Depletion of heavy elements toward the rho OPH dust cloud.

Abstract

Measurements of extinction toward the p Oph dust cloud have suggested that the grains are abnormally large, perhaps due to enhanced depletion of heavy atoms onto grain surfaces. In an attempt to measure directly the enhanced depletion, we have compared the depletions of interstellar Na, K, and Ca toward a Sco with the apparently more normal depletions of these elements toward Oph. Data for this comparison include published column densities from optical measurements and new 21 cm H I profiles. It is shown that the cold, dense cloud toward both stars produces self-absorption in the H I emission profiles. The temperature of the cold gas and the separate column densities for the cold and hot gas have been derived. The H I spin temperatures toward the stars agree with the kinetic temperatures found from H2 data. A model of the interstellar medium toward the two stars is developed and used in the analysis of depletions of heavy elements. The Na depletion toward a Sco is enhanced by about a factor of 4 over that toward Oph, but no enhanced depletion is found for K or Ca. Enhanced depletion onto positively charged grains in the high-density p Oph dust cloud could explain this result. Subject headings: nebulae: abundances - nebulae: general