Shulyak, Denis V.

[["dc.bibliographiccitation.artnumber","A88"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","520"],["dc.contributor.author","Shulyak, D."],["dc.contributor.author","Ryabchikova, Tanya"],["dc.contributor.author","Kildiyarova, R."],["dc.contributor.author","Kochukhov, Oleg"],["dc.date.accessioned","2018-11-07T08:39:29Z"],["dc.date.available","2018-11-07T08:39:29Z"],["dc.date.issued","2010"],["dc.description.abstract","Aims. Among the known Ap stars, HD 101065 is probably one of the most interesting objects, demonstrating very rich spectra of rare-earth elements (REE). Strongly peculiar photometric parameters of this star can not be fully reproduced by any modern theoretical calculations, even those accounting for realistic chemistry of its atmosphere. In this study we investigate a role of missing REE line opacity and construct a self-consistent atmospheric model based on accurate abundance and chemical stratification analysis. Methods. We employed the LLMODELS stellar model atmosphere code together with DDAFIT and SYNTHMAG software packages to derive homogeneous and stratified abundances for 52 chemical elements and to construct a self-consistent model of HD 101065 atmosphere. The opacity in REE lines is accounted for in details, by using up-to-date extensive theoretical calculations. Results. We show that REE elements play a key role in the radiative energy balance in the atmosphere of HD 101065, leading to the strong suppression of the Balmer jump and energy redistribution very different from that of normal stars. Introducing new line lists of REEs allowed us to reproduce, for the first time, spectral energy distribution of HD 101065 and achieve a better agreement between the unusually small observed Stromgren c(1) index and the model predictions. Using combined photometric and spectroscopic approaches and based on the iterative procedure of abundance and stratification analysis we find effective temperature of HD 101065 to be T(eff) = 6400 K."],["dc.identifier.doi","10.1051/0004-6361/200913750"],["dc.identifier.fs","581149"],["dc.identifier.isi","000283064200095"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9685"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/19008"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Realistic model atmosphere and revised abundances of the coolest Ap star HD 101065"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A151"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","537"],["dc.contributor.author","Nesvacil, N."],["dc.contributor.author","Lueftinger, T."],["dc.contributor.author","Shulyak, D."],["dc.contributor.author","Obbrugger, M."],["dc.contributor.author","Weiss, W."],["dc.contributor.author","Drake, N. A."],["dc.contributor.author","Hubrig, S."],["dc.contributor.author","Ryabchikova, Tanya"],["dc.contributor.author","Kochukhov, Oleg"],["dc.contributor.author","Piskunov, Nikolai E."],["dc.contributor.author","Polosukhina, N."],["dc.date.accessioned","2018-11-07T09:15:34Z"],["dc.date.available","2018-11-07T09:15:34Z"],["dc.date.issued","2012"],["dc.description.abstract","Context. In atmospheres of magnetic main-sequence stars, the diffusion of chemical elements leads to a number of observed anomalies, such as abundance spots across the stellar surface. Aims. The aim of this study was to derive a detailed picture of the surface abundance distribution of the magnetic chemically peculiar star HD 3980. Methods. Based on high-resolution, phase-resolved spectroscopic observations of the magnetic A-type star HD 3980, the inhomogeneous surface distribution of 13 chemical elements (Li, O, Si, Ca, Cr, Mn, Fe, La, Ce, Pr, Nd, Eu, and Gd) has been reconstructed. The INVERS12 code was used to invert the rotational variability in line profiles to elemental surface distributions. Results. Assuming a centered, dominantly dipolar magnetic field configuration, we find that Li, O, Mg, Pr, and Nd are mainly concentrated in the area of the magnetic poles and depleted in the regions around the magnetic equator. The high abundance spots of Si, La, Ce, Eu, and Gd are located between the magnetic poles and the magnetic equator. Except for La, which is clearly depleted in the area of the magnetic poles, no obvious correlation with the magnetic field has been found for these elements otherwise. Ca, Cr, and Fe appear enhanced along the rotational equator and the area around the magnetic poles. The intersection between the magnetic and the rotational equator constitutes an exception, especially for Ca and Cr, which are depleted in that region. Conclusions. No obvious correlation between the theoretically predicted abundance patterns and those determined in this study could be found. This can be attributed to a lack of up-to-date theoretical models, especially for rare earth elements."],["dc.identifier.doi","10.1051/0004-6361/201117097"],["dc.identifier.fs","596778"],["dc.identifier.isi","000300416800151"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9586"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/27720"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Multi-element