Oleg P. Kochukhov

Abundance stratification and pulsation in the atmosphere of the roAp star

We present the evidence for abundance stratication in the atmosphere of the rapidly oscillating Ap star Equ. Ca, Cr, Fe, Ba, Si, Na seem to be overabundant in deeper atmospheric layers, but normal to underabundant in the upper layers with a transition in the typical line forming region of 1:5 < log5000 < 0:5. This stratication prole agrees well with diusion theory for Ca and Cr, developed for cool magnetic stars with a weak mass loss of 2:5 10 15 M yr 1 . Pr and Nd from the rare earth elements have an opposite prole. Their abundance is more than 6 dex higher above log5000 8:0 than in the deeper atmospheric layers. We further discuss the implications of abundance stratication in the context of radial velocity amplitudes and phases observed by Kochukhov & Ryabchikova (2001) for a variety of spectral lines and elements using high spectral and time resolved, high S/N observations.

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We investigate magnetic field geometry and surface distribution of chemical elements in the rapidly oscillating Ap star HR 3831. Results of the model atmosphere analysis of the spectra of this star are combined with the Hipparcos parallax and evolutionary models to obtain new accurate estimates of the fundamental stellar parameters: Teff = 7650 K, log L/L� = 1.09, M/M� = 1.77 and an inclination angle i = 68 ◦ of the stellar axis of rotation. We find that the variation of the longitudinal magnetic field of HR 3831 and the results of our analysis of the magnetic intensification of Fe  lines in the spectrum of this star are consistent with a dipolar magnetic topology with a magnetic obliquity β = 87 ◦ and a polar strength Bp = 2. 5k G. We apply a multi-element abundance Doppler imaging inversion code for the analysis of the spectrum variability of HR 3831, and recover surface distributions of 17 chemical elements, including Li, C, O, Na, Mg, Si, Ca, Ti, Cr, Mn, Fe, Co, Ba, Y, Pr, Nd, Eu. Our study represents the most thorough examination of the surface chemical structure in a magnetic Ap star and provides important observational constraints for modelling radiative diffusion in magnetic stars. The exceedingly high quality of some of our spectroscopic data allowed us to reconstruct unprecedented details of abundance distributions, demonstrating a high level of complexity in the surface structure down to the resolution limit of the Doppler maps. The Doppler imaging analysis of HR 3831 forms a basis for subsequent detailed observational investigations and theoretical modelling of non-radial oscillations in this star. We discuss the compound effect of the chemical nonuniformities and pulsational velocity field on the rapid line profile variations, and assess the possibility of identifying pulsation modes by using spatial filtering produced by an inhomogeneous abundance distribution. The results of our study of the surface chemical structure suggest that differences in pulsational behaviour of lines of different ions observed for HR 3831 are not a consequence of horizontal atmospheric inhomogeneities, but predominantly a depth effect.

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The variability of the Hg ii � 3984 line in the primary of the binary starAnd was discovered through the examination of high-dispersion spectra with signal-to-noise ratios greater than 500. This first definitively identified spectrum variation in any mercury-manganese star is not due to the orbital motion of the compan- ion. Rather, the variation is produced by the combination of the 2.38236 day period of rotation of the pri- mary that we determined and a nonuniform surface distribution of mercury that is concentrated in its equatorial region. If the surface mercury distribution exhibits long-term stability, then it is likely that a weak magnetic field operates in its atmosphere, but if changes are observed in the line profile over a period of a few years, then these would constitute direct evidence for diffusion. Subject headings: binaries: general — binaries: spectroscopic — stars: chemically peculiar — stars: individual (� Andromedae)

Multi-element abundance Doppler imaging of the rapidly oscillating Ap star HR 3831

We present results of an abundance and stratification analysis of the weakly magnetic chemically peculiar star HD 204411 based on the echelle spectrum obtained with the high resolution spectrograph at the 3.55-m Telescopio Nazionale Galileo at the Observatorio del Roque de los Muchachos (La Palma, Spain). Atmospheric parameters obtained from the spec- troscopy and spectrophotometry together with the Hipparcos parallax show that this star has already left the Main Sequence band. The upper limit for the surface magnetic field derived from the differential broadening of the spectral lines with different magnetic sensitivity is 750 G, which agrees with the recent detection of the weak effective magnetic field in this star. The best fit to the observed spectral line profiles was obtained with a combination of the rotational velocity ve sin i = 5.4 km s −1 and the radial-tangential macroturbulence of 4.8 km s −1 . The average abundances of HD 204411 are typical for an Ap star of the Cr-type: C and O are deficient, Cr and Fe are strongly overabundant. Sr, Y, Zr and the rare-earths, which usually have large overabundances in cool Ap stars with strong magnetic fields, are either normal (Y, Ce) or only +0.5 dex overabundant in the weakly magnetic star HD 204411. The chemical stratification analysis was performed for 5 elements, Mg, Si, Ca, Cr and Fe. Si, Ca and Fe show a tendency to be concentrated below log τ5000 = −1, while for Mg we found marginal evidence for concentration in the upper atmosphere. This behaviour of Mg may be an artifact caused by the limited sample of spectral lines and poor atomic data available for the Mg  lines used in our analysis. Chromium, the most anomalous Fe-peak element, does not show significant abundance gradients in the line-forming region.

