arXiv subjects

O. Kochukhov

Publications and source records attributed to O. Kochukhov.

At least 163 records · Page 9Linked to original sources

arXiv2010

Modelling the light variability of the Ap star epsilon Ursae Majoris

We simulate the light variability of the Ap star epsUMa using the observed surface distributions of Fe, Cr, Ca, Mn, Mg, Sr and Ti obtained with the help of Doppler Imaging technique. Using all photometric data available we specified light variations of epsUMa modulated by its rotation from far UV to IR. We employed the LLmodels stellar model atmosphere code to predict the light variability in different photometric systems. The rotational period of epsUMa is refined to 5d088631(18). It is shown that the observed light variability can be explained as a result of the redistribution of radiative flux from the UV spectral region to the visual caused by the inhomogeneous surface distribution of chemical elements. Among seven mapped elements, only Fe and Cr significantly contribute to the amplitude of the observed light variability. In general, we find a very good agreement between theory and observations. We confirm the important role of Fe and Cr to the magnitude of the well-known depression around 5200 Å through the analysis of the peculiar $a$-parameter. Finally, we show that the abundance spots of considered elements cannot explain the observed variability in near UV and $β$ index which are likely due to some other causes. The inhomogeneous surface distribution of chemical elements can explain most of the observed light variability of the A-type CP star epsUMa.

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arXiv2010

Modelling the molecular Zeeman effect in M-dwarfs: methods and first results

We present first quantitative results of the surface magnetic field measurements in selected M-dwarfs based on detailed spectra synthesis conducted simultaneously in atomic and molecular lines of the FeH Wing-Ford $F^4\,Δ-X^4\,Δ$ transitions. A modified version of the Molecular Zeeman Library (MZL) was used to compute Landé g-factors for FeH lines in different Hund's cases. Magnetic spectra synthesis was performed with the Synmast code. We show that the implementation of different Hund's case for FeH states depending on their quantum numbers allows us to achieve a good fit to the majority of lines in a sunspot spectrum in an automatic regime. Strong magnetic fields are confirmed via the modelling of atomic and FeH lines for three M-dwarfs YZ~CMi, EV~Lac, and AD~Leo, but their mean intensities are found to be systematically lower than previously reported. A much weaker field ($1.7-2$~kG against $2.7$~kG) is required to fit FeH lines in the spectra of GJ~1224. Our method allows us to measure average magnetic fields in very low-mass stars from polarized radiative transfer. The obtained results indicate that the fields reported in earlier works were probably overestimated by about $15-30$\%. Higher quality observations are needed for more definite results.

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arXiv2010

A detailed spectropolarimetric analysis of the planet hosting star WASP-12

The knowledge of accurate stellar parameters is paramount in several fields of stellar astrophysics, particularly in the study of extrasolar planets, where often the star is the only visible component and therefore used to infer the planet's fundamental parameters. Another important aspect of the analysis of planetary systems is the stellar activity and the possible star-planet interaction. Here we present a self-consistent abundance analysis of the planet hosting star WASP-12 and a high-precision search for a structured stellar magnetic field on the basis of spectropolarimetric observations obtained with the ESPaDOnS spectropolarimeter. Our results show that the star does not have a structured magnetic field, and that the obtained fundamental parameters are in good agreement with what was previously published. In addition we derive improved constraints on the stellar age (1.0-2.65 Gyr), mass (1.23-1.49 M/M0), and distance (295-465 pc). WASP-12 is an ideal object to look for pollution signatures in the stellar atmosphere. We analyse the WASP-12 abundances as a function of the condensation temperature and compare them with those published by several other authors on planet hosting and non-planet hosting stars. We find hints of atmospheric pollution in WASP-12's photosphere, but are unable to reach firm conclusions with our present data. We conclude that a differential analysis based on WASP-12 twins will probably clarify if an atmospheric pollution is present, the nature of this pollution and its implications in the planet formation and evolution. We attempt also the direct detection of the circumstellar disk through infrared excess, but without success.

