arXiv subjects

S. Korotin

Publications and source records attributed to S. Korotin.

18 recordsLinked to original sources

arXiv2022

Non-LTE analysis of the Si II lines in iota Her with various atomic data sets

This study shows that the statistical equilibrium of Si II in the atmosphere of a B3 IV type star iota Her is extremely sensitive to a variation in photoionization cross-sections for the Si II levels. The difference in abundances derived from absorption lines of Si II between applying the data from two equal accuracy sources, namely, the Opacity Project (OP) and the NORAD database, amounts to 0.18 dex, on average. Using the hydrogenic approximation for photoionization cross-sections, we obtain the departure coefficients for the Si II \eu{4s}{2}{S}{}{} level, the source function for Si II 6371 A, and the abundance derived from this line, which are very similar to the corresponding values computed by Takeda (2022). We suppose that close-to-solar abundance obtained by Takeda (2022) from Si II 6371 A in iota Her is due to using the hydrogenic photoionization cross-sections for the Si II levels. However, emission lines of Si II observed in iota Her can only be reproduced with the OP photoionization cross-sections. Photoionization cross-sections for the Si II levels need further improvements.

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arXiv2022

Abundance of beryllium in the Sun and stars: The role of non-local thermodynamic equilibrium effects

Earlier studies have suggested that deviations from the local thermodynamic equilibrium (LTE) play a minor role in the formation of Be II 313 nm resonance lines in solar and stellar atmospheres. Recent improvements in the atomic data allow a more complete model atom of Be to be constructed and the validity of these claims to be reassessed using more up-to-date atomic physics. The main goal of this study therefore is to refocus on the role of non-local thermodynamic equilibrium (NLTE) effects in the formation of Be II 313.04 and 313.11 nm resonance lines in solar and stellar atmospheres. For this, we constructed a model atom of Be using new atomic data that recently became available. The model atom contains 98 levels and 383 radiative transitions of Be I and Be II and uses the most up-to-date collision rates with electrons and hydrogen. This makes it the most complete model atom used to determine 1D NLTE solar Be abundance and to study the role of NLTE effects in the formation of Be II 313 nm resonance lines. We find that deviations from LTE have a significant influence on the strength of the Be II 313 nm line in solar and stellar atmospheres. For the Sun, we obtained the 1D NLTE Be abundance of A(Be)_NLTE = 1.32+/-0.05, which is in excellent agreement with the meteoritic value of A(Be)=1.31+/-0.04. Importantly, we find that NLTE effects become significant in FGK stars. Moreover, there is a pronounced variation in 1D NLTE-LTE abundance corrections with the effective temperature and metallicity. Therefore, contrary to our previous understanding, the obtained results indicate that NLTE effects play an important role in Be line formation in stellar atmospheres and have to be properly taken into account in Be abundance studies, especially in metal-poor stars.

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arXiv2021

Abundance of zirconium in the globular cluster 47 Tuc: a possible Zr-Na correlation?

We determined abundances of Na and Zr in the atmospheres of 237 RGB stars in Galactic globular cluster (GGC) 47 Tuc (NGC 104), with a primary objective of investigating possible differences between the abundances of Zr in the first generation (1P) and second generation (2P) stars. For the abundance analysis, we used archival UVES/GIRAFFE spectra obtained during three different observing programmes. Abundances were determined from two Na I and three Zr I lines, using 1D hydrostatic ATLAS model atmospheres. The target stars for the abundance analysis were limited to those with 4200 $\leq$ $T_{\rm eff}$ $\leq$ 4800 K. This is the largest sample of GGC stars in which Na and Zr abundances have been studied so far. While our mean [Na/Fe] and [Zr/Fe] ratios agree well with those determined in the earlier studies, we find a weak but statistically significant correlation in the ${\rm [Zr/Fe]} - {\rm [Na/Fe]}$ plane. A comparison of the mean [Zr/Fe] abundance ratios in the 1P and 2P stars suggests a small but statistically significant Zr over-abundance in the 2P stars, $Δ{\rm [Zr/Fe]}_{\rm 2P-1P}\approx+0.06$ dex. Also, our analysis shows that stars enriched in both Zr and Na are more centrally concentrated. However, we find no correlation between their distance from the cluster centre and their full spatial velocity. While there may be some influence of CN line blends on the determined Zr abundances, it seems very unlikely that the detected Zr-Na correlation, for the slightly higher Zr abundances in the 2P stars, would be caused by the CN blending alone. The obtained results indicate that, in 47 Tuc, some amount of Zr should have been synthesised by the same polluters that enriched 2P stars with the light elements. Unfortunately, our data alone do not allow us to distinguish which of the two scenarios, or whether or not a combination of both, could have operated in this GGC.

