The Behavior of BpSi Stars in the Far UV—Paper I: HD 22920

Abstract The analysis of five spectra of the Bp Si star HD 22920 recorded by the SWP camera on board the International Ultraviolet Explorer reveals variations of the far-ultraviolet flux of this star throughout its recently derived rotation period of 3.947 days. Spectra separated by half of the rotation period show variations of the FUV flux shortwards of 1700 Å. The Si ii autoionization lines in the region 1350 to 1460 Å clearly vary in phase with the Si ii doublet near 1530 Å. The FUV variations are caused by variations of the FUV Si ii continuous and line opacity as spots of overabundant silicon come into the line of sight while the star rotates.