THE BETA-PICTORIS PHENOMENON AMONG YOUNG STARS .2. UV OBSERVATIONS OF THE HERBIG-AE-STAR UX-ORIONIS

IUE spectra of the Herbig Ae star UX Ori reveal the presence of large amplitude light and color changes, which are an extension to shorter wavelengths of the ''blueing'' effect seen in the optical. At optical maximum the UV spectrum of UX Ori is dominated by heavy line blanketing from accreting circumstellar gas with velocities as high as +200 km s(-1). At minimum light, prominent mid-UV emission from Fe II and Mg II is present, and the overall spectrum closely resembles spectra of more heavily embedded Herbig Ae stars. Comparison of the UV and optical data for UX Ori suggest that single parameter ISM-like extinction curves do not fit the observed data near optical maximum light, when contamination by scattered circumstellar dust is minimized. The best fit to the UV color-magnitude diagram is for circumstellar extinction dominated by silicate grains with a(min) greater than or equal to 0.15 mu m, and with a power law distribution n(alpha) = alpha(-2), suggesting that both the particle size distribution and grain chemistry in the inner disk have evolved considerably from those characteristic of molecular clouds in the 2-3 Myr since the formation of UX Ori.