# Spots structure and stratification of helium and silicon in the atmosphere of He-weak star HD 21699 - Astrophysics > Solar and Stellar Astrophysics

Spots structure and stratification of helium and silicon in the atmosphere of He-weak star HD 21699 - Astrophysics > Solar and Stellar Astrophysics - Descarga este documento en PDF. Documentación en PDF para descargar gratis. Disponible también para leer online.

Abstract: The magnetic star HD 21699 possesses a unique magnetic field structure wherethe magnetic dipole is displaced from the centre by 0.4 +- 0.1 of the stellarradius perpendicularly to the magnetic axis, as a result, the magnetic polesare situated close to one another on the stellar surface with an angularseparation of 55$^o$ and not 180$^o$ as seen in the case of a centred dipole.Respectively, the two magnetic poles form a large -magnetic spot-.High-resolution spectra were obtained allowing He I and Si II abundancevariations to be studied as a function of rotational phase. The results showthat the helium abundance is concentrated in one hemisphere of the star, nearthe magnetic poles and it is comparatively weaker in another hemisphere, wheremagnetic field lines are horizontal with respect to the stellar surface. At thesame time, the silicon abundance is greatest between longitudes of 180 -320$^o$, the same place where the helium abundance is the weakest. Theseabundance variations with rotational phase support predictions made by thetheory of atomic diffusion in the presence of a magnetic field. Simultaneously,these result support the possibility of the formation of unusual structures instellar magnetic fields. Analysis of vertical stratification of the silicon andhelium abundances shows that the boundaries of an abundance jump in the twostep model are similar for each element; $\tau {5000}$ = 0.8-1.2 for heliumand 0.5-1.3 for silicon. The elemental abundances in the layers of effectiveformation of selected absorption lines for various phases are also correlatedwith the excitation energies of low transition levels: abundances are enhancedfor higher excitation energy and higher optical depth within the applied modelatmosphere.

Autor: A.V.Shavrina MAO, Yu.V. Glagolevskij SAO, J. Silvester QU, RMC, G.A. Chuntonov SAO, V.R. Khalack UM, Ya.V. Pavlenko MAO

Fuente: https://arxiv.org/