arXiv subjects

S. A. Korotin

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

At least 19 recordsLinked to original sources

arXiv2026

Phosphorus in cool stars of various metallicities: The non-local thermodynamic equilibrium consideration

The phosphorus abundance distribution in field stars as a function of metallicity reveals a complex pattern. The LTE data for [P/Fe] in the low-metallicity range are sparse and scattered around [P/Fe]~ 0 dex. Near [Fe/H]~ -2 dex, the relative abundance [P/Fe] increases and reaches a maximum value of around [Fe/H]~ -1 dex. In this domain, P-rich stars and (super)phosphorus-rich stars are observed; the [P/Fe] value can exceed 1 dex. Until now, no attempts have been made to study the NLTE effects on the ultraviolet and infrared phosphorus lines in spectra of cool stars to test the robustness of the observed LTE phosphorus abundance distribution. We developed an atomic model of P I that can be used to analyze phosphorus lines in the spectra of cool dwarfs and giants in the NLTE approximation. The model was tested using the solar flux and intensity spectra, as well as the spectra of Procyon and sigma Boo. Profiles of 14 phosphorus lines in the infrared regions and equivalent widths were analyzed. Our NLTE phosphorus abundance in the Sun is (P/H)=5.35+/-0.04 dex. Using our NLTE model, we selected 12 ultraviolet and infrared phosphorus lines and calculated a grid of NLTE corrections for the following parameter ranges: Teff from 4000 to 6750 K, step 250 K; log g from 1 to 5 dex, step 1 dex; and Vt = 2 km/s, [Fe/H] from -3 to +0.5 dex, step 0.5 dex. The NLTE corrections were calculated for phosphorus abundance ratios of [P/Fe]=-0.4, 0.0, +0.4 dex. For the Sun, the NLTE correction is -0.08 dex. The grid of the NLTE corrections, as well as the direct line profile synthesis, were used to refine the literature data on the phosphorus abundance in metal-poor, intermediate-deficient, and solar-metallicity stars. NLTE corrections do not qualitatively alter the overall phosphorus abundance distribution over a wide metallicity range, and do not change the characteristic pattern of phosphorus-rich stars.

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arXiv2025

Influence of departures from LTE on determinations of the sulfur abundances in A-K type stars

The influence of departures from local thermodynamic equilibrium (LTE) on neutral sulfur lines is considered. A grid of corrections is proposed to take into account the influence of departures from LTE for neutral sulfur lines in the visible and infrared spectral regions, including the H-band. The grid is calculated using the atomic model of sulfur incorporating the most up-to-date collision rates with electrons and hydrogen. The inclusion of levels and transitions of ionized sulfur in the atomic model made it possible to expand the range of effective temperatures of stellar photospheres in the grid up to 10000 K. The atomic model was tested in determining the sulfur abundance of 13 stars and showed its adequacy in a wide range of fundamental stellar parameters. In the spectra of all test stars, the sulfur lines are fitted with similar abundances of the element, regardless of the degree of influence of the effects of deviation from LTE on a particular spectral line. For lines of several multiplets, the wavelengths and oscillator strengths were refined. A list of S I lines recommended for determining sulfur abundance has been created.

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arXiv2024

Peculiarities of the chemical enrichment of metal-poor Stars in the Milky Way Galaxy

