THE BETA-PICTORIS PHENOMENON AMONG YOUNG STARS .2. UV OBSERVATIONS OFTHE 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 w avelengths of the ''blueing'' effect seen in the optical. At optical m aximum 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 mor e heavily embedded Herbig Ae stars. Comparison of the UV and optical d ata for UX Ori suggest that single parameter ISM-like extinction curve s do not fit the observed data near optical maximum light, when contam ination by scattered circumstellar dust is minimized. The best fit to the UV color-magnitude diagram is for circumstellar extinction dominat ed 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 tha t 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.