresponding to different r- to s-process contributions and determine the corresponding abundances from the Ba II resonance lines in each sample star. Additionally, we determine Ba abundances from weak subordinate Ba II lines, which are unaffected by the adopted Ba isotope mixture. We then compare the Ba abundances derived from the subordinate lines with those from the Ba II resonance lines. We find a higher s-process contribution to Ba isotopes in stars with greater [Sr/Eu] and [Sr/Ba] overabundances, suggesting that the additional Sr synthesis was due to the early s-process occurring in massive stars. We estimate the [Sr/Ba] ratio produced by the early s-process and obtain [Sr/Ba]_earlyS = 1.1 +- 0.2. Regarding the potential synthesis of Sr and Ba in the i-process in massive stars, our results argue that there was no detectable contribution from this process within the error bars in our sample stars. In the early Galaxy, before significant main s-process enrichment, barium and strontium were produced primarily by the main r-process and the early s-process, which occurred in rapidly rotating massive stars.

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arXiv2024

HR-GO I: Comprehensive NLTE abundance analysis of the Cetus stream

Dwarf galaxy streams encode vast amounts of information essential to understanding early galaxy formation and nucleosynthesis channels. Due to the variation in the timescales of star formation history in their progenitors, stellar streams serve as `snapshots' that record different stages of galactic chemical evolution. This study focusses on the Cetus stream, stripped from a low-mass dwarf galaxy. We carried out a comprehensive analysis of the chemical composition of 22 member stars based on their high-resolution spectra. We derived abundances for up to 28 chemical species from C to Dy and, for 20 of them, we account for the departures from local thermodynamic equilibrium. We confirm that the Cetus stream has a mean metallicity of [Fe/H] = $-2.11$ $\pm$ 0.21. All observed Cetus stars are $α$ enhanced with [$α$/Fe] $\simeq$ 0.3. The absence of the $α$-`knee' implies that star formation stopped before iron production in type Ia supernovae (SNe Ia) became substantial. Neutron capture element abundances suggest that both the rapid (r-) and the main slow (s-) processes contributed to their origin. The decrease in [Eu/Ba] from a typical r-process value of [Eu/Ba] = 0.7 to 0.3 with increasing [Ba/H] indicates a distinct contribution of the r- and s-processes to the chemical composition of different Cetus stars. For barium, the r-process contribution varies from 100 % to 20 % in different sample stars, with an average value of 50 %. Our abundance analysis indicates that the star formation in the Cetus progenitor ceased after the onset of the main s-process in low- to intermediate-mass asymptotic giant branch stars but before SNe Ia played an important role. A distinct evolution scenario is revealed by comparing the abundances in the Ursa Minor dwarf spheroidal galaxy, showing the diversity in the chemical evolution of low-mass dwarf galaxies.

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arXiv2024

Chemical Evolution of R-process Elements in Stars (CERES). III. Chemical abundances of neutron capture elements from Ba to Eu

The chemical abundances of elements such as barium and the lanthanides are essential to understand the nucleosynthesis of heavy elements in the early Universe as well as the contribution of different neutron capture processes (for example slow versus rapid) at different epochs. The Chemical Evolution of R-process Elements in Stars (CERES) project aims to provide a homogeneous analysis of a sample of metal-poor stars ( [Fe/H]\<-1.5) to improve our understanding of the nucleosynthesis of neutron capture elements, in particular the r-process elements, in the early Galaxy. Our data consist of a sample of high resolution and high signal-to-noise ratio UVES spectra. The chemical abundances were derived through spectrum synthesis, using the same model atmospheres and stellar parameters as derived in the first paper of the CERES series. We measured chemical abundances or upper limits of seven heavy neutron capture elements (Ba, La, Ce, Pr, Nd, Sm, and Eu) for a sample of 52 metal-poor giant stars. We estimated through the mean shift clustering algorithm that at Ba/H =-2.4 and Fe/H =-2.4 a variation in the trend of X/Ba with X=La,Nd,Sm,Eu, versus Ba/H occurs. This result suggests that, for Ba/H $\<$$-2.4$, Ba nucleosynthesis in the Milky Way halo is primarily due to the $r$-process, while for Ba/H \<-2.4 the effect of the s-process contribution begins to be visible. In our sample, stars with Ba/Eu compatible with a Solar System pure r-process value (hereafter, r-pure) do not show any particular trend compared to other stars, suggesting r-pure stars may form in similar environments to stars with less pure r-process enrichments. Homogeneous investigations of high resolution and signal-to-noise ratio spectra are crucial for studying the heavy elements formation, as they provide abundances that can be used to test nucleosynthesis models as well as Galactic chemical evolution models.

