| 1. |
- Mikulasek, Z., et al.
(författare)
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Differential rotation in magnetic chemically peculiar stars
- 2018
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Ingår i: CONTRIBUTIONS OF THE ASTRONOMICAL OBSERVATORY SKALNATE PLESO. - : SLOVAK ACADEMY SCIENCES ASTRONOMICAL INST. - 1335-1842 .- 1336-0337. ; 48:1, s. 203-207
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic chemically peculiar (mCP) stars constitute about 10% of upper-main-sequence stars and are characterized by strong magnetic fields and abnormal photospheric abundances of some chemical elements. Most of them exhibit strictly periodic light, magnetic, radio, and spectral variations that can be fully explained by a rigidly rotating main-sequence star with persistent surface structures and a stable global magnetic field. Long-term observations of the phase curves of these variations enable us to investigate possible surface differential rotation with unprecedented accuracy and reliability. The analysis of the phase curves in the best-observed mCP stars indicates that the location and the contrast of photometric and spectroscopic spots as well as the geometry of the magnetic field remain constant for at least many decades. The strict periodicity of mCP variables supports the concept that the outer layers of upper-main-sequence stars do not rotate differentially. However, there is a small, inhomogeneous group consisting of a few mCP stars whose rotation periods vary on timescales of decades. The period oscillations may reflect real changes in the angular velocity of outer layers of the stars which are anchored by their global magnetic fields. In CU Vir, V901 On, and perhaps BS Cir, the rotational period variation indicates the presence of vertical differential rotation; however, its exact nature has remained elusive until now. The incidence of mCP stars with variable rotational periods is currently investigated using a sample of fifty newly identified Kepler mCP stars.
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| 2. |
- Vlemmings, Wouter, 1974
(författare)
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Magnetic fields around evolved stars
- 2018
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Ingår i: Contributions of the Astronomical Observatory Skalnate Pleso. - 1335-1842 .- 1336-0337. ; 48:1, s. 187-193
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Tidskriftsartikel (refereegranskat)abstract
- There has long been evidence of magnetic fields in the extended envelopes of asymptotic giant branch (AGB) and red supergiant (RSG) stars. These stars are important contributors to the enrichment of the interstellar medium by dust and heavy elements. Magnetic fields might play a role in the mass loss process responsible. Additionally, magnetic fields, typically in combination with binary companions, have often be suggested to be involved in shaping strongly a-spherical planetary nebulae (PNe). New telescopes and instruments are increasing our knowledge about magnetic fields around these evolved, mass losing, stars and their descendants.
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| 3. |
- Folsom, C. P., et al.
(författare)
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Magnetic, chemical and rotational properties of the Herbig Ae/Be binary system HD 72106
- 2008
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Ingår i: Contributions of the Astronomical Observatory Skalnaté Pleso. - 1335-1842. ; 38:2, s. 245-250
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Tidskriftsartikel (refereegranskat)abstract
- Recently, strong, globally-ordered magnetic fields have been detected in some Herbig Ae and Be (HAeBe) stars, suggesting a possible evolutionary connection to main sequence magnetic chemically peculiar Ap and Bp stars. We have undertaken a detailed study of the binary system HD 72106, which contains a B9 magnetic primary and a HAeBe secondary, using the ESPaDOnS spectropolarimeter mounted on the CFHT. A careful analysis of the very young primary reveals that it has an approximately dipolar magnetic field geometry, strong chemical peculiarities, and strong surface chemical abundance inhomogeneities. Thus the primary is very similar to an Ap/Bp star despite having completed less then 1.5% of its main sequence life, and possible still being on the pre-main sequence. In contrast, a similar analysis of the secondary reveals solar chemical abundances and no magnetic field.
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| 4. |
- Fossati, L., et al.
(författare)
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Chemical evolution of A- and B-type stars in open clusters : observed abundances vs. diffusion models
- 2008
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Ingår i: Contributions of the Astronomical Observatory Skalnaté Pleso. - 1335-1842. ; 38:2, s. 123-128
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Tidskriftsartikel (refereegranskat)abstract
- We have decided to address the problem of how abundances and peculiarities change during main sequence evolution. We have setup a program to measure the atmospheric abundance patterns from tens of A-type star members of clusters of different ages, and compare the results with theory predictions. In this paper we present the overall project and we focus on the results obtained for a sample of Am stars of the Praesepe cluster (log t = 8.85 +/- 0.15; Gonzalez-Garcia et al., 2006). We have obtained spectra for eight Am stars, two normal A-type stars and one blue straggler, that are probable members of the Praesepe cluster. For all of these stars we have determined fundamental parameters and photospheric abundances for a large number of chemical elements. For seven stars we also obtained spectra in circular polarisation and applied the LSD technique to measure the mean longitudinal magnetic field. We have found good agreement between abundance predictions of diffusion models and measured abundances, except for Na and S. Li appears to be overabundant in three stars of our sample. No magnetic field was detected in any of the analysed stars.
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| 5. |
- Kochukhov, Oleg, et al.
(författare)
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A search for very low amplitude magnetoacoustic pulsations with HARPS
- 2008
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Ingår i: Contributions of the Astronomical Observatory Skalnate Pleso. - 1335-1842. ; 38:2, s. 423-424
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Tidskriftsartikel (refereegranskat)abstract
- We have obtained time-resolved spectroscopic observations for a sample of 10 cool Ap stars using the ultra-stable spectrograph HARPS at the ESO 3.6-m telescope. The aim of our study was to search for low-amplitude oscillations in Ap stars with no or inconclusive evidence of pulsational variability. Here we report initial results of our investigation. We confirm the presence of approximate to 16-min period pulsations in beta CrB (HD 137909) and demonstrate multi-periodic character of oscillations in this star. Furthermore, we discovered very low amplitude 9-min pulsations in HD 75445 - an object spectroscopically very similar to known roAp stars.