The oldest stars in the Milky Way are metal-poor with [Fe/H] < -- 1.0, displaying peculiar elemental abundances compared to solar values. The relative variations in the chemical compositions among stars is also increasing with decreasing stellar metallicity, allowing for the pure signature of unique nucleosynthesis processes to be revealed. In this work, we report the atmospheric parameters, main dynamic properties, and the abundances of four metal-poor stars: HE 1523--0901, HD 6268, HD 121135, and HD 195636 (--1.5 > [Fe/H] >--3.0). The abundances were derived from spectra obtained with the HRS echelle spectrograph at the SALT, using both LTE and NLTE approaches. Based on their kinematical properties, we show that HE 1523--0901 and HD 195636 are halo stars with typical high velocities. In particular, HD 121135 displays a peculiar kinematical behaviour, making it unclear whether it is a halo or an accreted star. Furthermore, HD 6268 is possibly a rare prototype of very metal-poor thick disk stars. The abundances derived for our stars are compared with theoretical stellar models and with other stars with similar metallicity values from the literature. HD 121135 is Al-poor and Sc-poor, compared to stars observed in the same metallicity range (--1.62 > [Fe/H] >--1.12). The most metal-poor stars in our sample, HE 1523 -- 0901, HD 6268, and HD 195636, exhibit anomalies that are better explained by supernova models from fast-rotating stellar progenitors for elements up to the Fe group. Compared to other stars in the same metallicity range, their common biggest anomaly is represented by the low Sc abundances. If we consider the elements beyond Zn, HE 1523--0901 can be classified as an r-II star, HD 6268 as an r-I candidate, and HD 195636 and HD 121135 exhibiting a borderline r-process enrichment between limited-r and r-I star.

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arXiv2024

Lithium Cepheid V708 Car with an unusual chemical composition

The purpose of this work is to spectroscopically analyse the classical Cepheid V708 Car. A preliminary check of the spectrum of V708 Car showed that this is a lithium-rich supergiant. We also found that V708 Car has an unusual chemical composition in that the abundances of various elements correlate with their condensation temperatures. We tried to find an explanation of this feature, which is unusual for classical Cepheids. For the spectroscopic analysis, we used methods based on the assumption of local and non-local thermodynamic equilibrium. We determined the fundamental parameters of our program star V708 Car. This long-period Cepheid has a mass of about 12 M$_{\odot}$. We derived the abundances of 27 chemical elements in this star. They are clearly correlated with their condensation temperature: the higher the condensation temperature, the lower the abundance (there are exceptions for sodium and barium, however). We explain this peculiar chemical composition of the V708 Car atmosphere by the gas-dust separation in the envelope of this star. A similar mechanism leads to the observed peculiarities of the chemical composition of $λ$ Boo, W Vir, and asymptotic giant branch stars.

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arXiv2023

Unique distant classical Cepheid OGLE GD-CEP-1353 with anomalously high abundances of s- and r-process elements

While looking for recently discovered distant Cepheids with an interesting chemical composition, we noticed one star (OGLE GD-CEP-1353) with extremely large equivalent widths of spectral lines of heavy elements. The aim of this work is to perform an abundance analysis, and to find a possible explanation for the found chemical anomaly. Quantitative analysis of the equivalent widths and synthetic spectrum synthesis were used to derive abundances in this star. Both local and nonlocal thermodynamic equilibrium (LTE and NLTE) approximations were used in our analysis. Abundances of 28 chemical elements from carbon to thorium were derived. While light and iron peak elements show abundances typical for distant Cepheids (located in the outer disk), the s-process elements are overabundant about one dex. r-process elements are slightly less overabundant. This makes the star a unique Cepheid of our Galaxy.

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arXiv2023

Non-LTE abundance analysis of A-B stars with low rotational velocities. II. Do A-B stars with normal abundances exist?