The Variability of the Hg II λ3984 Line of the Mercury-Manganese Star α Andromedae

We present an analysis of 210 high-resolution time-resolved spectropolarimetric observations of the roAp star γ Equ obtained over three nights in August and September 2003. Radial velocity variations due to p-mode non-radial pulsations are clearly detected in the lines of rare-earth elements, in particular Pr  ,N d and Nd . In contrast, we find absolutely no evidence for the variation of the mean longitudinal magnetic field over the pulsation period in γ Equ at the level of 110-240 G which was recently reported by Leone & Kurtz (2003). Our investigation of the variability of circularly polarized profiles of 13 Nd  lines demonstrates that, at the 3σ confidence level, no magnetic field variation with an amplitude above ≈40−60 G was present in γ Equ during our monitoring of this star.

Abundances and chemical stratification analysis in the atmosphere of Cr-type Ap star HD 204411

We discuss the properties of atmospheres of chemically peculiar magnetic stars. The slow evolution of global mag- netic fields leads to the development of an induced electric current in all conductive atmospheric layers. The Lorentz force, which results from the interaction between a magnetic field and the induced current, may change the atmospheric structure and provide insight into the formation and evolution of stellar magnetic fields. We developed a model atmosphere code that takes into account the Lorentz force in the equation of hydrostatic equilibrium, and computed a number of model atmospheres for magnetic A and B stars. The surface distribution of a magnetic field was assumed to be a dipole, slightly distorted by the induced atmospheric electric current. The interaction between the magnetic field and electric current is modelled in detail, tak- ing into account microscopic properties of the stellar plasma. The presence of a significant Lorentz force leads to substantial modification of the atmospheric structure and in particular the pressure stratification, which in turn influences the formation of absorption spectral features, especially hydrogen Balmer lines. Furthermore, we find that rotational modulation of the disk- average parameters of a global stellar magnetic field causes characteristic rotational variability of hydrogen lines. With our model, observable effects correspond to induced electric currents of the order of 10 −11 cgs, which requires characteristic field evolution times 2-3 orders of magnitude shorter than the field decay time estimated for magnetic A and B stars assuming fossil dipolar field topology in the stellar interior. Using the computations of our model atmospheres we consider an observational aspect of the problem and attempt to interpret photometric data on the variability of hydrogen lines within the framework of simplest model of the evolution of global magnetic fields. With the hydrogen line data we find tentative support for the presence of a non-negligible Lorentz force in the atmospheres of some magnetic chemically peculiar stars.

No magnetic field variation with pulsation phase in the roAp star

We describe an analysis of the time-resolved measurements of the surface magnetic field in the roAp star y Equ. We have obtained a high-resolution and high signal-to-noise (S/N) spectroscopic time series, and the magnetic field was determined using Zeeman-resolved profiles of the Fe II 6149.25 A and Fe I 6173.34 A lines. Contrary to recent reports, we do not find any evidence of magnetic variability with pulsation phase, and derive an upper limit of 5-10 G for pulsational modulation of the surface magnetic field in y Equ.

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We present a novel technique aimed at the investigation of the geometry of surface velocity fluctuations in non-radially pulsating stars. Our method is based on the Doppler imaging principle and enables an accurate reconstruction of a two-dimensional map of the pulsational disturbances using spectroscopic time-series observations. No specific assumptions on the shape of the pulsation geometry are made. This opens a unique possibility to go beyond the traditional mode identification analyses and study complex rotationally and magnetically distorted non-radial pulsation modes which cannot be described by a single spherical harmonic function. Performance and intrinsic limitations of the new Doppler imaging procedure are assessed with inversions of simulated data. Based on the results of these numerical tests we conclude that a reliable recovery of the surface pulsation velocity structures can be achieved for all types of pulsation geometries accompanied by significant line profile variations. It is also demonstrated that the Doppler mapping of the pulsation velocity field is remarkably robust with respect to errors in the adopted stellar parameters.

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The Magnetism in Massive Stars (MiMeS) Project is a consensus collaboration among the foremost international researchers of the physics of hot, massive stars, with the basic aim of understanding the origin, evolution and impact of magnetic fields in these objects. The cornerstone of the project is the MiMeS Large Program at the Canada-France-Hawaii Telescope, which represents a dedication of 640 hours of telescope time from 2008-2012. The MiMeS Large Program will exploit the unique capabilities of the ESPaDOnS spectropolarimeter to obtain critical missing information about the poorly-studied magnetic properties of these important stars, to confront current models and to guide theory.

The influence of magnetic fields on the hydrostatic structure of the atmospheres of chemically peculiar stars

We present results of stratification analyses for Mg, Si, Ca, Cr and Fe in the atmo- spheres of two Cr-type sharp-lined peculiar stars HD 133792 and HD 204411. Even in the absence of strong magnetic fields their atmospheres are stable which is confirmed by zero microturbu- lence. Both stars are slightly evolved, their log g ≈ 3.5, whereas their effective temperatures are 9200 and 8400 K, respectively. We find a moderate Cr and Fe stratification. However the abundances of these elements do not fall below the solar value throughout the atmosphere. Si and Ca are strongly stratified, in particular in HD 133792, with large underabundances in the outer atmospheric layers. Abundance stratifications derived for weakly magnetic Ap stars are compared with the empirically determined stratification in the atmospheres of strongly magnetic Ap stars in the same temperature range.