Chemical and physical conditions in dense diffuse interstellar clouds

The primary goal was to analyze data from the International Ultraviolet Explorer (IUE) in order to determine interstellar abundances and depletions (from the gas onto dust grains) in lines of sight intersecting regions of differing physical conditions. In the Orion nebula, the dust grains are large and the far-UV extinction is correspondingly low. The depletions appear to be somewhat less than in environments where the far-UV extinction is high because small grains are present. This result suggests that the regions having small grains have more grain surface area available onto which gas-phase elements can be deposited.