Ultraviolet observations of two hot stars with infrared excesses from circumstellar dust.

ANS ultraviolet observations of two peculiar Be stars (HD 45677 and HD 50138) that exhibit large IR excesses due to heated circumstellar particles are presented. A theory is outlined concerning what one would expect to see when observing a B star surrounded by circumstellar dust. Absolute fluxes at central wavelengths of 1549, 1799, 2200, 2493, and 3294 A are reported in the OAO photometric system, and the energy distributions of the two stars over the wavelength range from 0.155 to 20 microns are examined. The results show that: (1) the excess IR radiation appears to be approximately equal to the missing visual and UV radiation for both stars; (2) neither star has a UV spectral energy distribution resembling those expected for stars surrounded by normal interstellar dust; (3) the spectrum of HD 45677 exhibits a 0.22-micron interstellar extinction bump that is most likely produced in the circumstellar dust shell, perhaps by graphite; and (4) the UV spectral energy distribution of HD 50138 changed significantly over an interval of 1 yr, possibly as a result of circumstellar dust absorption or variations in the amount of UV line blanketing or circumstellar gas emission.