Observational Constraints on Interstellar Depletion Mechanisms in Lines of Sight Exhibiting Peculiar Extinction Curves.

The nature of dust-gas interactions, which are capable of modifying the size distribution of the grains and thus causing changes in the selective extinction curve, are investigated through depletion studies. The gaseous abundances of 16 elements have been determined for several lines of sight toward moderately reddened stars, each having a so-called anomalous extinction curve. Four lines of sight in the (rho) Ophiuchi dark cloud complex as well as several lines of sight through the diffuse interstellar medium have also been analyzed for comparison. Two approaches are used to assess the strength of density-dependent depletion processes. First, the depletion pattern from element-to-element for each integrated line of sight is studied with particular emphasis being given to those species that are potential discriminators between the two major competing models of grain formation and growth. In the second approach, the relative abundances of neutral atoms, which are thought to form primarily in the densest portions of interstellar clouds, are studied. Both of these constraints are then compared to a theoretical extinction curve derived from a simple model for the size distribution of the grains based on the degree of mantling. In order to measure the weak absorption lines of the neutral atoms, it was necessary to develop special observational techniques to improve the signal-to-noise (S/N) ratio of the data and to develop statistical procedures to provide stronger constraints on the inferred column densities. This also required a careful examination of the detection processes and their associated sources of error. Although uncertain, it is concluded from this study that the integrated depletions toward these dense clouds are typically only marginally more (0.5 in the log) than the maximum values found for diffuse clouds. However, differential depletion between species, inferred from the abundances of the neutral atoms in the cloud cores, is greater by approximately a factor of 1.0 in the log. These results probably indicate that most, if not all, of the density-dependent depletion occurs in the cloud cores and that there is a density threshold below which dust-gas interactions are insignificant. Further, the amount of depletion of the refractory elements is not sufficient to modify the interstellar selective extinction law, unless comparable percentages of water or ammonia ices are also mantled. (Abstract shortened with permission of author.).