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| 6. |
- Kochukhov, Oleg
(författare)
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Magnetic fields, spots and weather in chemically peculiar stars
- 2008
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Ingår i: Contributions of the Astronomical Observatory Skalnaté Pleso. - 1335-1842. ; 38:2, s. 191-200
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Tidskriftsartikel (refereegranskat)abstract
- New observational techniques and sophisticated modelling methods have led to dramatic breakthroughs in our understanding of the interplay between the surface magnetism, atomic diffusion and atmospheric dynamics in chemically peculiar stars. Magnetic Doppler images, constructed using spectropolarimetric observations of Ap stars in all four Stokes parameters, reveal the presence of small-scale field topologies. Abundance Doppler mapping has been perfected to the level where distributions of many different chemical elements can be deduced self-consistently for one star. The inferred chemical spot structures are diverse and do not always trace underlying magnetic field geometry. Moreover, horizontal chemical inhomogeneities are discovered in nonmagnetic CP stars and evolving chemical spots are observed for the first time in the bright mercury-manganese star a And. These results show that in addition to magnetic fields, another important non-magnetic structure formation mechanism acts in CP stars.
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| 7. |
- Kochukhov, Oleg, et al.
(författare)
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New generation model atmospheres for chemically peculiar stars
- 2008
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Ingår i: Contributions of the Astronomical Observatory Skalnate Pleso. - 1335-1842. ; 38:2, s. 419-420
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Tidskriftsartikel (refereegranskat)abstract
- The atmospheric structure of chemically peculiar stars deviates from that of normal stars with similar fundamental parameters due to unusual chemistry, abundance inhomogeneities and the presence of strong magnetic field. These effects are not considered in the standard model atmospheres, possibly leading to large errors in stellar parameter determination and abundance analysis. To resolve this problem we used the state-of-the-art opacity sampling model atmosphere code LLmodels to calculate a comprehensive grid of new generation model atmospheres for magnetic CP stars. This grid covers the whole parameter space occupied by SrCrEu and Si-peculiar stars, taking into account the characteristic temperature dependence of the chemical abundances. Here we present the first results of our model atmosphere calculations.
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| 8. |
- Kochukhov, Oleg
(författare)
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Surface mapping of magnetic hot stars : Theories versus observations
- 2018
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Ingår i: CONTRIBUTIONS OF THE ASTRONOMICAL OBSERVATORY SKALNATE PLESO. - : SLOVAK ACADEMY SCIENCES ASTRONOMICAL INST. - 1335-1842. ; 48:1, s. 58-65
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Tidskriftsartikel (refereegranskat)abstract
- This review summarises results of recent magnetic and chemical abundance surface mapping studies of early-type stars. We discuss main trends uncovered by observational investigations and consider reliability of spectropolarimetric inversion techniques used to infer these results. A critical assessment of theoretical attempts to interpret empirical magnetic and chemical maps in the framework of, respectively, the fossil field and atomic diffusion theories is also presented. This confrontation of theory and observations demonstrates that 3D MHD models of fossil field relaxation are successful in matching the observed range of surface magnetic field geometries. At the same time, even the most recent time-dependent atomic diffusion calculations fail to reproduce diverse horizontal abundance distributions found in real magnetic hot stars.
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| 9. |
- Kochukhov, Oleg
(författare)
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Zeeman split lines in CP stars : a discovery of low-contrast fields in slow rotators?
- 2008
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Ingår i: Contributions of the Astronomical Observatory Skalnate Pleso. - 1335-1842. ; 38:2, s. 421-422
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Tidskriftsartikel (refereegranskat)abstract
- We show that high-resolution observations of resolved Zeeman split lines can be used to obtain new constraints on the stellar magnetic field geometry. In particular, the contrast of the field strength distribution over the stellar surface can be deduced from differential measurement of the second moment of the pi and sigma Zeeman components. Our analysis of the triplet lines in slowly rotating cool magnetic CP stars uncovers a surprisingly homogeneous field structure, inconsistent with any low-order multipolar geometry.
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| 10. |
- Krticka, J., et al.
(författare)
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The nature of light variations in magnetic hot stars
- 2018
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Ingår i: CONTRIBUTIONS OF THE ASTRONOMICAL OBSERVATORY SKALNATE PLESO. - : SLOVAK ACADEMY SCIENCES ASTRONOMICAL INST. - 1335-1842. ; 48:1, s. 170-174
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic stars show several types of light variability which is modulated by the stellar rotation. In chemically peculiar stars, the redistribution of the flux in the surface regions with peculiar chemical composition leads to the light variability with a typical amplitude of the order of hundredths of magnitude. The most efficient processes that cause the flux redistribution are bound-bound (line) transitions of iron and bound-free (ionization) transitions of silicon. This type of light variability typically leads to a complex dependence of the amplitude on the wavelength and shows antiphase light curves in the far ultraviolet and visual regions. In hot magnetic stars, the modulation of the stellar wind by the magnetic field and the wind blanketing cause the light variability with a typical amplitude of the order of millimagnitudes. We predict the light variations in selected magnetic hot stars and compare the simulated light curves with light variations derived from observations.
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