We present chemical composition and fundamental parameters (the effective temperature, surface gravity and radius) for four sharp-lined A-type stars $γ$ Gem (HD 41705), o Peg (HD 214994), $θ$ Vir (HD 114330) and $ν$ Cap (HD 193432). Our analysis is based on a self-consistent model fitting of high-resolution spectra and spectrophotometric observations over a wide wavelength range. We refined the fundamental parameters of the stars with the SME package and verified their accuracy by comparing with the spectral energy distribution and hydrogen line profiles. We found Teff/log g = 9190+/-130 K/3.56+/-0.08, 9600+/-50 K/3.81+/-0.04, 9600+/-140 K/3.61+/-0.12, and 10200+/-220 K/3.88+/-0.08 for $γ$ Gem, o Peg, $θ$ Vir and $ν$ Cap, respectively. Our detailed abundance analysis employs a hybrid technique for spectrum synthesis based on classical model atmospheres calculated in local thermodynamic equilibrium (LTE) assumption together with the non-LTE (NLTE) line formation for 18 of 26 investigated species. Comparison of the abundance patterns observed in A stars of different types (normal A, Am, Ap) with similar fundamental parameters reveals significant abundance diversity that cannot be explained by the current mechanisms of abundance peculiarity formation in stellar atmospheres. We found a rise of the heavy element (Zn, Sr, Y, Zr, Ba) abundance excess up to +1 dex with Teff increasing from 7200 to 10000 K, with a further decrease down to solar value at Teff = 13000 K, indicating that stars with solar element abundances can be found among late B-type stars.

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arXiv2023

Abundance of radioactive technetium in Przybylski's star revisited

We have searched for lines of the radioactive element technetium (Tc) in the spectrum of Przybylski's star (HD101065). The nuclei of this chemical element are formed in the slow process of capturing thermalized neutrons. The possible lines of Tc~I are heavily blended. We have synthesized the profile of one resonance line at 4297.06 Å~, which is also a part of the complex blend, and we arrived at a decision that it is not visible in the spectrum (as was first noted by Ryabchikova), casting doubt on the existence of technetium in the atmosphere of the Przybylski's star. Therefore, based on our calculated combined profile, which has been adjusted to the observed blend profile at ~4297.2 A (that may possibly contain the resonance technetium line 4297.06 A), we reduce the maximum technetium abundance to $\logε$(Tc/H) = 2.5. This value can be considered only as an upper limit of the technetium abundance in the Przybylski's star.

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arXiv2020

The CARMENES search for exoplanets around M dwarfs: Rubidium abundances in nearby cool stars

In this study, abundances of the neutron-capture elements Rb, Sr, and Zr are derived, for the first time, in a sample of nearby M dwarfs. We focus on stars in the metallicity range -0.5<[Fe/H]<+0.3, an interval poorly explored for Rb abundances in previous analyses. To do this we use high-resolution, high-signal-to-noise-ratio, optical and near-infrared spectra of 57 M dwarfs observed with CARMENES. The resulting [Sr/Fe] and [Zr/Fe] ratios for most M dwarfs are almost constant at about the solar value, and are identical to those found in GK dwarfs of the same metallicity. However, for Rb we find systematic underabundances ([Rb/Fe]<0.0) by a factor two on average. Furthermore, a tendency is found for Rb-but not for other heavy elements (Sr, Zr) -to increase with increasing metallicity such that [Rb/Fe]>0.0 is attained at metallicities higher than solar. These are surprising results, never seen for any other heavy element, and are difficult to understand within the formulation of the s- and r-processes, both contributing sources to the Galactic Rb abundance. We discuss the reliability of these findings for Rb in terms of non-LTE effects, stellar activity, or an anomalous Rb abundance in the Solar System, but no explanation is found. We then interpret the full observed [Rb/Fe] versus [Fe/H] trend within the framework of theoretical predictions from state-of-the-art chemical evolution models for heavy elements, but a simple interpretation is not found either. In particular, the possible secondary behaviour of the [Rb/Fe] ratio at super-solar metallicities would require a much larger production of Rb than currently predicted in AGB stars through the s-process without overproducing Sr and Zr.

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arXiv2020

Study of the departures from LTE in the unevolved stars infra-red spectra

We present a study of departures from Local Thermodynamic Equilibrium (LTE) in the formation of infra-red lines of Na I, Mg I, Al I, S I, K I and Sr II in unevolved stars of spectral types F,G,K and metallicities around the solar metallicity. The purpose of this investigation is to identify lines of these species that can be safely treated with the LTE approximation in the infra-red spectra of these types of stars. We employ a set of 40 stars observed with the GIANO spectrograph at the 3.5 m Telescopio Nazionale Galileo (TNG) and previously investigated by Caffau et al. We were able to identify many lines that can be treated in LTE for all the above-mentioned species, except for Sr II. The latter species can only be studied using three lines in the J-band, but all three of them display significant departures from LTE. With our small-size, but high-quality sample we can determine robustly the trends of the abundance ratios with metallicity, confirming the trends apparent from a sample that is larger by several orders of magnitude, but of lower quality in terms of resolution and S/N ratio.