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arXiv2021

Abundance of barium in the atmospheres of red giants in the Galactic globular cluster NGC 104 (47 Tuc)

Context. While most (if not all) Type I Galactic globular clusters (GGCs) are characterised by spreads in the abundances of light chemical elements (e.g. Li, N, O, Na, Mg, Al), it is not yet well established whether similar spreads may exist in s-process elements as well. Aims. We investigated the possible difference in Ba abundance between the primordial (1P) and polluted (2P) stars in the Galactic globular cluster (GGC) 47 Tuc (NGC 104). For this, we obtained homogeneous abundances of Fe, Na, and Ba in a sample of 261 red giant branch (RGB) stars which is the largest sample used for Na and Ba abundance analysis in any GGC so far. Methods. Abundances of Na and Ba were determined using archival GIRAFFE/VLT spectra and 1D non-local thermodynamic equilibrium (NLTE) abundance analysis methodology. Results. Contrary to the finding of Gratton et al. (2013), we did not detect any significant Ba-Na correlation or 2P-1P Ba abundance difference in the sample of 261 RGB stars in 47 Tuc. This corroborates the result of D'Orazi et al. (2010) who found no statistically significant Ba-Na correlation in 110 RGB stars in this GGC. The average barium-to-iron ratio obtained in the sample of 261 RGB stars, $\langle{\rm Ba/Fe}_{\rm 1D~NLTE}\rangle = -0.01\pm0.06$, agrees well with those determined in Galactic field stars at this metallicity and may therefore represent the abundance of primordial proto-cluster gas that has not been altered during the subsequent chemical evolution of the cluster.

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arXiv2021

Rubidium abundances in solar metallicity stars

A Rb deficiency by a factor two with respect to the Sun has been found in M dwarfs of solar metallicity. This deficiency is difficult to understand from both the observational and nucleosynthesis point of views. To test the reliability of this Rb deficiency, we study the Rb and Zr abundances in a sample of KM-type giant stars in a similar metallicity range extracted from the AMBRE Project. We derive Rb and Zr abundances in 54 giant stars with metallicity close to solar by spectral synthesis in LTE and NLTE. The impact of the Zeeman broadening in the RbI line is also studied. The LTE analysis results in a Rb deficiency in giant stars smaller than that obtained in M dwarfs, but the NLTE [Rb/Fe] ratios are very close to solar in the full metallicity range. This contrasts with the figure found in M dwarfs. We investigate the effect of gravitational settling and magnetic activity as possible causes of the Rb deficiency found in M dwarfs. While, the former phenomenon has a negligible impact on the surface Rb abundance, the existence of an average magnetic field with intensity typical of that observed in M dwarfs may result in systematic Rb abundance underestimations if the Zeeman broadening is not considered in the spectral synthesis. The new [Rb,Zr/Fe] vs. [Fe/H] relationships can be explained when the Rb production by rotating massive stars and low-and-intermediate mass stars are considered, without the need of any deviation from the standard s-process nucleosynthesis in AGB stars as previously suggested.

