Low Dispersion UV Spectral Variability in 3 Herbig Ae Stars Viewed Through Their Circumstellar Proto-Planetary Disks
Session 62 -- Very Young Stars
Display presentation, Thursday, 9:20-4:00, Pauley Room

## [62.13] Low Dispersion UV Spectral Variability in 3 Herbig Ae Stars Viewed Through Their Circumstellar Proto-Planetary Disks

C.A. Grady (ARC), M.R. P\'erez (CSC/IUE), P.S. Th\'e, D. de Winter (Amsterdam), F. Yusef-Zadeh (Northwestern), S.B. Johnson (Idaho State), V.P. Grinin (Crimean Astrophysical Observatory)

Recent studies of pre-Main Sequence (PMS) Herbig Ae/Be stars have suggested that these stars are surrounded by large, viscously heated accretion disks comparable in size to that observed around $\beta$ Pictoris. Approximately 27\% of these objects are known to exhibit large amplitude optical light and color changes consistent with aperiodic obscuration of the star and inner disk by dust clouds dominated by large grains. The most extensively studied of these objects exhibit variable linear polarization reaching 5-10\% at optical minimum, for which, as originally suggested by Grinin et al. (1991), dust clouds act as a "natural coronagraph" permitting us to view the immediate circumstellar environment of the star. We present IUE observations of three of these objects, UX Ori, BF Ori, and CQ Tau, demonstrating that at optical minimum the UV spectra exhibit a faint, blue continuum with prominent emission lines. At optical maximum, in contrast, the UV spectra exhibit unusually strong shell absorption features compared to main sequence stars of the same optical spectral type as the program stars or to A-shell stars lacking pronounced circumstellar dust envelopes. Intermediate state spectra show progressive weakening of the absorption as the continuum level decreases. The bulk of the continuum light variability between the optically-bright data and the intermediate state spectra is consistent with progressive extinction of the starlight by dust. The lack of absorption features in the optical minima spectra, together with the prominent emission lines suggests that at deep optical minimum the bulk of the observed continuum light is scattered into our line of sight rather than passing directly through the disk. The emission lines must then originate in either bipolar nebulosity or in a disk wind from a region which is unobscured by the circumstellar dust cloud.