L. A. Yakovina

National Academy of Sciences of Ukraine, Kiev, Misto Kyyiv, Ukraine

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Publications (38)42.77 Total impact

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    ABSTRACT: We determine lithium abundances in atmospheres of three carbon stars from synthetic spectrum fitting in the λλ 668–674 nm range using the Li I λ 670.8 nm resonance doublet. To produce synthetic spectra, we use a modified list of atomic lines from the VALD database and three alternative line lists of CN and C2 molecules which are modifications of line lists from the Jorgensen’s website (http://stella.nbi.dk) and from the Kurucz database (1993, CD-ROM nos. 1–23). The spectral lines from these lists were tested by matching synthetic spectra to observed spectra of the sun, Arcturus, and early R star HD 100764. We perform analysis of the blends involving the Li I λ 670.8 nm doublet in spectra of N stars AW Cyg and UX Dra. The lithium abundances in HD 100764, AW Cyg, and UX Dra are estimated to be lgN(Li) ≈ 2, −1.4, and −0.9, respectively. Discrepancies of lithium abundances lgN(Li) obtained with the help of molecular line lists do not seem to exceed 0.2 dex.
    Kinematics and Physics of Celestial Bodies 01/2011; 27(5):233-248. · 0.35 Impact Factor
  • Ya. V. Pavlenko, L. A. Yakovina
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    ABSTRACT: We calculated 240 model atmospheres of carbon giants with high carbon abundance (2800 K ≤ T eff ≤ 3400 K, 0.06 ≤ log(C/O) ≤ 2.7). This set of models was used for modelling of the energy distribution in the spectrum of an evolved carbon star DY Per demonstrating photometric features of the R CrB type stars. Most of the models are metal-poor (−3.5 ≤ [Fe/H] < 0) and a portion of them was calculated with the hydrogen deficiency (1/9 ≤ H/He < 9/1). The calculations of the models were carried out taking into account specific for carbon stars sources of opacity in the frame of the classic approaches. The opacity sampling method was used to calculate the opacity due to the atomic and molecular line absorption.
    Kinematics and Physics of Celestial Bodies 12/2009; 25(6):302-308. · 0.35 Impact Factor
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    ABSTRACT: We analyse the spectral energy distribution of the evolved carbon giant DY Per with a spectral synthesis technique. The red giant shows the photometric features of R CrB type stars. We derive the atmospheric parameters of DY Per using three variants of molecular line lists. We estimate Teff to be in the range 2900 < Teff < 3300 K. We adopted log g = 0. The star may be metal deficient and hydrogen deficient. The maximum possible carbon abundance in the star, [C]=0.94, provides the following atmospheric parameters: Tef=3100 K, [Fe/H] = 0, log(C/O)=0.6, [N/Fe] = 0, [H/He] = 0, with Jorgensen's line lists for the molecules C2 and CN. Comment: 4 pages, 2 tables, 3 figs, accepted by Odessa asronomical observatory Publ., 2009
    05/2009;
  • L. A. Yakovina, A. F. Pugach, Ya. V. Pavlenko
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    ABSTRACT: We have derived the atmospheric parameters of the R CrB carbon star DY Per. The spectrum of DY Per near its maximum brightness was obtained using the 2.6-m ZTSh telescope of the Crimean Astrophysical Observatory, and has a resolution of about 1.74 Å per pixel. We compare the absolute observed and theoretical spectral energy distributions (SEDs) of DY Per for λλ 430–730 nm for the first time. The model atmospheres were computed using a code written by Ya.V. Pavlenko in the classical approximation, taking into account the main opacity sources in carbon-star atmospheres. The theoretical SEDs were computed using the list of atomic lines from the VALD database and the molecular line lists from CD-ROM No. 18 of Kurucz’s database. The estimated by spectral synthesis effective temperature of DY Per is in the range of 2900–3100 K, if we assume log g = 0. We find a metal deficiency in the atmosphere of DY Per. Quantitative estimates of the overall metallicity, carbon and nitrogen abundances, and the H/He ratio are somewhat uncertain: −2 ≤ [Fe/H] ≤ −0.5, 0.65 ≤ [C/Fe] ≤ 1.35, 0.0 ≤ [N/Fe] ≤ 0.8, 1/9 ≤ H/He ≤ 9/1. These high H/He values do not quite agree with qualitative observational estimates made by other authors. Our results confirm that DY Per is a unique stellar object. This is the coolest and possibly the most metal-poor of all known R CrB stars. We conclude that the large deficiencies of metals and hydrogen lead to [C/Fe] values in the atmosphere of DY Per characteristic of R CrB stars.
