Tavrian National University

Simferopol’, Ukraine

Tavrian National University

Simferopol’, Ukraine
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Lehmann H.,Thuringer Landessternwarte Tautenburg | Tkachenko A.,Thuringer Landessternwarte Tautenburg | Semaan T.,University Paris Diderot | Gutierrez-Soto J.,University Paris Diderot | And 6 more authors.
Astronomy and Astrophysics | Year: 2011

Context. For asteroseismic modelling, analysis of the high-accuracy light curves delivered by the Kepler satellite mission needs support by ground-based, multi-colour and spectroscopic observations. Aims. We determine the fundamental parameters of SPB and β Cep candidate stars observed by the Kepler satellite mission and estimate the expected types of non-radial pulsators. Methods. We compared newly obtained high-resolution spectra with synthetic spectra computed on a grid of stellar parameters assuming LTE, and checked for NLTE effects for the hottest stars. For comparison, we determined T eff independently from fitting the spectral energy distribution of the stars obtained from the available photometry. Results. We determine T eff, log g, microturbulent velocity, v sin i, metallicity, and elemental abundance for 14 of the 16 candidate stars. Two stars are spectroscopic binaries. No significant influence of NLTE effects on the results could be found. For hot stars, we find systematic deviations in the determined effective temperatures from those given in the Kepler Input Catalogue. The deviations are confirmed by the results obtained from ground-based photometry. Five stars show reduced metallicity, two stars are He-strong, one is He-weak, and one is Si-strong. Two of the stars could be β Cep/SPB hybrid pulsators, four SPB pulsators, and five more stars are located close to the borders of the SPB instability region. © ESO, 2011.

Kolenberg K.,University of Vienna | Fossati L.,Open University Milton Keynes | Shulyak D.,University of Gottingen | Pikall H.,University of Vienna | And 3 more authors.
Astronomy and Astrophysics | Year: 2010

Context.The knowledge of accurate stellar parameters is a keystone in several fields of stellar astrophysics, such as asteroseismology and stellar evolution. Although the fundamental parameters can be derived from both spectroscopy and multicolour photometry, the results obtained are sometimes affected by systematic uncertainties. Stellar pulsation reaches high amplitudes in RR Lyrae stars, and as a consequence the stellar parameters vary significantly over the pulsation cycle. The abundances of the star, however, are not expected to change. Aims.We present a self-consistent spectral analysis of the pulsating star RR Lyr, which is the primary target of our study of the Blazhko effect. Methods.We used high-resolution and high signal-to-noise ratio spectra to carry out a consistent parameter determination and abundance analysis for RR Lyr. The LLmodels code was employed for model atmosphere calculations, while the synth3 and width9 codes were used for line profile calculations and LTE abundance analysis. We describe in detail the methodology adopted to derive the fundamental parameters and the abundances. From a set of available high-resolution spectra of RR Lyr, we selected the phase of maximum radius at which the spectra are least disturbed by the pulsation. Using the abundances determined at this phase as a starting point, we expect to be able to determine the fundamental parameters determined at other phases more accurately. Results.The set of fundamental parameters obtained in this work fits the observed spectrum accurately. From the abundance analysis, we find clear indications of a depth-dependent microturbulent velocity, that we quantify. Conclusions. We confirm the importance of a consistent analysis of relevant spectroscopic features, the application of advanced model atmospheres, and the use of up-to-date atomic line data for determining stellar parameters. These results are crucial for further studies, e.g., detailed theoretical modelling of the observed pulsations. © 2010 ESO.

Tkachenko A.,Catholic University of Leuven | Degroote P.,Catholic University of Leuven | Aerts C.,Catholic University of Leuven | Aerts C.,Radboud University Nijmegen | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