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arXiv2019

Elemental abundances in the center of the Galactic Nuclear Disc

We have made the first attempt to derive the chemical properties of the Galactic disc at its very central part using high-resolution infrared spectroscopic observations of four classical Cepheids. Those stars are located at Galactocentric distances smaller than 1 kpc. All investigated stars show near-to-solar elemental abundances. By combining these new data with our previous studies, this result suggests that the radial distribution of iron abundance on a logarithmic scale gradually increases from the outskirts of the Galactic disc to Galactocentric distances of about 2-4 kpc, reaching there a maximal value of about +0.4 dex, and then declines sharply to about the solar value at the Galactic Center.

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arXiv2019

Oxygen abundance and the N/C vs N/O relation for AFG supergiants and bright giants

Non-LTE analysis (LTE is local thermodynamic equilibrium) of the oxygen abundances for 51 Galactic A-, F- and G-type supergiants and bright giants is performed. In contrast with carbon and nitrogen, oxygen does not show any significant systematic anomalies in their abundances log E(O). There is no marked difference from the initial oxygen abundance within errors of the log E(O) determination across the Teff interval from 4500 to 8500 K and the log g interval from 1.2 to 2.9 dex. This result agrees well with theoretical predictions for stellar models with rotation. With our new data for oxygen and our earlier non-LTE determinations of the N and C abundances for stars from the same sample, we constructed the [N/C] vs [N/O] relation for 17 stars. This relation is known to be a sensitive indicator of stellar evolution. A pronounced correlation between [N/C] vs [N/O] is found; the observed [N/C] increase from 0 to 1.6 dex is accompanied by the [N/O] increase from 0 to 0.9 dex. When comparing the observed [N/C] vs [N/O] relation with the theoretical one, we show that this relation reflects a strong dependence of the evolutionary changes in CNO abundances on the initial rotation velocities of stars. Given that the initial rotational velocities of these stars are expected to satisfy V0<150 km/s, it is found that they are mostly the post first dredge-up (post-FDU) objects. It is important that just such initial velocities V0 are typical for about 80% of stars in question (i.e. for stars with masses 4-19 M_sun). A constancy of the total C+N+O abundance during stellar evolution is confirmed. The mean value log E(C+N+O)=8.97+/-0.08 found for AFG supergiants and bright giants seems to be very close to the initial value 8.92 (the Sun) or 8.94 (the unevolved B-type MS stars).

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arXiv2018

Copper abundance from Cu I and Cu II lines in metal-poor star spectra: NLTE vs LTE

We checked consistency between the copper abundance derived in six metal-poor stars using UV Cu II lines (which are assumed to form in LTE) and UV Cu I lines (treated in NLTE). Our program stars cover the atmosphere parameters which are typical for intermediate temperature dwarfs (effective temperature is in the range from approximately 5800 to 6100 K, surface garvity is from 3.6 to 4.5, metallicity is from about -1 to -2.6 dex). We obtained a good agreement between abundance from these two sets of the lines, and this testifies about reliability of our NLTE copper atomic model. We confirmed that no underabundace of this element is seen at low metallicities (the mean [Cu/Fe] value is about -0.2 dex, while as it follows from the previous LTE studies copper behaves as a secondary element and [Cu/Fe] ratio in the range of [Fe/H from -2 to -3 dex should be about -1 dex). According to our NLTE data the copper behaves as a primary element at low metallicity regime. We also conclude that our new NLTE copper abundance in metal-poor stars requires significant reconsideration of this element yields in the explosive nucleosynthesis.

