Possible causes of the appearance of broad depressions in the ultraviolet spectra of B and A stars. I. Chemical composition anomalies of stellar atmospheres

A brief review of observational data on the distribution of energy in the ultraviolet spectra of hot stars indicates the existence of broad depressions of the UV flux in some normal B- and A-stars and especially in Ap-stars. One possible cause of the observed depressions might be high abundances of metals in the stellar atmospheres. To test this hypothesis, the authors computed synthetic spectra in three UV intervals (2350 - 2380, 2245 - 2275 and 1810 - 1840 Å) and in one segment in the visible region (4500 - 4530 Å) with various chemical compositions. It is shown that the large UV flux deficiencies of Ap-stars can be explained fully by the high abundances of the metals that are characteristic for stars of this type. In the case of normal stars, reasonable variations of the metal abundances can explain the appearance of a deficiency (or excess) of the UV flux only for A-stars and possibly for the latest B-stars; this mechanism fails at higher temperatures.