We present a detailed analysis and interpretation of the high-mass binary V380 Cyg, based on high-precision space photometry gathered with the Kepler space mission as well as highresolution ground-based spectroscopy obtained with the HERMES spectrograph attached to the 1.2 m Mercator telescope. We derive a precise orbital solution and the full physical properties of the system, including dynamical component mass estimates of 11.43 ± 0.19 and 7.00 ± 0.14 M⊙ for the primary and secondary, respectively. Our frequency analysis reveals the rotation frequency of the primary in both the photometric and spectroscopic data and additional low-amplitude stochastic variability at low frequency in the space photometry with characteristics that are compatible with recent theoretical predictions for gravity-mode oscillations excited either by the convective core or by sub-surface convective layers. Doppler imaging analysis of the silicon lines of the primary suggests the presence of two high-contrast stellar surface abundance spots which are located either at the same latitude or longitude. Comparison of the observed properties of the binary with present-day single-star evolutionary models shows that the latter are inadequate and lack a serious amount of near-core mixing. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Kochukhov O.,Uppsala University | Alentiev D.,Tavrian National University | Alentiev D.,University of Porto | Ryabchikova T.,Russian Academy of Sciences | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We have carried out a survey of short-period pulsations among a sample of carefully chosen cool Ap stars using time-resolved observations with the Ultraviolet and Visual Echelle Spectrograph at the European Southern Observatory ESO 8 m Very Large Telescope. Here, we report the discovery of pulsations with amplitudes 50-100 m s-1 and periods 7-12 min in HD 132205, HD 148593 and HD 151860. These objects are therefore established as new rapidly oscillating Ap (roAp) stars. In addition, we independently confirm the presence of pulsations in HD 69013, HD 96237 and HD 143487 and detect, for the first time, radial velocity oscillations in two previously known photometric roAp stars HD 119027 and HD 185256. At the same time, no pulsation variability is found for HD 5823, HD 178892 and HD 185204. All of the newly discovered roAp stars were previously classified as non-pulsating based on the low-precision ground-based photometric surveys. This shows that such observations cannot be used to reliably distinguish between pulsating and non-pulsating stars and that all cool Ap stars may harbour p-mode pulsations of different amplitudes. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Temnenko V.,Tavrian National University
Electronic Journal of Theoretical Physics | Year: 2015

The formulation of classical singlet-triplet (electroweak) theory, in which point particles are replaced by continual current fields, has been presented. The expression for the Lagrangian of the theory and complementary algebraic and differential constraints, imposed on the currents of the theory, have been suggested. Classification of stationary and wave states for the singlet-triplet theory has been given. Stationary states correspond to massive particles with the current zone of a finite volume. The theory contains a number of wave states, both one-sector (singlet or triplet waves) and compound two-sector ones (singlet-triplet waves). Wave states differ in number of currents: zero-current waves (free singlet or free triplet waves), onecurrent, two-current, three-current and four-current ones. Wave states also differ in character of four-dimensional wave vector (the waves with time-like and space-like wave vector). Some forms of waves may have negative density of energy. Some wave states can be treated as classical models of a neutrino. Neutrino states are classified in accordance with the character of the current which forms the state: singlet (maxwellian) neutrino, Yang-Mills triplet neutrino, Maxwell-Yang-Mills singlet-triplet neutrino. © Electronic Journal of Theoretical Physics.

Tkachenko A.,Catholic University of Leuven | Van Reeth T.,Catholic University of Leuven | Tsymbal V.,Tavrian National University | Aerts C.,Catholic University of Leuven | And 3 more authors.
Astronomy and Astrophysics | Year: 2013

Context. The MOST, CoRoT, and Kepler space missions have led to the discovery of a large number of intriguing, and in some cases unique, objects among which are pulsating stars, stars hosting exoplanets, binaries, etc. Although the space missions have delivered photometric data of unprecedented quality, these data are lacking any spectral information and we are still in need of ground-based spectroscopic and/or multicolour photometric follow-up observations for a solid interpretation. Aims. The faintness of most of the observed stars and the required high signal-to-noise ratio (S/N) of spectroscopic data both imply the need to use large telescopes, access to which is limited. In this paper, we look for an alternative, and aim for the development of a technique that allows the denoising of the originally low S/N (typically, below 80) spectroscopic data, making observations of faint targets with small telescopes possible and effective. Methods. We present a generalization of the original least-squares deconvolution (LSD) method by implementing a multicomponent average profile and a line strengths correction algorithm. We tested the method on simulated and real spectra of single and binary stars, among which are two intrinsically variable objects. Results. The method was successfully tested on the high-resolution spectra of Vega and a Kepler star, KIC 04749989. Application to the two pulsating stars, 20 Cvn and HD 189631, showed that the technique is also applicable to intrinsically variable stars: the results of frequency analysis and mode identification from the LSD model spectra for both objects are in good agreement with the findings from literature. Depending on the S/N of the original data and spectral characteristics of a star, the gain in S/N in the LSD model spectrum typically ranges from 5 to 15 times. Conclusions. The technique introduced in this paper allows an effective denoising of the originally low S/N spectroscopic data. The high S/N spectra obtained this way can be used to determine fundamental parameters and chemical composition of the stars. The restored LSD model spectra contain all the information on line profile variations present in the original spectra of pulsating stars, for example. The method is applicable to both high- (>30 000) and low- (<30 000) resolution spectra, although the information that can be extracted from the latter is limited by the resolving power itself. © ESO, 2013.

