2023RNAAS...7..117M

The analysis of spectra of the B8 He-weak Si star HD 21699 recorded by the SWP camera on board the International Ultraviolet Explorer in 1987 reveals large amplitude variations of the far-ultraviolet spectral energy distribution over a time interval of about four days in 1987 October. In contrast, the level of the mid-UV flux did not vary between phases of FUV flux maximum and minimum which suggests that changes of the silicon continuous and line opacity play an important role in the variations. The lines of C IV doublet near 1550 Å are stronger and have an extended blue wing at phase of far-UV maximum. The optical brightness recorded by the Fine Error Sensor on board IUE is probably anticorrelated with the FUV flux, reflecting the redistribution of the FUV flux toward longer wavelengths.