    Astronomy Reports 01/2009; 53(3):187-202. · 0.76 Impact Factor
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    ABSTRACT: The method and results of the computation of the model atmospheres and spectral energy distributions of chemically peculiar stars are discussed. The models and spectra are computed with special consideration of the particular problems encountered for peculiar and hydrogen deficient stars in the later stages of evolution. We present some computed model atmospheres and fits to observed spectra of Sakurai's object, V838 Mon, and RS Oph.
    11/2008; 401:124.
  • Ya. V. Pavlenko, L. A. Yakovina
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    ABSTRACT: We calculated 240 model atmospheres of carbon giants of high carbon abundances (2800
    Odessa Astronomical Publications. 01/2008;
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    ABSTRACT: Context: The recurrent nova RS Ophiuchi undergoes nova eruptions every ~10-20 years as a result of thermonuclear runaway on the surface of a white dwarf close to the Chandrasekhar limit. Both the progress of the eruption and its aftermath depend on the (poorly known) composition of the red giant in the RS Oph system. Aims: Our aim is to understand better the effect of the giant secondary on the recurrent nova eruption. Methods: Synthetic spectra were computed for a grid of M-giant model atmospheres having a range of effective temperatures 3200 < T_eff < 4400 K, gravities 0 < log g < 1 and abundances -4 < [Fe/H] < 0.5, and fit to infrared spectra of RS Oph as it returned to quiescence after its 2006 eruption. We have modelled the infrared spectrum in the range 1.4-2.5 mum to determine metallicity and effective temperature of the red giant. Results: We find T_eff = 4100 ± 100 K, log g = 0.0 ± 0.5, [Fe/H] = 0.0 ± 0.5, [C/H] = -0.8 ± 0.2, [N/H] = +0.6 ± 0.3 in the atmosphere of the secondary, and demonstrate that inclusion of some dust ``veiling'' in the spectra cannot improve our fits.
    Astronomy and Astrophysics 01/2008; 485(2):541-545. · 5.08 Impact Factor
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    ABSTRACT: We compute opacities for the electronic molecular band systems A{^6}\sum{^+}-\times{^6}\sum{^+} of CrH and CrD, and A{^2}\prod-\times{^2}\sum{^+} of MgH and MgD. The opacities are computed by making use of existing spectroscopic constants for MgH and CrH. These constants are adjusted for the different reduced masses of MgD and CrD. Frank-Condon factors are used to provide intensities for the individual vibronic bands. These results are used in the computation of synthetic spectra between T{_e_f_f}= 1800 and 1200 K with an emphasis on the realization of `deuterium test', first proposed by Bejar et al. to distinguish brown dwarfs from planetary mass objects. We discuss the possible use of CrD and MgD electronic bands for the `deuterium test'. We find CrD to be the more promising of the two deuterides, potentially, the most useful bands of CrH/CrD are the \bigtriangleup v=+1 and \bigtriangleup v=-1 at 0.795 and 0.968 μm.
    Monthly Notices of the Royal Astronomical Society 01/2007; · 5.52 Impact Factor
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    ABSTRACT: We present the results of modelling the 0.45-1 mum spectral energy distribution of V838 Mon for 2002 November. Synthetic spectra were calculated using the NextGen model atmospheres of Hauschildt et al. (1999, ApJ, 512, 377), which incorporate line lists for H2O, TiO, CrH, FeH, CO, and MgH, as well as the VALD atomic line list. Fits to the observed spectra show that, in 2002 November, the effective temperature of V838 Mon was approximately 2000±100 K. Our theoretical spectra show a comparatively weak dependence on log g. Preliminary analysis of the hot star observed together with V838 Mon shows it to be a normal B3V dwarf.