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arXiv2017

Barium abundance in the central part of the dSph Fornax galaxy

We revisited barium abundance in a sample of giant stars in the dwarf spheroidal Fornax galaxy. Non-local thermodynamic equilibrium (NLTE) synthesis was used to derive the barium abundance from two Ba II lines. Our new NLTE result for barium shows that in the range of metallicities from -1 to -0.5 the abundance of this element is almost the same as in the stars of the Milky Way. We conclude that the evolution of barium abundance in the dSph Fornax galaxy resembles that of the Milky Way at the epoch of the corresponding metallicity level.

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arXiv2017

Observing the metal-poor solar neighbourhood: a comparison of galactic chemical evolution predictions

Atmospheric parameters and chemical compositions for ten stars with metallicities in the region of -2.2< [Fe/H] <-0.6 were precisely determined using high resolution, high signal to noise, spectra. For each star the abundances, for 14 to 27 elements, were derived using both LTE and NLTE approaches. In particular, differences by assuming LTE or NLTE are about 0.10 dex; depending on [Fe/H], Teff, gravity and element lines used in the analysis. We find that the O abundance has the largest error, ranging from 0.10 and 0.2 dex. The best measured elements are Cr, Fe, and Mn; with errors etween 0.03 and 0.11 dex. The stars in our sample were included in previous different observational work. We provide a consistent data analysis. The data dispersion introduced in the literature by different techniques and assumptions used by the different authors is within the observational errors, excepting for HD103095. We compare these results with stellar observations from different data sets and a number of theoretical galactic chemical evolution (GCE) simulations. We find a large scatter in the GCE results, used to study the origin of the elements. Within this scatter as found in previous GCE simulations, we cannot reproduce the evolution of the elemental ratios [Sc/Fe], [Ti/Fe], and [V/Fe] at different metallicities. The stellar yields from core collapse supernovae (CCSN) are likely primarily responsible for this discrepancy. Possible solutions and open problems are discussed.

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arXiv2016

Oxygen, $α$-element and iron abundance distributions in the inner part of the Galactic thin disc. II

We have derived the abundances of 36 chemical elements in one Cepheid star, ASAS 181024--2049.6, located R$_{\rm G}= 2.53$ kpc from the Galactic center. This star falls within a region of the inner thin disc poorly sampled in Cepheids. Our spectral analysis shows that iron, magnesium, silicon, calcium and titanium LTE abundances in that star support the presence of a plateau-like abundance distribution in the thin disc within 5 kpc of the Galactic center, as previously suggested by \cite{Maret15}. If confirmed, the flattening of the abundance gradient within that region could be the result of a decrease in the star formation rate due to dynamic effects, possibly from the central Galactic bar.

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arXiv2015

Mn abundances in the stars of the Galactic disc with metallicities -1.0 < [Fe/H] < 0.3

In this work we present and discuss the observations of the Mn abundances for 247 FGK dwarfs, located in the Galactic disc with metallicity -1<Fe/H]<+0.3. The observed stars belong to the substructures of the Galaxy thick and thin discs, and to the Hercules stream. The observations were conducted using the 1.93 m telescope at Observatoire de Haute-Provence (OHP, France) equipped with the echelle type spectrographs ELODIE and SOPHIE. The abundances were derived under the LTE approximation, with an average error for the [Mn/Fe] ratio of 0.10 dex. For most of the stars in the sample Mn abundances are not available in the literature. We obtain an evolution of [Mn/Fe] ratio with the metallicity [Fe/H] consistent with previous data compilations. In particular, within the metallicity range covered by our stellar sample the [Mn/Fe] ratio is increasing with the increase of metallicity. This due to the contribution to the Galactic chemical evolution of Mn and Fe from thermonuclear supernovae. We confirm the baseline scenario where most of the Mn in the Galactic disc and in the Sun is made by thermonuclear supernovae. In particular, the effective contribution from core-collapse supernovae to the Mn in the Solar system is about 10-20%. However, present uncertainties affecting the production of Mn and Fe in thermonuclear supernovae are limiting the constraining power of the observed [Mn/Fe] trend in the Galactic discs on, e.g., the frequency of different thermonuclear supernovae populations. The different production of these two elements in different types of thermonuclear supernovae needs to be disentangled by the dependence of their relative production on the metallicity of the supernova progenitor.