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arXiv2019

Probing the existence of very massive first stars

We present a novel approach aimed at identifying the key chemical elements to search for the (missing) descendants of very massive first stars exploding as Pair Instability Supernovae (PISN). Our simple and general method consists in a parametric study accounting for the unknowns related to early cosmic star-formation and metal-enrichment. Our approach allow us to define the most likely [Fe/H] and abundance ratios of long-lived stars born in inter-stellar media polluted by the nucleosynthetic products of PISN at a > 90%, 70%, and 50% level. In agreement with previous works, we show that the descendants of very massive first stars can be most likely found at [Fe/H]~ -2. Further, we demonstrate that to search for an under-abundance of [(N, Cu, Zn)/Fe] < 0 is the key to identify these rare descendants. The "killing elements" N, Zn, and Cu are not produced by PISN, so that their sub-Solar abundance with respect to iron persists in environments polluted by further generations of normal core-collapse supernovae up to a 50% level. We show that the star BD +80 245, which has [Fe/H]= -2.2, [N/Fe]= -0.79, [Cu/Fe]=-0.75, and [Zn/Fe]= -0.12 can be the smoking gun of the chemical imprint from very massive first stars. To this end we acquired new spectra for BD +80 245 and re-analysed those available from the literature accounting for Non-Local Thermodynamic Equilibrium corrections for Cu. We discuss how to find more of these missing descendants in ongoing and future surveys to tightly constrain the mass distribution of the first stars.

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arXiv2018

Systematic investigation of chemical abundances derived using IR spectra obtained with GIANO

Detailed chemical abundances of Galactic stars are needed in order to improve our knowledge of the formation and evolution of our galaxy, the Milky Way. We took advantage of the GIANO archive spectra to select a sample of Galactic disc stars in order to derive their chemical inventory and to compare the abundances we derived from these infrared spectra to the chemical pattern derived from optical spectra. We analysed high-quality spectra of 40 stars observed with GIANO. We derived the stellar parameters from the photometry and the Gaia data-release 2 (DR2) parallax; the chemical abundances were derived with the code MyGIsFOS. For a subsample of stars we compared the chemical pattern derived from the GIANO spectra with the abundances derived from optical spectra. We derived P abundances for all 40 stars, increasing the number of Galactic stars for which phosphorus abundance is known. We could derive abundances of 14 elements, 8 of which are also derived from optical spectra. The comparison of the abundances derived from infrared and optical spectra is very good. The chemical pattern of these stars is the one expected for Galactic disc stars and is in agreement with the results from the literature. GIANO is providing the astronomical community with an extremely useful instrument, able to produce spectra with high resolution and a wide wavelength range in the infrared.

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arXiv2018

Abundances of Mg and K in the atmospheres of turn-off stars in Galactic globular cluster 47 Tucanae

We determined abundances of Mg and K in the atmospheres of 53 (Mg) and 75 (K) turn-off (TO) stars of the Galactic globular cluster 47 Tuc. The obtained abundances, together with those of Li, O, and Na, were used to search for possible relations between the abundances of K and other light elements, Li, O, Na, and Mg, as well as the connections between the chemical composition of TO stars and their kinematical properties. Abundances of Mg and K were determined using archival VLT FLAMES/GIRAFFE spectra, in combination with the 1D NLTE spectral synthesis methodology. Spectral line profiles were computed with the MULTI code, using ATLAS9 stellar model atmospheres. We also utilized 3D hydrodynamical CO5BOLD and 1D hydrostatic LHD model atmospheres for computing 3D-1D LTE abundance corrections for the spectral lines of Mg and K, in order to assess the influence of convection on their formation in the atmospheres of TO stars. The determined average abundance-to-iron ratios and their RMS variations due to star-to-star abundance spreads were $\langle{\rm[Mg/Fe]}\rangle^{\rm 1D NLTE}=0.47\pm0.12$, and $\langle{\rm [K/Fe]}\rangle^{\rm 1D NLTE}=0.39\pm0.09$. Although the data suggest the possible existence of a weak correlation in the [K/Fe]-[Na/Fe] plane, its statistical significance is low. No statistically significant relations between the abundance of K and other light elements were detected. Also, we did not find any significant correlations or anti-correlations between the [Mg/Fe] and [K/Fe] ratios and projected distance from the cluster center. Similarly, no relations between the absolute radial velocities of individual stars and abundances of Mg and K in their atmospheres were detected. The 3D-1D abundance corrections were found to be small ($\leq 0.1$\,dex) for the lines of Mg and K used in this study, thus indicating that the influence of convection on their formation is small.

