THE GROWTH OF SOLIDS, DESTRUCTION OF MOLECULES, AND SHIELDING OF RADIATION IN THE YOUNG STELLAR DISK OF HD-45677

Ultraviolet spectra (912-3300 Angstrom) from the Astro-1 and IUE space missions of two Herbig Be stars, HD 45677 and HD 200775, show that th e HD 200775 intrinsic continuum shape generally coincides with a T = 2 0,000 K Kurucz model and that the HD 45677 continuum exhibits addition al line blanketing; extinction, and emission from gas and dust in its disk and bipolar wind. With log N(H I)= 21.4 +/- 0.1 cm(-1), our measu red upper limit on the fraction of HD 45677 disk H atoms in molecules (f < 1.5 x 10(-2)) is much less than that seen in general Galactic mol ecular clouds (f approximate to 0.5). Also, our derived HD 45677 circu mstellar dust extinction for lambda > 1400 Angstrom is produced by an inhomogeneous disk of material: during periods of moderate visual exti nction (A(v) approximate to 1.0 +/- 0.3 mag) the disk as a whole cause s mid-UV extinction with an R(v) x 4-5 (values found in Galactic molec ular clouds), yet at the thickest optical depths, there are regions wi th larger grains (inferred R(v) > 7) than those in molecular clouds (R (v) < 5.6). Alternately, we find in the FUV (1300-1000 Angstrom) a str ong, steeply curved circumstellar extinction for HD 45677 that has a v ery similar shape as extinction produced by interstellar polycyclic ar omatic hydrocarbon molecules (PAHs) in an illuminated molecular cloud (NGC 2023). The PAHs around HD 45677, like NGC 2023, are more abundant relative to the large grains (A lambda A(v)) than predicted by the ex tinction parameter R(v) because HD 45677 has evaporated the PAHs off l arge grains. With a measured FUV depth corresponding to tau(lambda) ap proximate to 1.4-3.5, the excess PAHs around HD 45677 partly shield th e outer disk from FUV radiation. Both the disk gas and dust have evolv ed greatly since the formation of the midmass star HD 45677 from molec ular clouds: in some regions, the disk grains have grown to micron siz es suitable for the formation of planetesimals, the ISM H-2 has been p hotodissociated into H I in the disk, and CO (McGregor, Hyland, and Hi llier 1988) has disappeared. In addition, the molecular PAHs that are condensed onto grains in molecular clouds have photoevaporated off gra ins into the gaseous disk of HD 45677, becoming dehydrogenated and ion ized. Thus, because of strong FUV radiation from HD 45677 and moderate FUV shielding, the disk around this intermediate-mass star differs dr amatically from the condensate-rich, molecular gas around low-mass sta rs and would seem to lead to a different chemical and physical evoluti on toward planets than those around low-mass stars such as our Sun.