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arXiv2024

Revision of calcium and scandium abundances in Am stars based on NLTE calculations and comparison with diffusion stellar evolution models

The homogeneous data sets for the calcium and scandium abundances accounting for departures from LTE were obtained for a sample of 54 metallic-line (Am) stars. The Ca and Sc abundances were found to correlate with effective temperature Teff, the abundance growth with increasing Teff being higher in stars with surface gravity log g < 4 than in those with log g > 4. No correlation was found between Ca or Sc abundances and the iron abundance or the velocity of axial rotation. Am stars exhibit on average the higher values of [Ca/H] than those of [Sc/H] as well as the abundance ratio [Ca/Sc] = 0.41 +/- 0.30. However, at Teff > 9500 K there is an allusion to the systematic difference between Am stars with surface gravity log g > 4 and log g < 4. The iron excess is nearly the same in the range 7200 K <= Teff <= 10030 K. Evolution diffusion models computed with the code MESA for stars with masses from 1.5 to 2Msun show the surface abundances that are in good agreement with Ca and Fe abundances observed in Am stars of the three open clusters with the age > 600 Myr. Additional mechanisms of chemical separation should be considered for explanation of the Am phenomenon in young stars of the Pleiades cluster. We tested the published diffusion stellar evolution models. The diffusion models by Richer et al. (2000) and Hui-Bon-Hoa et al. (2022) are shown to agree with observations of Am stars in the open clusters at large values of the free turbulence parameter: omega=1000 for Ca and Fe, omega=500 for Sc. There is no model with the mass and age of the Am-type star Sirius that could reproduce its surface abundances from He to Ni. The results presented in the paper may be of importance for understanding the chemical peculiarity of Am stars.

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arXiv2024

Non-LTE abundances of nitrogen in the Sun and reference A-F type stars

A new model atom of nitrogen was developed using the energy levels of N I from laboratory measurements and also predicted in atomic structure calculations and the most up-to-date atomic data for computing radiative and collisional rates of the transitions. Solar abundance $\log\varepsilon_{\odot,N}$(1D NLTE) = 7.92$\pm$0.03 was determined from lines of N I by applying the synthetic spectrum method with the plane-parallel (1D) MARCS model atmosphere and taking into account the departures from local thermodynamic equilibrium (non-LTE = NLTE effects). Having implemented the 3D-corrections of Amarsi et al. (2020), we obtained $\log\varepsilon_{\odot,N}$(NLTE+3D) = 7.88$\pm$0.03 for the Sun. Based on high spectral resolution spectra, the NLTE abundances of nitrogen were derived for 11 unevolved A-F type stars with reliable atmospheric parameters. Non-LTE leads to strengthened N I lines, and the non-LTE effects grow with increasing effective temperature. For each star, non-LTE leads to smaller abundance error compared to the LTE case. For superficially normal A stars, non-LTE removes the enhancements relative to the solar nitrogen abundance obtained in the LTE case. A $λ$ Boo-type star HD 172167 (Vega) also has close-to-solar N abundance. The four Am stars reveal a scatter of the N abundances, from [N/H] = -0.44 to [N/H] = 0.39. The N abundances of the Sun and superficially normal A stars are consistent within 0.09 dex with the nitrogen abundance of the interstellar gas and the early B-type stars.