Cunha M.S.,University of Porto | Cunha M.S.,University of Sydney | Alentiev D.,Tavrian National University | Brandao I.M.,University of Porto | Perraut K.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory
Monthly Notices of the Royal Astronomical Society | Year: 2013

Rapidly oscillating Ap stars are unique objects in the potential they offer to study the interplay between a number of important physical phenomena, in particular, pulsations, magnetic fields, diffusion and convection. Nevertheless, the simple understanding of how the observed pulsations are excited in these stars is still in progress. In this work, we perform a test to what is possibly the most widely accepted excitation theory for this class of stellar pulsators. The test is based on the study of a subset of members of this class for which stringent data on the fundamental parameters are available thanks to interferometry. For three out of the four stars considered in this study, we find that linear, non-adiabatic models with envelope convection suppressed around the magnetic poles can reproduce well the frequency region where oscillations are observed. For the fourth star in our sample no agreement is found, indicating that a new excitation mechanism must be considered. For the three stars whose observed frequencies can be explained by the excitation models under discussion, we derive the minimum angular extent of the region where convection must be suppressed. Finally, we find that the frequency regions where modes are expected to be excited in these models are very sensitive to the stellar radius. This opens the interesting possibility of determining this quantity and related ones, such as the effective temperature or luminosity, from comparison between model predictions and observations, in other targets for which these parameters are not well determined. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Sviridenko A.,Tavrian National University | Shcherbina O.,University of Vienna
Memetic Computing | Year: 2012

Five ordering algorithms for the nonserial dynamic programming algorithm for solving sparse discrete optimization problems are compared in this paper. The benchmarking reveals that the ordering of the variables has a significant impact on the run-time of these algorithms. In addition, it is shown that different orderings are most effective for different classes of problems. Finally, it is shown that, amongst the algorithms considered here, heuristics based on maximum cardinality search and minimum fill-in perform best for solving the discrete optimization problems considered in this paper. © 2012 Springer-Verlag.

Alentiev D.,Tavrian National University | Alentiev D.,University of Porto | Kochukhov O.,Uppsala University | Ryabchikova T.,Russian Academy of Sciences | And 3 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2012

We present the discovery of a long-period, rapidly oscillating Ap (roAp) star, HD177765. Using high-resolution time-series observations obtained with theUltraviolet andVisual Echelle Spectrograph at the European Southern Observatory Very Large Telescope, we found radial velocity variations with amplitudes 7-150 ms-1 and a period of 23.6 min, exceeding that of any previously known roAp star. The largest pulsation amplitudes are observed for Eu III, Ce III and for the narrow core of Há. We derived the atmospheric parameters and chemical composition of HD177765, showing this star to be similar to other long-period roAp stars. Comparison with theoretical pulsational models indicates an advanced evolutionary state for HD177765. Abundance analyses of this and other roAp stars suggest a systematic variation with age of the rare-earth line anomalies seen in cool Ap stars. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Temnenko V.,Tavrian National University
Electronic Journal of Theoretical Physics | Year: 2014

The formulation of classical electrodynamics, in which point charges are replaced by distributed current field, has been given. It has been demonstrated that 4-vector of current inside non-point particle is space-like and must be considered as a primary value which is not reduced to a charge carrier motion. A closed system of equations, describing simultaneous evolution of current and electro-magnetic field, has been introduced. The formula that makes calculation of mass ratio of different particles possible has been derived. There has been demonstrated the necessity to count space-time curvature inside non-point particles. The possibility of approximate preservation of the conception of point particle with spin moment with quasi-stationary description of the particle's movement in a weak electro-magnetic field has been shown. The solution of one of the particular problems of the constructed theory allows to suggest the existence of a new particle - heavy photon. One of the possible classical neutrino models, Maxwell neutrino, has been described. The theory requires the introduction of a fundamental constant which has the dimension of length. © Electronic Journal of Theoretical Physics.

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