absorption against background stars with diffuse interstellar medium material 

 along the line of sight is important to assess the possibility of the ubiquitous 

 presence of PAHs in the diffuse interstellar medium. Such absorption studies of 

 the outflow from carbon-rich, late-type giants could elucidate their formation 

 process. 



Important progress in the study of interstellar dust can also be expected from 

 visible and ultraviolet studies. In particular, ultraviolet extinction and polariza- 

 tion studies of stars behind dark clouds represent a largely unexplored area, and 

 with both HST and ASTRO it will be possible to make such studies and derive 

 average grain sizes for such regions. This could answer the important question, 

 "Do interstellar grains coagulate into fluffy aggregates inside molecular clouds or 

 does grain growth mainly take place through grain mantle formation?" It may 

 also be possible to study how the composition of the grain surface influences the 

 rate of grain growth. The HST could also search for structure in the ultraviolet 

 extinction curve, similar to the diffuse interstellar bands in the visible. Optical 

 and ultraviolet spectroscopy with HST of high-velocity shocks (100-200 km/sec) 

 can yield the elemental abundances in the post-shock gas and provide insight 

 into the destruction of interstellar grains. Finally, with the ASTRO satellite, it 

 will perhaps be possible to study the dust extinction curve toward nearby stars 

 for wavelengths shorter than the Lyman limit. Presently no information of this 

 sort is available. Many materials have electronic transitions in this wavelength 

 region that may show up as prominent extinction bumps for small particles. 



The observational projects outlined here require much laboratory and 

 theoretical effort. Studies of the condensation process in the outflow from late- 

 type giants, planetary nebulae, novae, and supernovae have to be undertaken. In 

 particular, the role of large aromatic molecules in the formation of interstellar 

 carbon grains should be investigated and their physical and chemical properties 

 determined in the laboratory. Laboratory spectra of candidate materials have to 

 be measured for a successful interpretation of interstellar spectra. Laboratory 

 studies are a prerequisite for our understanding of the role of grain-mantle pho- 

 tolysis in the interstellar medium. Such studies may also provide insights into the 

 interchange of molecules between the gas phase and the solid phase in molecular 

 clouds. 



Suggestions for Further Reading 



Nuth, J. A., IV; and Stencel, R. A., eds.: Interrelationships among Circum- 

 stellar, Interstellar, and Interplanetary Dust. NASA CP-2403, 1986. 



Tielens, A. G. G. M.; and Allamondola, L. J.: The Composition, Structure, 

 and Chemistry of Interstellar Dust: In Physical Processes in the Interstellar 

 Medium, D. Hollenbach and H. Thronson, eds., 1986. 



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