Journal Article

Spots structure and stratification of helium and silicon in the atmosphere of He-weak star HD 21699

A. V. Shavrina, Yu. V. Glagolevskij, J. Silvester, G. A. Chuntonov, V. R. Khalack and Ya. V. Pavlenko

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 401, issue 3, pages 1882-1888
Published in print January 2010 | ISSN: 0035-8711
Published online January 2010 | e-ISSN: 1365-2966 | DOI: https://dx.doi.org/10.1111/j.1365-2966.2009.15765.x
Spots structure and stratification of helium and silicon in the atmosphere of He-weak star HD 21699

Show Summary Details

Preview

The magnetic star HD 21699 possesses a unique magnetic field structure where the magnetic dipole is displaced from the centre by 0.4 ± 0.1 of the stellar radius (perpendicularly to the magnetic axis), as a result the magnetic poles are situated close to one another on the stellar surface with an angular separation of 55° and not 180° 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 abundance variations to be studied as a function of rotational phase. The results show that the helium abundance is concentrated in one hemisphere of the star, near the magnetic poles, and it is comparatively weaker in another hemisphere, where magnetic field lines are horizontal with respect to the stellar surface. At the same time, the silicon abundance is greatest between longitudes of 180°–320°, the same place where the helium abundance is the weakest. These abundance variations (with rotational phase) support predictions made by the theory of atomic diffusion in the presence of a magnetic field. Simultaneously, these results support the possibility of the formation of unusual structures in stellar magnetic fields. The analysis of vertical stratification of the silicon and helium abundances shows that the boundaries of an abundance jump (in the two-step model) are similar for each element; τ5000= 0.8–1.2 for helium and 0.5–1.3 for silicon. The elemental abundances in the layers of effective formation of selected absorption lines for various phases are also correlated with the excitation energies of low transition levels: abundances are enhanced for higher excitation energy and higher optical depth within the applied model atmosphere.

Keywords: stars: atmosphere; stars: chemically peculiar; stars: individual: HD 21699; stars: magnetic fields

Journal Article.  4347 words.  Illustrated.

Subjects: Astronomy and Astrophysics

Full text: subscription required

How to subscribe Recommend to my Librarian

Users without a subscription are not able to see the full content. Please, subscribe or login to access all content.