Interstellar dust in the rho OPH dark cloud.

This is the authors' abstract. We don't add key points for this paper.

The dark cloud near p Oph is shown to be an ideal region for studying several characteristics of the dust component of the interstellar medium. Infrared and optical photometry and measurements of the wavelength dependence of polarization suggest that the grains are larger in the higher- density regions of the p Oph cloud. It is argued that this size increase results from depletion of heavy elements from the gas onto the grains. Given our observations, the adopted depletion model predicts an age for the cloud of t > 7 >c 10 years/ , where a is the probability per grain-atom collision that a heavy-element atom will stick to the grain. From independent estimates of the cloud age, the plausible range for a is 0.1 < < 1. This result suggests that heavy-element depletion is a fairly efficient process and can be important in determining the thermal balance in interstellar clouds. The observed polarization is highest for the most heavily obscured stars, which suggests that the grains are well aligned even in the densest observable regions of the cloud. A magnetic field appears to be the most likely source of grain alignment, and at the high densities prevailing near the cloud center, fields as high as several times 10- gauss may be required. Subject headings: associations - interstellar matter - interstellar reddening - polarization