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arXiv2018

Metal-Poor Type II Cepheids with Periods Less Than Three Days

We have analysed 10 high resolution spectra of Type II Cepheids with periods less than 3 days. We find that they clearly separate into two groups: those with near or slightly below solar metallicities, and those with [Fe/H] between --1.5 and --2.0. While the former are usually called BL~Her stars, we suggest that the latter be called UY~Eri stars. The UY~Eri subclass appears to be similar to the short period variables in globular clusters of the Galactic Halo. Globular clusters with [Fe/H] $\textgreater$ --1.0 almost never have Type II Cepheids.

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arXiv2017

Galactic evolution of Copper in the light of NLTE computations

We have developed a model atom for Cu with which we perform statistical equilibrium computations that allow us to compute the line formation of Cu I lines in stellar atmospheres without assuming Local Thermodynamic Equilibrium (LTE). We validate this model atom by reproducing the observed line profiles of the Sun, Procyon and eleven metal-poor stars. Our sample of stars includes both dwarfs and giants. Over a wide range of stellar parameters we obtain excellent agreement among different Cu I lines. The eleven metal-poor stars have iron abundances in the range -4.2 <= [Fe/H] <= -1.4, the weighted mean of the [Cu/Fe] ratios is -0.22 dex, with a scatter of -0.15 dex. This is very different from the results from LTE analysis (the difference between NLTE and LTE abundances reaches 1 dex) and in spite of the small size of our sample it prompts for a revision of the Galactic evolution of Cu.

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arXiv2017

Abundances of Na, Mg, and K in the atmospheres of red giant branch stars of Galactic globular cluster 47 Tucanae

We study the abundances of Na, Mg, and K in the atmospheres of 32 RGB stars in the Galactic globular cluster (GGC) 47 Tuc, with the goal to investigate the possible existence of Na-K and Mg-K correlations/anti-correlations, similar to those that were recently discovered in two other GGCs, NGC 2419 and 2808. The abundances of K, Na, and Mg were determined using high-resolution 2df spectra obtained with the AAT. The 1D NLTE abundance estimates were obtained using 1D hydrostatic ATLAS9 model atmospheres and spectral line profiles synthesized with the MULTI package. We also used 3D hydrodynamical CO5BOLD and 1D hydrostatic LHD model atmospheres to compute 3D-1D LTE abundance corrections, $Δ_{\rm 3D-1D~LTE}$, for the spectral lines of Na, Mg, and K used in our study. These abundance corrections were used to understand the role of convection in the formation of spectral lines, as well as to estimate the differences in the abundances obtained with the 3D hydrodynamical and 1D hydrostatic model atmospheres. The average element-to-iron abundance ratios and their RMS variations due to star-to-star abundance spreads determined in our sample of RGB stars were $\langle{\rm [Na/Fe]}\rangle^{\rm 1D~NLTE}=0.42\pm0.13$, $\langle{\rm [Mg/Fe]}\rangle^{\rm 1D~NLTE}=0.41\pm0.11$, and $\langle{\rm [K/Fe]}\rangle^{\rm 1D~NLTE}=0.05\pm0.14$. We found no statistically significant relations between the abundances of the three elements studied here. Also, there were no abundance trends with the distance from the cluster center, nor any statistically significant relations between the abundance/abundance ratios and absolute radial velocities of individual stars. All these facts suggest the similarity of K abundance in stars that belong to different generations in 47 Tuc which, in turn, may hint that evolution of K in this particular cluster was unrelated to the nucleosynthesis of Na and/or Mg.