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arXiv2010

Magnetic field, chemical composition and line profile variability of the peculiar eclipsing binary star AR Aur

AR Aur is the only eclipsing binary known to contain a HgMn star, making it an ideal case for a detailed study of the HgMn phenomenon. HgMn stars are a poorly understood class of chemically peculiar stars, which have traditionally been thought not to possess significant magnetic fields. However, the recent discovery of line profile variability in some HgMn stars, apparently attributable to surface abundance patches, has brought this belief into question. In this paper we investigate the chemical abundances, line profile variability, and magnetic field of the primary and secondary of the AR Aur system, using a series of high resolution spectropolarimetric observations. We find the primary is indeed a HgMn star, and present the most precise abundances yet determined for this star. We find the secondary is a weak Am star, and is possibly still on the pre-main sequence. Line profile variability was observed in a range of lines in the primary, and is attributed to inhomogeneous surface distributions of some elements. No magnetic field was detected in any observation of either stars, with an upper limit on the longitudinal magnetic field in both stars of 100 G. Modeling of the phase-resolve longitudinal field measurements leads to a 3 sigma upper limit on any dipole surface magnetic field of about 400 G.

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arXiv2010

Towards pulsation mode identification in 3-D: theoretical simulations of line profile variations in roAp stars

Time-resolved spectroscopic observations of rapidly oscillating Ap (roAp) stars show a complex picture of propagating magneto-acoustic pulsation waves, with amplitude and phase strongly changing as a function of atmospheric height. We have recently conducted numerical, non-linear MHD simulations to get an insight into the complex atmospheric dynamics of magnetic pulsators. Here we use the resulting time-dependent atmospheric structure and velocity field to predict line profile variations for roAp stars. These calculations use realistic atmospheric structure, account for vertical chemical stratification and treat the line formation in pulsating stellar atmosphere without relying on the simplistic single-layer approximation universally adopted for non-radial pulsators. The new theoretical calculations provide an essential tool for interpreting the puzzling complexity of the spectroscopic pulsations in roAp stars.

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arXiv2010

Realistic model atmosphere and revised abundances of the coolest Ap star HD101065

Among the known Ap stars, HD101065 is probably one of the most interesting objects, demonstrating very rich spectra of rare-earth elements (REE). Strongly peculiar photometric parameters of this star that 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. 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 HD101065 atmosphere. The opacity in REE lines is accounted for in details, by using up-to-date extensive theoretical calculations. We show that REE elements play a key role in the radiative energy balance in the atmosphere of HD101065, 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 HD101065 and achieve a better agreement between the unusually small observed Strömgren c1 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 HD101065 to be Teff=6400K.

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arXiv2010

Magnetic Doppler imaging of alpha^2 Canum Venaticorum in all four Stokes parameters. Unveiling the hidden complexity of stellar magnetic fields

Strong organized magnetic fields have been studied in the upper main sequence chemically peculiar stars for more than half a century. However, only recently have observational methods and numerical techniques become sufficiently mature to allow us to record and interpret high-resolution four Stokes parameter spectra, leading to the first assumption-free magnetic field models of these stars. Here we present a detailed magnetic Doppler imaging analysis of the spectropolarimetric observations of the prototypical magnetic Ap star alpha^2 CVn. The surface abundance distributions of Fe and Cr and a full vector map of the stellar magnetic field are reconstructed in a self-consistent inversion using our state-of-the-art magnetic Doppler imaging code Invers10. We succeeded in reproducing most of the details of the available spectropolarimetric observations of alpha^2 CVn with a magnetic map which combines a global dipolar-like field topology with localized spots of higher field intensity. We demonstrate that these small-scale magnetic structures are inevitably required to fit the linear polarization spectra; however, their presence cannot be inferred from the Stokes I and V observations alone. Our magnetic Doppler imaging analysis of alpha^2 CVn and previous results for 53 Cam support the view that the upper main sequence stars can harbour fairly complex surface magnetic fields which resemble oblique dipoles only at the largest spatial scales. Spectra in all four Stokes parameters are absolutely essential to unveil and meaningfully characterize this field complexity in Ap stars. We therefore suggest that understanding magnetism of stars in other parts of the H-R diagram is similarly incomplete without investigation of their linear polarization spectra.