Doppler imaging of the CP2 star HD 3980"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A14"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","551"],["dc.contributor.author","Shulyak, D."],["dc.contributor.author","Ryabchikova, Tanya"],["dc.contributor.author","Kochukhov, Oleg"],["dc.date.accessioned","2018-11-07T09:27:21Z"],["dc.date.available","2018-11-07T09:27:21Z"],["dc.date.issued","2013"],["dc.description.abstract","Aims. As a well-established procedure for the vast majority of normal main-sequence stars, determination of atmospheric and stellar parameters turns to be a challenging process in case of magnetic chemically peculiar stars. Inhomogeneous distribution of chemical elements and strong magnetic fields make most of the standard photometric and spectroscopic calibrations inapplicable for this class of stars. In this work we make use of available observed energy distributions calibrated to absolute units, stellar parallaxes, high-resolution spectroscopic observations, and advanced stellar atmosphere models to derive parameters of three bright Ap stars: 33 Lib, gamma Equ, and beta CrB. Methods. Model atmospheres and fluxes were computed with the LLmodels code. SYNTH3 and SYNTHMAG codes were used to compute profiles of individual spectral lines involved in abundance analysis. Results. For each of the stars, we construct a self-consistent atmospheric models assuming normal and depleted helium compositions and derive empirically stratification profiles of certain elements. The effective temperatures and surface gravities are found from the simultaneous fit to spectroscopic, photometric, and spectrophotometric observations calibrated to absolute units. We show that using advanced model atmospheres and accurate stellar parallaxes allows one to derive stellar radii with high accuracy, and these are consistent with those obtained from independent but more complicated interferometric observations."],["dc.identifier.doi","10.1051/0004-6361/201220425"],["dc.identifier.isi","000316460600014"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/10021"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/30512"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Fundamental parameters of bright Ap stars from wide-range energy distributions and advanced atmospheric models"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A28"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","552"],["dc.contributor.author","Nesvacil, N."],["dc.contributor.author","Shulyak, D."],["dc.contributor.author","Ryabchikova, Tanya"],["dc.contributor.author","Kochukhov, Oleg"],["dc.contributor.author","Akberov, A."],["dc.contributor.author","Weiss, W."],["dc.date.accessioned","2018-11-07T09:26:23Z"],["dc.date.available","2018-11-07T09:26:23Z"],["dc.date.issued","2013"],["dc.description.abstract","Context. Chemically peculiar A-type (Ap) stars are a subgroup of the CP2 stars that exhibit anomalous overabundances of numerous elements, e.g. Fe, Cr, Sr, and rare earth elements. The pulsating subgroup of Ap stars, the roAp stars, present ideal laboratories to observe and model pulsational signatures, as well as the interplay of the pulsations with strong magnetic fields and vertical abundance gradients. Aims. Based on high-resolution spectroscopic observations and observed stellar energy distributions, we construct a self-consistent model atmosphere for the roAp star 10 Aquilae (HD 176232). It accounts for modulations of the temperature-pressure structure caused by vertical abundance gradients. We demonstrate that such an analysis can be used to determine precisely the fundamental atmospheric parameters required for pulsation modelling. Methods. Average abundances were derived for 56 species. For Mg, Si, Ca, Cr, Fe, Co, Sr, Pr, and Nd, vertical stratification profiles were empirically derived using the DDAFIT minimisation routine together with the magnetic spectrum synthesis code SYNTHMAG. Model atmospheres were computed with the LLMODELS code, which accounts for the individual abundances and stratification of chemical elements. Results. For the final model atmosphere, T-eff = 7550 K and log (g) = 3.8 were adopted. While Mg, Si, Co, and Cr exhibit steep abundance gradients, Ca, Fe, and Sr showed much wider abundance gradients between log tau(5000) = -1.5 and 0.5. Elements Mg and Co were found to be the least stratified, while Ca and Sr showed strong depth variations in abundance of up to approximate to 6 dex."],["dc.identifier.doi","10.1051/0004-6361/201220320"],["dc.identifier.isi","000317912000028"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/10129"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/30284"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","A self-consistent chemically stratified atmosphere model for the roAp star 10 Aquilae"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]