Line variations in the ultra-violet spectrum of the Ap-Si star HD 25823.

Abstract

Spectral variations of the Ap-Si star HD 25823 are investigated in the UV range 1250-1900 A, from a set of five IUE images recorded during one rotational period. It is found that line intensities, as well as other wide absorption features, vary versus the phase (up to 20 percent) with different behaviors related to the absorbing elements. The equivalent widths of the strongest lines of silicon and gallium are plotted in function of the phase, showing opposite variations of these two elements. These observations confirm previous theoretical predictions of the radiative diffusion of the elements through the stellar atmosphere in presence of an horizontal magnetic field. The results are interpreted on the basis of the oblique rotator model and they appear consistent with the known variation of the effective magnetic field. Different time variations for other elements are pointed out, especially for carbon, iron and titanium.