Ultraviolet photometry from the Orbiting Astronomical Observatory. VIII. The blue Ap stars.

The Wisconsin Experiment Package filter photometers on OAO-2 were used to obtain data for a carefully selected set of 24 blue (B - V < 0.00) Ap stars and 31 comparison standard B and A dwarfs and giants for a program of relative photometry over the effective wavelength range 1430-4250 A. The Ap stars observed include members of the Si, Hg-Mn, and Sr-Cr-Eu peculiarity classes, and most of them are too blue in B - V for their published MK spectral classes. The close similarity, deduced from ground-based observations, between the blue Ap stars and normal B stars of similar UB V colors is not maintained in the ultraviolet. The blue Ap stars emit less ultraviolet flux than normal stars of like UB V colors. They are underluminous and cool in effective temperature for their colors. The Hg-Mn stars are less flux-deficient in the ultraviolet, for their UBVcolors, than are Si and Sr-Cr-Eu stars. In most cases the ultraviolet flux distribution of a blue Ap star closely resembles that of a normal star of similar MK spectral class. These results are confirmed by ultraviolet spectrophotometry at 10 and 20 A resolution of the Si star 0 Aur, data also obtained with OAO-2. Interesting differences in detail between 0 Aur and normal stars are evident in the latter data. The possible role of enhanced ultraviolet opacities in modifying the shape of the emergent flux envelope of an Ap star and in modifying its atmospheric structure, relative to normal B stars, is discussed. Subject headings: peculiar A stars - spectrophotometry - ultraviolet