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arXiv2009

Surface structure of the CoRoT CP2 target star HD 50773

We compare surface maps of the chemically peculiar star HD 50773 produced with a Bayesian technique and based on high quality CoRoT photometry with those derived from rotation phase resolved spectropolarimetry. The goal is to investigate the correlation of surface brightness with surface chemical abundance distribution and the stellar magnetic field. The rotational period of the star was determined from a nearly 60 day long continuous light curve obtained during the initial run of CoRoT. Using a Bayesian approach to star-spot modelling, which in this work is applied for the first time for the photometric mapping of a CP star, we derived longitudes, latitudes and radii of four different spot areas. Additional parameters like stellar inclination and the spot's intensities were also determined. The CoRoT observations triggered an extensive ground-based spectroscopic and spectropolarimetric observing campaign and enabled us to obtain 19 different high resolution spectra in Stokes parameters I and V with NARVAL, ESPaDOnS, and SemelPol spectropolarimeters. Doppler and Magnetic Doppler imaging techniques allowed us to derive the magnetic field geometry of the star and the surface abundance distributions of Mg, Si, Ca, Ti, Cr, Fe, Ni, Y, and Cu. We find a dominant dipolar structure of the surface magnetic field. The CoRoT light curve variations and abundances of most elements mapped are correlated with the aforementioned geometry: Cr, Fe, and Si are enhanced around the magnetic poles and coincide with the bright regions on the surface of HD 50773 as predicted by our light curve synthesis and confirmed by photometric imaging.

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arXiv2009

Magnetic Doppler imaging of the roAp star HD 24712

We present the first magnetic Doppler images of a rapidly oscillating Ap (roAp) star. We deduce information about magnetic field geometry and abundance distributions of a number of chemical elements on the surface of the hitherto best studied roAp star, HD 24712, using the magnetic Doppler imaging (MDI) code, INVERS10, which allows us to reconstruct simultaneously and consistently the magnetic field geometry and elemental abundance distributions on a stellar surface. For this purpose we analyse time series spectra obtained in Stokes I and V parameters with the SOFIN polarimeter at the Nordic Optical Telescope and recover surface abundance structures of sixteen different chemical elements, respectively ions, including Mg, Ca, Sc, Ti, Cr, Fe, Co, Ni, Y, La, Ce, Pr, Nd, Gd, Tb, and Dy. For the rare earth elements (REE) Pr and Nd separate maps were obtained using lines of the first and the second ionization stage. We find and confirm a clear dipolar structure of the surface magnetic field and an unexpected correlation of elemental abundances with respect to this field: one group of elements accumulates solely where the positive magnetic pole is visible, whereas the other group avoids this region and is enhanced where the magnetic equatorial region dominates the visible stellar surface. We also observe relative shifts of abundance enhancement- or depletion regions between the various elements exhibiting otherwise similar behaviour.

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arXiv2009

The Lorentz force in atmospheres of CP stars: 56 Arietis

The presence of electric currents in the atmospheres of magnetic chemically peculiar (mCP) stars could bring important theoretical constrains about the nature and evolution of magnetic field in these stars. The Lorentz force, which results from the interaction between the magnetic field and the induced currents, modifies the atmospheric structure and induces characteristic rotational variability of pressure-sensitive spectroscopic features, that can be analysed using phase-resolved spectroscopic observations. In this work we continue the presentation of results of the magnetic pressure studies in mCP stars focusing on the high-resolution spectroscopic observations of Bp star 56Ari. We have detected a significant variability of the Halpha, Hbeta, and Hgamma spectral lines during full rotation cycle of the star. Then these observations are interpreted in the framework of the model atmosphere analysis, which accounts for the Lorentz force effects. We used the LLmodels stellar model atmosphere code for the calculation of the magnetic pressure effects in the atmosphere of 56Ari taking into account realistic chemistry of the star and accurate computations of the microscopic plasma properties. The Synth3 code was employed to simulate phase-resolved variability of Balmer lines. We demonstrate that the model with the outward-directed Lorentz force in the dipole+quadrupole configuration is likely to reproduce the observed hydrogen lines variation. These results present strong evidences for the presence of non-zero global electric currents in the atmosphere of this early-type magnetic star.

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arXiv2009

Discovery of a magnetic field in the O9 sub-giant star HD 57682 by the MiMeS Collaboration

We report the detection of a strong, organised magnetic field in the O9IV star HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the 3.6-m Canada-France-Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Program. From the fitting of our spectra using NLTE model atmospheres we determined that HD 57682 is a $17^{+19}_{-9}$ M$_{\odot}$ star with a radius of $7.0^{+2.4}_{-1.8}$ R$_\odot$, and a relatively low mass-loss rate of $1.4^{+3.1}_{-0.95}\times10^{-9}$ M$_{\odot}$ yr$^{-1}$. The photospheric absorption lines are narrow, and we use the Fourier transform technique to infer $v\sin i=15\pm3$ km s$^{-1}$. This $v\sin i$ implies a maximum rotational period of 31.5 d, a value qualitatively consistent with the observed variability of the optical absorption and emission lines, as well as the Stokes $V$ profiles and longitudinal field. Using a Bayesian analysis of the velocity-resolved Stokes $V$ profiles to infer the magnetic field characteristics, we tentatively derive a dipole field strength of $1680^{+134}_{-356}$ G. The derived field strength and wind characteristics imply a wind that is strongly confined by the magnetic field.