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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. IV. Segue 1, Triangulum II, and Coma Berenices UFDs

We present atmospheric parameters and abundances for chemical elements from carbon to barium in metal-poor stars in Segue 1 (seven stars), Coma Berenices (three stars), and Triangulum II (one star) ultra-faint dwarf galaxies (UFDs). The effective temperatures rely on new photometric observations in the visible and infra-red bands, obtained with the 2.5 m telescope of the SAI MSU Caucasian observatory. Abundances of up to fourteen chemical elements were derived under the non-local thermodynamic equilibrium (NLTE) line formation, and LTE abundances were obtained for up to five more elements. For the first time we present abundance of oxygen in Seg 1 S1 and S4, silicon in ComaBer S2 and Tri II S40, potassium in Seg 1 S1-S6 and ComaBer S1-S3, and barium in Seg 1 S7. Three stars in Segue 1, two stars in Coma Berenices, and Triangulum II star have very low [Na/Mg] of -1.08 to -1.67 dex, which is usually attributed in the literature to an odd-even effect produced by nucleosynthesis in massive metal-free stars. We interpret this chemical property as a footprint of first stars, which is not blurred due to a small number of nucleosynthesis events that contributed to chemical abundance patterns of the sample stars. Our NLTE abundances of Sr and Ba in Coma Berenices, Segue 1, and Triangulum II report on lower [Sr/Ba] abundance ratio in the UFDs compared to that in classical dwarf spheroidal galaxies and the Milky Way halo. However, in UFDs, just as in massive galaxies, [Sr/Ba] is not constant and it can be higher than the pure r-process ratio. We suggest a hypothesis of Sr production in metal-poor binaries at the earliest epoch of galactic evolution.

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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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arXiv2019

Contribution of type Ia supernovae to the chemical enrichment of the ultra-faint dwarf galaxy Bootes I

For three stars in the ultra-faint dwarf (UFD) galaxy Bootes I we have determined the atmospheric parameters, performed a new reduction of high-resolution spectra from the Subaru archive, and derived the abundances of eight chemical elements without using the LTE assumption. As a result, among the galaxies of its class Bootes I now has the largest sample of stars (11) with a homogeneous set of atmospheric parameters and chemical abundances, and this makes it the most promising for studying the chemical evolution of UFD galaxies. We show that in the range -3<[Fe/H]<-2 for each of the three $α$--process elements, magnesium, calcium, and titanium, a transition from their overabundance relative to iron with [$α$/Fe]$\approx$0.3 to the solar [$α$/Fe] ratio occurs. This most likely suggests the commenced production of iron in type Ia supernovae. The behaviour of the carbon, sodium, nickel, and barium abundances does not differ from that in more massive galaxies, our Galaxy and classical dwarf spheroidal galaxies.

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arXiv2016

NLTE carbon abundance determination in selected A- and B-type stars and the interpretation of C\ione\ emission lines

We constructed a comprehensive model atom for C\ione\ -- C\ii\ using the most up-to-date atomic data available and evaluated the non-local thermodynamic equilibrium (NLTE) line formation for C\ione\ and C\ii\ in classical 1D models representing the atmospheres of A and late B-type stars. Our NLTE calculations predict the emission that appears at effective temperature of 9250 to 10\,500~K depending on log~$g$ in the C\ione\ 8335, 9405\,Å singlet lines and at \Teff~$>$~15\,000~K (log~$g$ = 4) in the C\ione\ 9061 -- 9111\,Å\,, 9603 -- 9658\,Å\, triplet lines. A prerequisite of the emission phenomenon is the overionization-recombination mechanism resulting in a depopulation of the lower levels of C\ione\ to a greater extent than the upper levels. Extra depopulation of the lower levels of the transitions corresponding to the near-infrared lines, is caused by photon loss in the UV lines C\ione\ 2479, 1930, and 1657\,Å. We analysed the lines of C\ione\ and C\ii\ in Vega, HD~73666, Sirius, 21~Peg, $π$~Cet, HD~22136, and $ι$ Her taking advantage of their observed high-resolution spectra. The C\ione\ emission lines were detected in the four hottest stars, and they were well reproduced in our NLTE calculations. For each star, the mean NLTE abundances from lines of the two ionization stages, C\ione\ and C\ii, including the C\ione\ emission lines, were found to be consistent. We show that the predicted C\ione\ emission phenomenon depends strongly on whether accurate or approximate electron-impact excitation rates are applied.

