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

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$^o$ 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 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$^o$, 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 result support the possibility of the formation of unusual structures in stellar magnetic fields. 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; $\tau_{5000}$ = 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.Comment: accepted by MN, 7 pagers, 10 figs, 3 table

Astrophysics in 2005

We bring you, as usual, the Sun and Moon and stars, plus some galaxies and a new section on astrobiology. Some highlights are short (the newly identified class of gamma-ray bursts, and the Deep Impact on Comet 9P/ Tempel 1), some long (the age of the universe, which will be found to have the Earth at its center), and a few metonymic, for instance the term "down-sizing" to describe the evolution of star formation rates with redshift

Comments on the magnetic field structure of the star CU Virginis

The model of the magnetic field structure of the CP2 star CU Vir (HD 124224) constructed by the method of “magnetic charge distribution" (MCD-method) has shown that it is consistent with the model of a displaced dipole. The displacement from the center of the star to the negative monopole is $d = 0.3$ of the radius, the inclination angle of the dipole axis to the rotational axis is $\beta = 87^{\rm o}$, and the field strength at the poles amounts to $B_{\rm p}(-) = 7.9$ kG and $B_{\rm p}(+) = 1.2$ kG. The mean surface magnetic field varies within 1.2–3.2 kG. The dipole axis points away from the zero meridian by an angle of $+30^{\rm o}$. Using the MCD-method we derived the distribution of the field intensity over the surface, which has been compared to the distribution of the chemical elements He and Si, taken from literature. Silicon has turned out to concentrate around the strong negative magnetic pole, whereas helium concentrates in the region of the weak positive pole, where the orientation of magnetic lines of force is mostly vertical. The presence of a double silicon spot suggests a more complex magnetic field structure than the dipolar one, however, the small number of data makes it impossible so far to confirm such an assumption