    Astronomy and Astrophysics 01/2006; 460(1):245-250. · 5.08 Impact Factor
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    ABSTRACT: We compare high resolution infrared observations of the CO 3-1 bands in the 2.297-2.310 micron region of M dwarfs and one L dwarf with theoretical expectations. We find a good match between the observational and synthetic spectra throughout the 2000-3500K temperature regime investigated. Nonetheless, for the 2500-3500 K temperature range the temperatures that we derive from synthetic spectral fits are higher than expected from more empirical methods by several hundred K. In order to reconcile our findings with the empirical temperature scale it is necessary to invoke warming of the model atmosphere used to construct the synthetic spectra. We consider that the most likely reason for the back-warming is missing high temperature opacity due to water vapour. We compare the water vapour opacity of the Partridge & Schwenke (1997) line list used for the model atmosphere with the output from a preliminary calculation by Barber & Tennyson (2004). While the Partridge & Schwenke line list is a reasonable spectroscopic match for the new line list at 2000 K, by 4000 K it is missing around 25% of the water vapour opacity. We thus consider that the offset between empirical and synthetic temperature scales is explained by the lack of hot water vapour used for computation of the synthetic spectra. For our coolest objects with temperatures below 2500 K we find best fits when using synthetic spectra which include dust emission. Our spectra also allow us to constrain the rotational velocities of our sources, and these velocities are consistent with the broad trend of rotational velocities increasing from M to L. Comment: Accepted for publication in MNRAS (9 pages, 33 figures)
    Monthly Notices of the Royal Astronomical Society 12/2004; · 5.52 Impact Factor
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    ABSTRACT: We present the results of our analysis of a high resolution (R ~ 30000) infrared spectrum of V4334 Sgr (Sakurai's Object) around the first overtone CO bands, obtained in 1998 July. The 12CO and 13CO bands are well-resolved, and we compute synthetic hydrogen-deficient model atmosphere spectra to determine the 12C/13C ratio. We find 12C/13C ~ 4 +/-1, consistent with the interpretation of V4334 Sgr as an object that has undergone a very late thermal pulse. Comment: 5 pages, 7 figs, accepted by AAL
    Astronomy and Astrophysics 02/2004; · 5.08 Impact Factor
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    Larisa A. Yakovina, Yakiv V. Pavlenko, Carlos Abia
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    ABSTRACT: Lithium abundances in the atmospheres of the super Li-rich C-giants WZ Cas and WX Cyg are derived by the spectral synthesis technique {\bf using} the Li I resonance line at \lambda 670.8 nm and {\bf three subordinate} lines at \lambda\lambda 812.6, 610.4 and 497.2 nm. The differences between the Li abundances derived from the \lambda 670.8 nm line and the \lambda\lambda 497.2, 812.6 nm lines do not exceed $\pm$ 0.5 dex. The lithium line at \la610.4 nm provides typically lower abundances than the resonance line (by $\approx$ 1 dex). The mean LTE and NLTE Li abundances from three Li I lines (excluding \la610.4 nm) are 4.7, 4.9 for WZ Cas, and 4.6, 4.8 for WX Cyg, respectively. Comment: 9 pages, 3 tables, 2 figs. To be published in AS&S
    Astrophysics and Space Science 08/2002; · 2.06 Impact Factor
  • L. A. Yakovina, Ya. V. Pavlenko
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    ABSTRACT: Problems and procedure of the lithium abundance determination in the atmospheres of superlithium C giants using the resonance Li I line lambda 670.8 nm and subordinate lines lambdalambda 812.6, 610.4, and 497.2 nm are considered. LTE and NLTE lithium abundances from these lines in the atmospheres of J stars WZ Cas log N(Li) = 4.7, 4.9 and WX Cyg (4.6, 4.8) were determined. We found that among the subordinate Li I lines the line lambda 497.2 nm is the best lithium abundance indicator in the spectra of superlithium giants.
    Kinematika i Fizika Nebesnykh Tel. 04/2002;
  • Ya. Pavlenko, L. A. Yakovina, H. W. Duerbeck
    01/2001;
  • L. Yakovina, Ya. Pavlenko
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    ABSTRACT: Subordinate Li I line λ 460.3 nm is quite strong and sensitive on lithium abundances at log N(Li) =3-5 that are typical in atmospheres of super lithium-rich AGB giants. However, this line cannot be a good indicator of lithium abundances due to severe blending, especially by line Fe I λ 460.294 nm. On the other hand, a sensitivity of calculated blend containing Li I line λ 460.3 nm on log N(Li) increases for more complete system of continuum opacity sources in the blue and if use this indicator for metal-deficient stars.
