arXiv subjects

S. M. Andrievsky

Publications and source records attributed to S. M. Andrievsky.

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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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

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

Comet 29P/Schwassmann-Wachmann 1 dust environment from photometric observation at the SOAR Telescope

We report photometric observations of comet 29P/Schwassmann-Wachmann 1 made on August 12, 2016 with the broadband B, V, R and I filters and the SOAR 4.1-meter telescope (Chile). We find the comet active at that time. Enhanced images obtained in all filters reveal three radial features in the 29P/ Schwassmann-Wachmann 1 coma, regardless of the image-processing algorithm. Using a high-resolution spectrum of comet 29P/Schwassmann-Wachmann 1 reported by Ivanova et al. (2018) on the same date, we estimate the relative contribution of the gaseous emission and the continuum to the total response measured with our broadband B and V filters. The gaseous-emission contribution appears to be very small 2.5%. We compute the dust production Af\r{ho} in 29P/ Schwassmann-Wachmann 1 for the four filters and find its growth with the wavelength, from 3,393 cm in the B filter to 8,561 cm in the I filter. We model the color slope of dust in Comet 29P/Schwassmann-Wachmann 1 using agglomerated debris particles. Simultaneous analysis of the color slope in the B-R and R-I pairs suggests a single dominant chemical species of 29P/Schwassmann-Wachmann 1 dust particles consisting of Fe-Mg silicates and obeying a power-law size distribution with index n=2.55. This conclusion is consistent with the previous thermal-emission study of 29P/Schwassmann-Wachmann 1.

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arXiv2020

Photometric and spectroscopic analysis of Comet 29P/Schwassmann-Wachmann 1 activity

We carried out photometric and spectroscopic observations of comet 29P/Schwassmann-Wachmann 1 at the 6-m BTA telescope (SAO RAS, Russia) and the 1.6-m telescope of the National Laboratory for Astro-physics (LNA, Brazil) on February 20, 2012, and on May 31, 2011, respectively. The spectra revealed the presence of CO+ and N2+ emissions in the cometary coma at a distance of 5.25 AU from the Sun. The ratio [N2+]/[CO+] within the projected slit is 0.013. The images obtained through BVR filters showed a bright, dust coma, indicating a high level of activity. We estimated a colour index and a colour excess for the comet. The parameter Af\r{ho}, which is used as an indicator of a cometary activity, was measured to be 2584 cm in the reference optical aperture of \r{ho}=10^4 km. The dust production constituted 33 kg/s and 9.3*10^3 kg/s, it was obtained using different methods. We also investigated the morphology of the comet using image enhancement techniques and found two jets in the coma.

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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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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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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

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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arXiv2015

Chemical abundances of giant stars in the Crater stellar system

We obtained spectra for two giants of Crater (Crater J113613-105227 and Crater J113615-105244) using X-Shooter at the VLT. The spectra have been analysed with the MyGIsFoS code using a grid of synthetic spectra computed from one dimensional, Local Thermodynamic Equilibrium (LTE) model atmospheres. Effective temperature and surface gravity have been derived from photometry measured from images obtained by the Dark Energy Survey. The radial velocities are 144.3+-4.0 km/s for Crater J113613-105227 and and 134.1+-4.0 km/s for Crater J113615-105244. The metallicities are [Fe/H]=-1.73 and [Fe/H]=-1.67, respectively. Beside the iron abundance we could determine abundances for nine elements: Na, Mg, Ca, Ti, V, Cr, Mn, Ni and Ba. For Na and Ba we took into account deviations from LTE, since the corrections are significant. The abundance ratios are similar in the two stars and resemble those of Galactic stars of the same metallicity. On the deep photometric images we could detect several stars that lie to the blue of the turn-off. conclusions heading (optional), leave it empty if necessary The radial velocities imply that both stars are members of the Crater stellar system. The difference in velocity between the two taken at face value, implies a velocity dispersion > 3.7 km/s at 95% confidence level. Our spectroscopic metallicities are in excellent agreement with that determined by previous investigations using photometry. Our deep photometry and the spectroscopic metallicity imply an age of 7 Gyr for the main population of the system. The stars to the blue of the turn-off can be interpreted as a younger population, of the same metallicity and an age of 2.2 Gyr. Finally, spatial and kinematical parameters support the idea that this system is associated to the galaxies Leo~IV and Leo~V. All the observations favour the interpretation of Crater as a dwarf galaxy. (Abridged).

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arXiv2015

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

We derived elemental abundances in 27 Cepheids, the great majority situated within a zone of Galactocentric distances ranging from 5 to 7 kpc. One star of our sample, SU Sct, has a Galactocentric distance of about 3 kpc, and thus falls in a poorly investigated region of the inner thin disc. Our new results, combined with data on abundances in the very central part of our Galaxy taken from literature, show that iron, magnesium, silicon, sulfur, calcium and titanium LTE abundance radial distributions, as well as NLTE distribution of oxygen reveal a plateau-like structure or even positive abundance gradient in the region extending from the Galactic center to about 5 kpc.

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arXiv2014

Oxygen abundance distribution in Galactic disc

We have performed a NLTE analysis of the infrared oxygen triplet for a large number of cepheid spectra obtained with the Hobby-Eberly telescope. These data were combined with our previous NLTE results for the stars observed with Max Planck Gesellschaft telescope with the aim to investigate oxygen abundance distribution in Galactic thin disc. We find the slope of the radial (O/H) gradient value to be equal -0.058 dex/kpc. Nevertheless, we found that there could be a hint that the distribution might become flatter in the outer parts of the disc. This is also supported by other authors who studied open clusters, planetary nebulae and H II regions. Some mechanisms of flattening are discussed.

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arXiv2014

Light element abundances in the Galactic globular cluster 47 Tuc

It is very likely that most (perhaps all) Galactic globular clusters (GGCs) have experienced two or even more star-formation episodes. This is indicated, in particular, by peculiar chemical composition of the cluster stars which show large variation in the abundances of light elements, such as Li, C, N, O, Na, Mg, and Al. We studied the abundances of Na, Mg, and Al in the atmospheres of 103 red giant branch stars located below (49 stars) and above (54 stars) the RGB bump in the GGC 47 Tuc. Our results show that the spread of [Na/Fe] abundance ratios is about three times larger than that of [Mg/Fe]. Our data also confirm the existence of weak Na-Al correlation, similar to the one observed in other GGCs. At the same time, we find no evidence for the existence of three populations of stars characterized with different abundances of aluminum, as reported recently by Carretta et al. (2013).

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arXiv2012

Barium Abundances in Cepheids

We derived the barium atmospheric abundances for a large sample of Cepheids, comprising 270 stars. The sample covers a large range of galactocentric distances, from about 4 to 15 kpc, so that it is appropriated to investigate the existence of radial barium abundance gradients in the galactic disc. In fact, this is the first time that such a comprehensive analysis of the distribution of barium abundances in the galactic disc is carried out. As a result, we conclude that the Ba abundance distribution can be characterized by a zero gradient. This result is compared with derived gradients for other elements, and some reasons are briefly discussed for the independence of the barium abundances upon galactocentric distances.

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