| 1. |
- Kochukhov, Oleg, et al.
(author)
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Chemical stratification in the atmosphere of Ap star HD 133792. Regularized solution of the vertical inversion problem
- 2006
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 460:3, s. 831-842
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Journal article (peer-reviewed)abstract
- Context. High spectral resolution studies of cool Ap stars reveal conspicuous anomalies of the shape and strength of many absorption lines. This is a signature of large atmospheric chemical gradients ( chemical stratification) produced by the selective radiative levitation and gravitational settling of chemical species. Aims. Previous observational studies of the chemical stratification in Ap stars were limited to fitting simple parametrized chemical profiles. Here we present a new approach to mapping the vertical chemical structures in stellar atmospheres. Methods. We have developed a regularized chemical inversion procedure that uses all information available in high-resolution stellar spectra. The new technique for the first time allowed us to recover chemical profiles without making a priori assumptions about the shape of chemical distributions. We have derived average abundances and applied the vertical inversion procedure to the high-resolution VLT UVES spectra of the weakly magnetic, cool Ap star HD133792. Results. Our spectroscopic analysis yielded improved estimates of the atmospheric parameters of HD133792. We show that this star has negligible v(e) sin i and the mean magnetic field modulus < B > = 1.1 +/- 0.1 kG. We have derived average abundances for 43 ions and obtained vertical distributions of Ca, Si, Mg, Fe, Cr, and Sr. All these elements except Mg show high overabundance in the deep layers and solar or sub-solar composition in the upper atmosphere of HD133792. In contrast, the Mg abundance increases with height. Conclusions. We find that transition from the metal-enhanced to metal-depleted zones typically occurs in a rather narrow range of depths in the atmosphere of HD133792. Based on the derived photospheric abundances, we conclude that HD133792 belongs to the rare group of evolved cool Ap stars, which possesses very large Fe-peak enhancement, but lacks a prominent overabundance of the rare-earth elements.
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| 2. |
- Hareter, M., et al.
(author)
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MOST discovers a multimode delta Scuti star in a triple system : HD 61199
- 2008
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 492:1, s. 185-195
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Journal article (peer-reviewed)abstract
- Context. A field star, HD61199 (V approximate to 8), simultaneously observed with Procyon by the MOST (Microvariability & Oscillations of STars) satellite in continuous runs of 34, 17, and 34 days in 2004, 2005, and 2007, was found to pulsate in 11 frequencies in the delta Scuti range with amplitudes from 1.7 down to 0.09 mmag. The photometry also showed variations with a period of about four days. To investigate the nature of the longer period, 45 days of time-resolved spectroscopy was obtained at the Thuringer Landessternwarte Tautenburg in 2004. The radial velocity measurements indicate that HD61199 is a triple system. Aims. A delta Scuti pulsator with a rich eigenspectrum in a multiple system is promising for asteroseismology. Our objectives were to identify which of the stars in the system is the delta Scuti variable and to obtain the orbital elements of the system and the fundamental parameters of the individual components, which are constrained by the pulsation frequencies of the delta Scuti star.Methods. Classical Fourier techniques and least-squares multi-sinusoidal fits were applied to the MOST photometry to identify the pulsation frequencies. The groundbased spectroscopy was analysed with least-squares-deconvolution (LSD) techniques, and the orbital elements derived with the KOREL and ORBITX routines. Asteroseismic models were also generated. Results. The photometric and spectroscopic data are compatible with a triple system consisting of a close binary with an orbital period of 3.57 days and a d Scuti companion (HD61199 A) as the most luminous component. The d Scuti star is a rapid rotator with about v sin i = 130 km s(-1) and an upper mass limit of about 2.1 M-circle dot. For the close binary components, we find they are of nearly equal mass, with lower mass limits of about 0.7 M-circle dot. Comparisons to synthetic spectra indicate these stars have a late-F spectral type. The observed oscillation frequencies are compared to pulsation models to further constrain the evolutionary state and mass of HD61199 A. The orbit frequency of the close binary corresponds to the difference of the two d Scuti frequencies with the highest amplitudes a coincidence that is remarkable, but not explained.
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| 3. |
- Shulyak, D., et al.
(author)
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The Lorentz force in atmospheres of CP stars : theta Aurigae
- 2007
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 464:3, s. 1089-1099
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Journal article (peer-reviewed)abstract
- Several dynamical processes may induce considerable electric currents in the atmospheres of magnetic chemically peculiar (CP) 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 the hydrogen Balmer lines. To study this phenomena we have initiated a systematic spectroscopic survey of the Balmer lines variation in magnetic CP stars. In this paper we continue presentation of results of the program focusing on the high-resolution spectral observations of A0p star \aur (HD 40312). We have detected a significant variability of the H$\alpha$, H$\beta$, and H$\gamma$ spectral lines during full rotation cycle of the star. This variability is interpreted in the framework of the model atmosphere analysis, which accounts for the Lorentz force effects. Both the inward and outward directed Lorentz forces are considered under the assumption of the axisymmetric dipole or dipole+quadrupole magnetic field configurations. We demonstrate that only the model with the outward directed Lorentz force in the dipole+quadrupole configuration is able to reproduce the observed hydrogen line variation. These results present new strong evidences for the presence of non-zero global electric currents in the atmosphere of an early-type magnetic star.
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