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arXiv2009

Time-resolved photometric and spectroscopic analysis of a luminous Ap star HD103498

We present the results on the photometric and spectroscopic monitoring of a luminous Ap star HD103498. The time-series photometric observations were carried out on 17 nights using three-channel fast photometer attached to the 1.04-m optical telescope at ARIES, Nainital. The photometric data of five nights of year 2007 show clear signature of 15-min periodicity. However, the follow-up observations during 2007--2009 could not repeated any such periodicity. To confirm the photometric light variations, the time-series spectroscopic observations were carried out with the 2.56-m Nordic Optical Telescope (NOT) at La Palma on February 2, 2009. Any radial velocity variations were absent in this data set which is in full agreement with the photometric observations taken near the same night. Model atmosphere and abundance analysis of HD103498 show that the star is evolved from the Main Sequence and its atmospheric abundances are similar to two other evolved Ap stars HD133792 and HD204411: large overabundances of Si, Cr, and Fe and moderate overabundances of the rare-earth elements. These chemical properties and a higher effective temperature distinguish HD103498 from any known roAp star.

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arXiv2009

Simulations of magneto-acoustic pulsations in atmospheres of rapidly oscillating Ap stars

Rapidly oscillating Ap stars exhibit an astrophysically interesting combination of strong, dipolar-like magnetic fields and high-overtone p-mode pulsations similar to the Sun. Recent time-resolved spectroscopy of these stars unravelled a complex picture of propagating magneto-acoustic pulsation waves, with amplitude and phase strongly changing as a function of atmospheric height. To interpret these observations and gain a new insight into the atmospheric dynamics of roAp stars we have carried out 2-D time-dependent, non-linear magneto-hydrodynamical simulations of waves for a realistic atmospheric stratification of a cool Ap star. We explore a grid of simulations in a wide parameter space, treating oscillations of the velocity, magnetic field and thermodynamic quantities in a self-consistent manner. Our simulations foster a new understanding of the influence of the atmosphere and the magnetic field on the propagation and reflection properties of magneto-acoustic waves, formation of node surfaces, and relative variation of different quantities. Our simulations reproduce all main features of the observed pulsational behavior of roAp stars. We show, for the first time, that the overall depth dependence of the pulsations in roAp atmospheres is strongly influenced by the density inversion at the photospheric base.

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arXiv2009

The chemical abundance analysis of normal early A- and late B-type stars

Modern spectroscopy of early-type stars often aims at studying complex physical phenomena. Comparatively less attention is paid to identifying and studying the "normal" A- and B-type stars and testing how the basic atomic parameters and standard spectral analysis allow one to fit the observations. We wish to stablish whether the chemical composition of the solar photosphere can be regarded as a reference for early A- and late B-type stars. We have obtained optical high-resolution, high signal-to-noise ratio spectra of three slowly rotating early-type stars (HD 145788, 21 Peg and pi Cet) that show no obvious sign of chemical peculiarity, and performed a very accurate LTE abundance analysis of up to 38 ions of 26 elements (for 21 Peg), using a vast amount of spectral lines visible in the spectral region covered by our spectra. We provide an exhaustive description of the abundance characteristics of the three analysed stars with a critical review of the line parameters used to derive the abundances. We compiled a table of atomic data for more than 1100 measured lines that may be used in the future as a reference. The abundances we obtained for He, C, Al, S, V, Cr, Mn, Fe, Ni, Sr, Y, and Zr are compatible with the solar ones derived with recent 3D radiative-hydrodynamical simulations of the solar photosphere. The abundances of the remaining studied elements show some degree of discrepancy compared to the solar photosphere. Those of N, Na, Mg, Si, Ca, Ti, and Nd may well be ascribed to non-LTE effects; for P, Cl, Sc and Co, non-LTE effects are totally unknown; O, Ne, Ar, and Ba show discrepancies that cannot be ascribed to non-LTE effects. The discrepancies obtained for O (in two stars) and Ne agree with very recent non-LTE abundance analysis of early B-type stars in the solar neighbourhood.