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arXiv2017

The Gaia-ESO Survey: Galactic evolution of sulphur and zinc

Due to their volatile nature, when sulfur and zinc are observed in external galaxies, their determined abundances represent the gas-phase abundances in the interstellar medium. This implies that they can be used as tracers of the chemical enrichment of matter in the Universe at high redshift. Comparable observations in stars are more difficult and, until recently, plagued by small number statistics. We wish to exploit the Gaia ESO Survey (GES) data to study the behaviour of sulfur and zinc abundances of a large number of Galactic stars, in a homogeneous way. By using the UVES spectra of the GES sample, we are able to assemble a sample of 1301 Galactic stars, including stars in open and globular clusters in which both sulfur and zinc were measured. We confirm the results from the literature that sulfur behaves as an alpha-element. We find a large scatter in [Zn/Fe] ratios among giant stars around solar metallicity. The lower ratios are observed in giant stars at Galactocentric distances less than 7.5 kpc. No such effect is observed among dwarf stars, since they do not extend to that radius. Given the sample selection, giants and dwarfs are observed at different Galactic locations, and it is plausible, and compatible with simple calculations, that Zn-poor giants trace a younger population more polluted by SN Ia yields. It is necessary to extend observations in order to observe both giants and dwarfs at the same Galactic location. Further theoretical work on the evolution of zinc is also necessary.

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arXiv2015

GIANO Y-band spectroscopy of dwarf stars: Phosphorus, Sulphur, and Strontium abundances

In recent years a number of poorly studied chemical elements, such as phosphorus, sulphur, and strontium, have received special attention as important tracers of the Galactic chemical evolution. By exploiting the capabilities of the infrared echelle spectrograph GIANO mounted at the Telescopio Nazionale Galileo, we acquired high resolution spectra of four Galactic dwarf stars spanning the metallicity range between about one-third and twice the solar value. We performed a detailed feasibility study about the effectiveness of the P, S, and Sr line diagnostics in the Y band between 1.03 and 1.10 microm. Accurate chemical abundances have been derived using one-dimensional model atmospheres computed in local thermodynamic equilibrium (LTE). We computed the line formation assuming LTE for P, while we performed non-LTE analysis to derive S and Sr abundances. We were able to derive phosphorus abundance for three stars and an upper limit for one star, while we obtained the abundance of sulphur and strontium for all of the stars. We find [P/Fe] and [S/Fe] abundance ratios consistent with solar-scaled or slightly depleted values, while the [Sr/Fe] abundance ratios are more scattered (by +/-0.2 dex) around the solar-scaled value. This is fully consistent with previous studies using both optical and infrared spectroscopy. We verified that high-resolution, Y-band spectroscopy as provided by GIANO is a powerful tool to study the chemical evolution of P, S, and Sr in dwarf stars.

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arXiv2014

New insights on Ba over-abundance in open clusters. Evidence for the intermediate neutron-capture process at play?

Recently an increasing number of studies were devoted to measure the abundances of neutron-capture elements heavier than iron in stars belonging to Galactic Open Clusters (OCs). OCs span a sizeable range in metallicity -0.6<[Fe/H]<+0.4), and they show abundances of light elements similar to disk stars of the same age. A different pattern is observed for heavy elements. A large scatter is observed for Ba, with most OCs showing [Ba/Fe] and [Ba/La] overabundant with respect to the Sun. The origin of this overabundance is not clearly understood. With the goal of providing new observational insights we determined radial velocities, atmospheric parameters and chemical composition of 27 giant stars members of five OCs: Cr 110, Cr 261, NGC 2477, NGC 2506 and NGC 5822. We used high-resolution spectra obtained with the UVES spectrograph at ESO Paranal. We perform a detailed spectroscopic analysis of these stars to measure the abundance of up to 22 elements per star. We study the dependence of element abundance on metallicity and age with unprecedented detail, complementing our analysis with data culled from the literature. We confirm the trend of Ba overabundance in OCs, and show its large dispersion for clusters younger than ~4 Gyr. Finally, the implications of our results for stellar nucleosynthesis are discussed. We show in this work that the Ba enrichment compared to other neutron-capture elements in OCs cannot be explained by the contributions from the slow neutron-capture process and the rapid neutron-capture process. Instead, we argue that this anomalous signature can be explained by assuming an additional contribution by the intermediate neutron-capture process.

