The Growth of Solids and Radiation Shielding in the Young Stellar Disk of HD~45677
Session 41 -- Circumstellar Disks
Display presentation, Thursday, January 13, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [41.10] The Growth of Solids and Radiation Shielding in the Young Stellar Disk of HD~45677

T. Brown (Johns Hopkins Univ.), R. H. Buss, Jr. (Johns Hopkins Univ.), C. Grady (Applied Research Corp.), K. Bjorkman (Univ. of Wisconsin), R. Schulte-Ladbeck (Univ. of Pittsburg)

To investigate the properties of a probable proto-planetary system, HD~45677, we acquired FUV spectra from the ASTRO-1 and IUE space missions of two Herbig Be stars: HD~45677 and HD 200775 -- young B2~Ve and B3~Ve stars with circumstellar disks. HD~45677 is viewed edge-on through the disk, while the HD~200775 sight-line apparently avoids the disk. After correcting for interstellar extinction and absorption toward both stars, we use HD~200775 as a comparison to HD~45677 in order to derive the HD~45677 disk absorption and to place an upper limit on the molecular hydrogen [N(H$_{2}$) $< 10^{19}$~cm$^{-2}$] through the HD~45677 disk. We also measure N(HI)~$= 2.5 \times 10^{21}$~cm$^{-2}$ through the HD~45677 disk.

Since the HD~45677 spectrum was taken during moderate disk extinction ($A_{V} \approx 0.7$) between optical minimum and maximum of this variably-obscured star, the FUV dust absorption and gas densities characterize the general disk properties. Like the dense clumps observed by Sitko et~al.\ (1993), the general disk dust absorption is almost flat up to about 1400~\AA, implying that the disk grains ($R_{V} \approx 6.2$) are larger than those in molecular clouds ($R_{V} < 5.6$). Moreover, the inferred fraction ($f < 7.9 \times 10^{-3}$) of H-atoms in molecules compared to the total H-atoms is much less in the HD~45677 disk than in molecular clouds ($f \approx 0.5$). Because the disk FUV absorption rises from about 1400--1000~\AA\ with the same shape as that of hydrocarbon molecules, such as PAHs, there are large molecules in the disk. These hydrocarbons produce more absorption than predicted by the $R_{V} = 6.2$ and can shield the outer parts of the proto-planetary disk from FUV radiation, though not enough to prevent the photodissociation of H$_{2}$. Thus, both the disk gas and dust have evolved since the formation of HD~45677 out of the Galactic medium, becoming more atomic and larger in the general disk around HD~45677.