    Odessa Astronomical Publications. 01/2001;
  • L. A. Yakovina, Ya. V. Pavlenko
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    ABSTRACT: Seven Li I lines in the optical and near IR region are considered as lithium abundance indicators in the atmospheres of super-Li-rich cool carbon stars. The resonance Li I line lambda 670.8 nm is extremely sensitive to the lithium abundance at logN(Li) = 4.0...5.0 due to the sensitivity to logN(Li) of the line wings mainly. This requires a high accuracy of their calculations. The subordinate Li I lines lambda 610.4 and lambda 812.6 ­¬ are saturated and show a rather weak dependence on logN(Li) in the range 4.0...5.0. Weaker subordinate Li I lines lambdalambda 497.2, 460.3, 427.3, and 413.3 nm can be potentially good lithium abundance indicators at logN(Li) = 4.0...5.0. However, only the line Li I lambda 497.2 nm can be used for lithium abundance determinations in stars with normal metallicities because of strong blending of three other lines and a "violet depression" in the spectra of the majority of carbon giants.
    Kinematika i Fizika Nebesnykh Tel. 01/2001;
  • Ya. Pavlenko, L. A. Yakovina
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    ABSTRACT: Results of the computation of spectral energy distributions (SED) in spectra of Sakurai's object (V4334 Sgr) are discussed. Hydrogen deficient and carbon-reach model atmospheres with Teff 4000-6200 K and log{g} = 1.0 - 0.0 are computed by the technique of opacity sampling taking into account molecular and atomic absorption for abundances determined by Asplund et al. (1997) and T. Kipper & Klochkova (1997). The computed SED's (lambdalambda 300-1000 nm) are fitted to the observed ones. Our conclusion is as follows: a) the theoretical SED's of Sakurai's object show a strong dependence on Teff; b) in the frame of our self-consistent approach we estimate Teff, log{g}, EB-V from the fit of computed SEDs to the observed spectra.
    Kinematika i Fizika Nebesnykh Tel Supplement. 09/2000;
  • Ya. V. Pavlenko, L. A. Yakovina
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    ABSTRACT: Theoretical spectral energy distributions for Sakurai's object at 300–1000 nm are derived. A model-atmosphere grid with T eff=5000–6250 K and logg=0.0–1.0 is computed for the chemical composition of Sakurai's object using opacity sampling including molecular and atomic absorption. Opacity due to absorption in 20 band systems of diatomic molecules is computed using the JOLA technique. The theoretical fluxes are compared with the observed energy distribution in a spectrum of Sakurai's object taken in April 1997. It is shown that (a) the theoretical energy distributions agree well qualitatively with the observed spectrum and depend strongly on the effective temperature; (b) C2 and CN molecular bands are dominant in the visible and near-infrared spectrum, while atomic absorption is important at UV and blue wavelengths; and (c) comparison of the observed and computed spectra yields an effective temperature for Sakurai's object in April 1997 T eff≈5250–5500 K. The dependence of the computed spectra at 300–1000 nm on the input parameters and adopted approximations is also discussed.
    Astronomy Reports 01/2000; 44(4):209-218. · 0.76 Impact Factor
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    Ya. V. Pavlenko, L. A. Yakovina, H. W. Duerbeck
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    ABSTRACT: Theoretical spectral energy distributions were computed for a grid of hydrogen-deficient and carbon-rich model atmospheres of T(eff) in the range of 5000-6250 K and log g = 1.0 - 0.0 by the technique of opacity sampling, taking into account continuous, molecular band and atomic line absorption. These energy distributions were compared with the spectrum of V4334 Sgr (Sakurai's object) of April, 1997 in the wavelength interval 300-1000 nm. We show that (1) the shape of the theoretical spectra depends strongly on T(eff) but only very weakly on the hydrogen abundance; (2) the comparison of the observed and computed spectra permits to estimate T(eff) approximately 5500 K for V4334 Sgr in April, 1997, and its interstellar reddening (plus a possible circumstellar contribution) E(B-V) approximately 0.70. Comment: 7 pages, 8 figures, LaTeX, accepted by Astronomy and Astrophysics
    Astronomy and Astrophysics 12/1999; · 5.08 Impact Factor
  • Ya. Pavlenko, L. Yakovina, I.S. Savanov
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    ABSTRACT: Without Abstract
    Astrophysics and Space Science 06/1999; 265(1):445-447. · 2.06 Impact Factor