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- Kochukhov, Oleg, et al.
(författare)
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Line profile variations in rapidly oscillating Ap stars : Resolution of the enigma
- 2007
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 376:2, s. 651-672
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Tidskriftsartikel (refereegranskat)abstract
- We have carried out the first survey of the pulsational line profile variability in rapidly oscillating Ap (roAp) stars. We analysed high signal-to-noise ratio time-series observations of 10 sharp-lined roAp stars obtained with the high-resolution spectrographs attached to the Very Large Telescope and Canada–France–Hawaii Telescope. We investigated in detail the variations of Pr iii, Nd ii, Nd iii and Tb iii lines and discovered a prominent change of the profile variability pattern with height in the atmospheres of all studied roAp stars. We show that, in every investigated star, profile variability of at least one rare-earth ion is characterized by unusual blue-to-red moving features, which we previously discovered in the time-resolved spectra of the roAp star γ Equ. This behaviour is common in rapidly rotating non-radial pulsators but is inexplicable in the framework of the standard oblique pulsator model of slowly rotating roAp stars. Using analysis of the line profile moments and spectrum synthesis calculations, we demonstrate that unusual oscillations in spectral lines of roAp stars arise from the pulsational modulation of linewidths. This variation occurs approximately in quadrature with the radial velocity changes, and its amplitude rapidly increases with height in stellar atmosphere. We propose that the linewidth modulation is a consequence of the periodic expansion and compression of turbulent layers in the upper atmospheres of roAp stars. Thus, the line profile changes observed in slowly rotating magnetic pulsators should be interpreted as a superposition of two types of variability: the usual time-dependent velocity field due to an oblique low-order pulsation mode and an additional linewidth modulation, synchronized with the changes of stellar radius. Our explanation of the line profile variations of roAp stars solves the long-standing observational puzzle and opens new possibilities for constraining geometric and physical properties of the stellar magnetoacoustic pulsations.
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| 2. |
- Ryabchikova, T., et al.
(författare)
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Pulsation in the atmosphere of the roAp star HD 24712 : I. Spectroscopic observations and radial velocity measurements
- 2007
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 462:3, s. 1103-1112
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Tidskriftsartikel (refereegranskat)abstract
- Aims.We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712.Methods.For this purpose we analyzed spectra collected during 2001-2004. An extensive data set was obtained in 2004 simultaneously with the photometry of the Canadian MOST mini-satellite. This allows us to connect directly atmospheric dynamics observed as radial velocity variations with light variations seen in photometry. Results.We directly derived for the first time and for different chemical elements, respectively ions, phase shifts between photometric and radial velocity pulsation maxima indicating, as we suggest, different line formation depths in the atmosphere. This allowed us to estimate for the first time the propagation velocity of a pulsation wave in the outer stellar atmosphere of a roAp star to be slightly lower than the sound speed. We confirm large pulsation amplitudes (150-400 m s-1) for REE lines and the H core, while spectral lines of the other elements (Mg, Si, Ca, and Fe-peak elements) have nearly constant velocities. We did not find different pulsation amplitudes and phases for the lines of rare-earth elements before and after the Balmer jump, which supports the hypothesis of REE concentration in the upper atmosphere above the hydrogen line-forming layers. We also discuss radial velocity amplitudes and phases measured for individual spectral lines as tools for a 3D tomography of the atmosphere of HD 24712.
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| 3. |
- Shulyak, D., et al.
(författare)
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The Lorentz force in atmospheres of CP stars : theta Aurigae
- 2007
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 464:3, s. 1089-1099
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Ryabchikova, T., et al.
(författare)
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Pulsation tomography of rapidly oscillating Ap stars : Resolving the third dimension in peculiar pulsating stellar atmospheres
- 2007
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 473:3, s. 907-922
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We present detailed analysis of the vertical pulsation mode cross-section in ten rapidly oscillating Ap (roAp) stars based onspectroscopic time-series observations. The aim of this analysis is to derive from observations a complete picture of how the amplitudeand phase of magnetoacoustic waves depend on depth.Methods. We use the unique properties of roAp stars, in particular chemical stratification, to resolve the vertical structure of p-modes.Our approach consists of characterising pulsational behaviour of a carefully chosen, but extensive sample of spectral lines. We analysethe resulting amplitude-phase diagrams and interpret observations in terms of pulsation wave propagation.Results. We find common features in the pulsational behaviour of roAp stars. Within a sample of representative elements the lowestamplitudes are detected for Eu (and Fe in 33 Lib and in HD 19918), then pulsations go through the layers where Hα core, Nd,and Pr lines are formed. There RV amplitude reaches its maximum, and after that decreases in most stars. The maximum RV of thesecond REE ions is always delayed relative to the first ions. The largest phase shifts are detected in Tb and Th lines. Pulsationalvariability of the Th lines is detected here for the first time. The Y lines deviate from this picture, showing even lower amplitudesthan Eu lines but half a period phase shift relative to other weakly pulsating lines. We measured an extra broadening, equivalent toa macroturbulent velocity from 4 to 11–12 km s−1 (where maximum values are observed for Tb and Th lines), for pulsating REElines. The surface magnetic field strength is derived for the first time for three roAp stars: HD 9289 (2 kG), HD 12932 (1.7 kG), andHD 19918 (1.6 kG).Conclusions. The roAp stars exhibit similarity in the depth-dependence of pulsation phase and amplitude, indicating similar chemicalstratification and comparable vertical mode cross-sections. In general, pulsations waves are represented by a superposition of therunning and standing wave components. In the atmospheres of roAp stars with the pulsation frequency below the acoustic cut-offfrequency, pulsations have a standing-wave character in the deeper layers and behave like a running wave in the outer layers. CoolerroAp stars develop a running wave higher in the atmosphere. In stars with pulsation frequency close to the acoustic cut-off one,pulsation waves have a running character starting from deep layers. The transition from standing to running wave is accompanied byan increase in the turbulent broadening of spectral lines.
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