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arXiv2014

The chemical composition of red giants in 47 Tucanae I: Fundamental parameters and chemical abundance patterns

Context: The study of chemical abundance patterns in globular clusters is of key importance to constrain the different candidates for intra-cluster pollution of light elements. Aims: We aim at deriving accurate abundances for a large range of elements in the globular cluster 47 Tucanae (NGC 104) to add new constraints to the pollution scenarios for this particular cluster, expanding the range of previously derived element abundances. Methods: Using tailored 1D LTE atmospheric models together with a combination of equivalent width measurements, LTE, and NLTE synthesis we derive stellar parameters and element abundances from high-resolution, high signal-to-noise spectra of 13 red giant stars near the tip of the RGB. Results: We derive abundances of a total 27 elements (O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ru, Ba, La, Ce, Pr, Nd, Eu, Dy). Departures from LTE were taken into account for Na, Al and Ba. We find a mean [Fe/H] = $-0.78\pm0.07$ and $[α/{\rm Fe}]=0.34\pm0.03$ in good agreement with previous studies. The remaining elements show good agreement with the literature, but the inclusion of NLTE for Al has a significant impact on the behaviour of this key element. Conclusions: We confirm the presence of an Na-O anti-correlation in 47 Tucanae found by several other works. Our NLTE analysis of Al shifts the [Al/Fe] to lower values, indicating that this may be overestimated in earlier works. No evidence for an intrinsic variation is found in any of the remaining elements.

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arXiv2013

Barium and Yttrium abundance in intermediate-age and old open clusters

Barium is a neutron capture element, that, in open clusters, is frequently over-abundant with respect to the Iron. A clear explanation for this is still missing. Additionally, its gradient across the Galactic disk is poorly constrained. We measure the abundance of yttrium and barium using the synthetic spectrum method from UVES high-resolution spectra of eight distant open clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, Berkeley 75, NGC 6192, NGC 6404, and NGC 6583. The barium abundance was estimated using NLTE approximation. We confirm that Barium is indeed over-abundant in most clusters, especially young clusters. Finally, we investigated the trend of yttrium and barium abundances as a function of distance in the Galaxy and ages. Several scenarios for the barium over-abundance are then discussed.

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arXiv2011

The NLTE Barium Abundance in Dwarf Stars in the Metallicity Range of -1 < [Fe/H] < +0.3

We present the results of determination of the barium abundance considering the non-LTE (NLTE) effects in 172 dwarf stars in the metallicity range of -1< [Fe/H] <+0.3, assigned to different Galactic substructures by kinematic criteria. We used a model of the Ba atom with 31 levels of Ba I and 101 levels of Ba II. The atmosphere models for the investigated stars were computed using the ATLAS9 code modified by new opacity distribution functions. The NLTE profiles of the unblended Ba II (4554 A, 5853 A, 6496 A) were computed and then compared to those observed. The line 6141 A was also used, but with an allowance for its correlation with the iron line. The average barium abundances in the thin and thick discs are 0.01 +/- 0.08 and -0.03 +/- 0.07, respectively. The comparison to the calculations of the Galactic chemical evolution by Serminato et al. (2009) was conducted. The trend obtained for the Ba abundance versus [Fe/H] suggests a complex barium production process in the thin and thick discs.

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arXiv2011

Spectroscopy of high proper motion stars in the ground--based UV

Based on high quality spectral data (spectral resolution R>60000) within the wavelength range of 3550-5000 AA we determined main parameters (effective temperature, surface gravity, microturbulent velocity, and chemical element abundances including heavy metals from Sr to Dy) for 14 metal-deficient G-K stars with large proper motions. The stars we studied have a wide range of metallicity: [Fe/H]=-0.3 ÷-2.9. Abundances of Mg, Al, Sr and Ba were calculated with non-LTE line-formation effects accounted for. Abundances both of the radioactive element Th and r-process element Eu were determined using synthetic spectrum calculations. We selected stars that belong to different galactic populations according to the kinematical criterion and parameters determined by us. We found that the studied stars with large proper motions refer to different components of the Galaxy: thin, thick disks and halo. The chemical composition of the star BD+80 245 located far from the galactic plane agrees with its belonging to the accreted halo. For the giant HD115444 we obtained [Fe/H]=-2.91, underabundance of Mn, overabundance of heavy metals from Ba to Dy, and, especially high excess of the r-process element Europium: [Eu/Fe]=+1.26. Contrary to its chemical composition typical for halo stars, HD115444 belongs to the disc population according to its kinematic parameters.

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