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arXiv2016

A NLTE line formation for neutral and singly-ionised titanium in model atmospheres of the reference A-K stars

We construct a comprehensive model atom for TiI-II using more than 3600 measured and predicted energy levels of TiI and 1800 energy levels of TiII, and quantum mechanical photoionisation cross-sections. Non-local thermodynamical equilibrium (NLTE) line formation for the two ions of titanium is treated through a wide range of spectral types from A to K, including metal-poor stars with [Fe/H] down to -2.6 dex. NLTE leads to systematically depleted total absorption in the TiI lines and to positive abundance corrections. The magnitude of NLTE abundance corrections is smaller compared to the available literature data for the cool (FGK type) atmospheres. NLTE leads to strengthened TiII lines and to negative NLTE abundance corrections. For the first time, we performed the NLTE calculations for TiI-II in the 6500 K < Teff < 13000 K range. The deviations from LTE grow towards higher effective temperature. For four A type stars we derived the NLTE and LTE abundances from lines of TiI and TiII. In LTE an abundance discrepancy of up to 0.22 dex was obtained between TiI and TiII, and it vanishes in NLTE. For other four A-B stars, with only TiII lines observed, NLTE leads to smaller line-to-line scatter compared to LTE. An efficiency of inelastic TiI + HI collisions was empirically estimated from analysis of TiI and TiII lines in 17 cool stars in a wide metallicity range -2.6 < [Fe/H] < 0.0. Consistent NLTE abundances from lines of TiI and TiII were obtained applying classical Drawinian rates for the stars with log g > 4.1, and neglecting inelastic collisions with HI for the VMP giant HD 122563. For the very metal-poor turn-off stars ([Fe/H] < -2 and log g < 4.1), the abundance difference TiI-TiII was obtained to be positive already in LTE and it increases in NLTE. The accurate collisional data for TiI and TiII are desired to find a clue to this problem.

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arXiv2015

Carbon abundances of the reference late-type stars from 1D analysis of atomic C I and molecular CH lines

A comprehensive model atom was constructed for C I using the most up-to-date atomic data. We evaluated non-local thermodynamical equilibrium (NLTE) line formation for neutral carbon in classical 1D models representing atmospheres of late-type stars, where carbon abundance varies from solar value down to [C/H] = $-$3. NLTE leads to stronger C I lines compared with their LTE strength and negative NLTE abundance corrections, $Δ_{\rm NLTE}$. The deviations from LTE are large for the strong lines in the infrared (IR), with $Δ_{\rm NLTE}$ = $-$0.10 dex to $-$0.45 dex depending on stellar parameters, and they are minor for the weak lines in the visible spectral range, with |$Δ_{\rm NLTE}| \le$ 0.03 dex. The NLTE abundance corrections were found to be dependent of the carbon abundance in the model. As the first application of the treated model atom, carbon NLTE abundances were determined for the Sun and eight late-type stars with well-determined stellar parameters that cover the $-2.56 \le$ [Fe/H] $\le -1.02$ metallicity range. Consistent abundances from the visible and IR lines were found for the Sun and the most metal-rich star of our sample, when applying a scaling factor of S_H = 0.3 to the Drawinian rates of C+H collisions. Carbon abundances were also derived from the molecular CH lines and, for each star, they agree with that from the atomic C I lines. We present the NLTE abundance corrections for lines of C I in the grid of model atmospheres applicable to the carbon-enhanced (CEMP) stars.