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arXiv2009

Model atmospheres of chemically peculiar stars: Self-consistent empirical stratified model of HD24712

High-resolution spectra of some chemically peculiar stars clearly demonstrate the presence of strong abundance gradients in their atmospheres. However, these inhomogeneities are usually ignored in the standard scheme of model atmosphere calculations, braking the consistency between model structure and spectroscopically derived abundance pattern. In this paper we present first empirical self-consistent stellar atmosphere model of roAp star HD24712, with stratification of chemical elements included, and which is derived directly from the observed profiles of spectral lines without time-consuming simulations of physical mechanisms responsible for these anomalies. We used the LLmodels stellar model atmosphere code and DDAFIT minimization tool for analysis of chemical elements stratification and construction of self-consistent atmospheric model. Empirical determination of Pr and Nd stratification in the atmosphere of HD24712 is based on NLTE line formation for Prii/iii and Ndii/iii with the use of the DETAIL code. Based on iterative procedure of stratification analysis and subsequent re-calculation of model atmosphere structure we constructed a self-consistent model of HD24712, i.e. the model which temperature-pressure structure is consistent with results of stratification analysis. It is shown that stratification of chemical elements leads to the considerable changes in model structure as to compare with non-stratified homogeneous case. We find that accumulation of REE elements allows for the inverse temperature gradient to be present in upper atmosphere of the star with the maximum temperature increase of about 600K.

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arXiv2009

A self-consistent empirical model atmosphere, abundance and stratification analysis of the benchmark roAp star alpha Circini

Chemically peculiar (CP) stars are unique natural laboratories for investigation of the microscopic diffusion processes of chemical elements. The element segregation under the influence of gravity and radiation pressure leads to the appearance of strong abundance gradients in the atmospheres of CP stars. Consequently, the atmospheric temperature-pressure structure of these objects could deviate significantly from the atmospheres of normal stars with homogeneous abundances. In this study we performed a self-consistent, empirical model atmosphere study of the brightest rapidly oscillating Ap star alpha Cir. We account for chemical stratification in the model atmosphere calculations and assess the importance of non-uniformed vertical element distribution on the model structure, energy distribution and hydrogen line profiles. Based on an iterative procedure of the chemical abundance analysis of 52 ions of 35 elements, stratification modeling of 4 elements (Si, Ca, Cr and Fe) and subsequent re-calculations of the atmospheric structure, we derived a new model atmosphere of alpha Cir, which is consistent with the inferred atmospheric chemistry of the star. We find Teff=7500 K, logg=4.1, and demonstrate that chemical stratification has a noticeable impact on the model structure and modifies the formation of the hydrogen Balmer lines. Our spectroscopically determined Teff of alpha Cir agrees with the fundamental effective temperature of this star. This shows that temperatures inferred in detailed spectroscopic analyses of cool magnetic CP stars are not affected by a large systematic bias.

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arXiv2009

Asteroseismology of chemically peculiar stars

Pulsational variability is observed in several types of main sequence stars with anomalous chemical abundances. In this contribution I summarize the relationship between pulsations and chemical peculiarities, giving special emphasis to rapid oscillations in magnetic Ap stars. These magneto-acoustic pulsators provide unique opportunities to study the interaction of pulsations, chemical inhomogeneities, and strong magnetic fields. Time-series monitoring of rapidly oscillating Ap stars using high-resolution spectrometers at large telescopes and ultra-precise space photometry has led to a number of important breakthroughs in our understanding of these interesting objects. Interpretation of the roAp frequency spectra has allowed constraining fundamental stellar parameters and probing poorly known properties of the stellar interiors. At the same time, investigation of the pulsational wave propagation in chemically stratified atmospheres of roAp stars has been used as a novel asteroseismic tool to study pulsations as a function of atmospheric height and to map in detail the horizontal structure of the magnetically-distorted p-modes.

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arXiv2009

Simulations of magneto-hydrodynamic waves in atmospheres of roAp stars

We report 2D time-dependent non-linear magneto-hydrodynamical simulations of waves in the atmospheres of roAp stars. We explore a grid of simulations in a wide parameter space. The aim of our study is to understand the influence of the atmosphere and the magnetic field on the propagation and reflection properties of magneto-acoustic waves, formation of shocks and node layers.

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