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Simferopol’, Ukraine

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.

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.

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.

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.

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.

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