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arXiv2015

High-resolution abundance analysis of HD 140283

HD 140283 is a reference subgiant that is metal poor and confirmed to be a very old star. The abundances of this type of old star can constrain the nature and nucleosynthesis processes that occurred in its (even older) progenitors. The present study may shed light on nucleosynthesis processes yielding heavy elements early in the Galaxy. A detailed abundance analysis of a high-quality spectrum is carried out, with the intent of providing a reference on stellar lines and abundances of a very old, metal-poor subgiant. We aim to derive abundances from most available and measurable spectral lines. The analysis is carried out using high-resolution (R = 81 000) and high signal-to-noise ratio (800 < S/N/pixel < 3400) spectrum, in the wavelength range 3700 - 10475, obtained with a seven-hour exposure time, using the ESPaDOnS at the CFHT. The calculations in LTE were performed with the OSMARCS 1D atmospheric model and the spectrum synthesis code Turbospectrum, while the analysis in NLTE is based on the MULTI code. We present LTE abundances for 26 elements, and NLTE calculations for the species C I, O I, Na I, Mg I, Al I, K I, Ca I, Sr II, and Ba II lines. The abundance analysis provided an extensive line list suitable for metal-poor subgiant stars. The results for Li, CNO, alpha-, and iron peak elements are in good agreement with literature. The newly NLTE Ba abundance, along with a NLTE Eu correction and a 3D Ba correction from literature, leads to [Eu/Ba] = +0.59 +/- 0.18. This result confirms a dominant r-process contribution, possibly together with a very small contribution from the main s-process, to the neutron-capture elements in HD 140283. Overabundances of the lighter heavy elements and the high abundances derived for Ba, La, and Ce favour the operation of the weak r-process in HD 140283.

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arXiv2015

Grid of theoretical NLTE equivalent widths of four Ba II lines and barium abundance in cool stars

We present a grid of computed non-local thermodynamic equilibrium (NLTE) equivalent widths (EW) and NLTE abundance corrections for four Ba II lines: 4554, 5853, 6141, and 6496 A. The grid can be useful in deriving the NLTE barium abundance in stars having parameters in the following ranges: effective temperature from 4000 K to 6500 K, surface gravity log g from 0 to 5, microturbulent velocity 0 km s^-1 to 3 km s^-1, metallicity [Fe/H] from -2 to +0.5, and [Ba/Fe] from -0.4 to +0.6. The NLTE abundance can be either derived by EW interpolation (using the observed Ba II line EW) or by using the NLTE correction applied to a previously determined LTE abundance. Ba II line equivalent widths and the NLTE corrections were calculated using the updated MULTI code and the Ba II atomic model that was previously applied to determine the NLTE barium abundance in different types of stars. The grid is available on-line through the web, and we find that the grid Ba NLTE corrections are almost as accurate as direct NLTE profile fitting (to within 0.05-0.08 dex). For the weakest Ba II line (5853 A) the LTE abundances almost agree with the NLTE abundances, whereas the other three Ba II lines, 4554, 6141, and 6496 A, need NLTE corrections even at the highest metallicities tested here. The 4554 A line is extremely strong and should not be used for abundance analysis above [Fe/H]= -1. Furthermore, we tested the impact of different model atmospheres and spectrum synthesis codes and found average differences of 0.06 dex and 0.09 dex, respectively, for all four lines. At these metallicities we find an average Delta NLTE of +/-0.1 dex for the three useful Ba lines for subsolar cool dwarfs.

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