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arXiv2014

Non-LTE Sodium Abundance in Galactic Thick- and Thin-Disk Red Giants

We evaluate non-local thermodynamical equilibrium (non-LTE) line formation for neutral sodium in model atmospheres of the 79 red giants using the model atom that incorporates the best available atomic data. The non-LTE abundances of Na were determined from Na I 6154, 6161 Å for the 38 stars of the thin disk (15 of them the BaII stars), 15 stars of the thick disk, 13 stars of Hercules stream and 13 transition stars which can be identified with neither thin disk nor thick disk. For Na I 6154, 6161 Å non-LTE abundance corrections amount to -0.06 to -0.24 dex depending on stellar parameters. We found no difference in [Na/Fe] abundance between the thick disk and thin disk and the obtained abundances are close to the solar one. We confirmed a weak excess of [Na/Fe] in BaII stars. Stars of the Hercules stream reveal [Na/Fe] abundances close to the solar one. The obtained results can be used to constrain the nucleosynthesis models for Na.

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arXiv2013

Influence of Departures from LTE on Oxygen Abundance Determination

We performed non-LTE calculations for O I with the plane-parallel model atmospheres for a set of stellar parameters corresponding to A-K type stars. The model atom of Przybilla et al. (2000) was updated using the best theoretical and experimental atomic data available so far. Non-LTE leads to strengthening the O I lines, and the difference between the non-LTE and LTE abundances (non-LTE correction) is negative. The departures from LTE grow toward higher effective temperature and lower surface gravity. In the entire temperature range and log g = 4, the non-LTE correction does not exceed 0.05 dex in absolute value for O I lines in the visible spectral range. The non-LTE corrections are significantly larger for the infrared O I 7771-5 A lines and reach -1.9 dex in the model atmosphere with Teff = 10000 K and log g = 2. To differentiate the effects of inelastic collisions with electrons and neutral hydrogen atoms on the statistical equilibrium (SE) of O I, we derived the oxygen abundance for the three well studied A-type stars Vega, Sirius, and HD 32115. For each star, non-LTE leads to smaller difference between the infrared and visible lines. For example, for Vega, this difference reduces from 1.17 dex in LTE down to 0.14 dex when ignoring LTE. To remove the difference between the infrared and visible lines in A-type stars completely, one needs to reduce the used electron-impact excitation rates by Barklem (2007) by a factor of 4. In the case of Procyon and the Sun, inelastic collisions with H I affect the SE of O I, and agreement between the abundances from different lines is achieved when using the Drawin's formalism to compute collisional rates. The solar mean oxygen abundance from the O I 6300, 6158, 7771-5, and 8446 A lines is log epsilon = 8.74+-0.05, when using the MAFAGS-OS solar model atmosphere and log epsilon = 8.78+-0.03, when applying the 3D corrections from the literature.

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arXiv2009

Nucleosynthesis Modes in the High-Entropy-Wind of Type II Supernovae: Comparison of Calculations with Halo-Star Observations

While the high-entropy wind (HEW) of Type II supernovae remains one of the more promising sites for the rapid neutron-capture (r-) process, hydrodynamic simulations have yet to reproduce the astrophysical conditions under which the latter occurs. We have performed large-scale network calculations within an extended parameter range of the HEW, seeking to identify or to constrain the necessary conditions for a full reproduction of all r-process residuals N_{r,\odot}=N_{\odot}-N_{s,\odot} by comparing the results with recent astronomical observations. A superposition of weighted entropy trajectories results in an excellent reproduction of the overall N_{r,\odot}-pattern beyond Sn. For the lighter elements, from the Fe-group via Sr-Y-Zr to Ag, our HEW calculations indicate a transition from the need for clearly different sources (conditions/sites) to a possible co-production with r-process elements, provided that a range of entropies are contributing. This explains recent halo-star observations of a clear non-correlation of Zn and Ge and a weak correlation of Sr - Zr with heavier r-process elements. Moreover, new observational data on Ru and Pd seem to confirm also a partial correlation with Sr as well as the main r-process elements (e.g. Eu).

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