Dust Grain Size Distributions and the Abundance of Refractory Elements in the Diffuse Interstellar Medium

Abstract

We have produced models for the interactions between grains in the diffuse interstellar medium (ISM) with each other and between grains and gas atoms. The models seek to explain two powerful diagnostics of grain and gas interactions: (a) the observed gas-phase abundances of the most highly depleted elements, and (b) the interstellar extinction law per H atom. We consider detailed models of interactions only between diffuse clouds [n(H) = 40 cm(-3)] and the warm neutral intercloud gas [WNM; n(H) = 0.4 cm(-3)]. Grains and gas circulate from one to the other. Larger grains (a greater than or equal to 0.02 mu m) are assumed to be composite and huffy (consisting of collections of smaller particles, with about 50% vacuum). In the clouds the observations require that grains have a nonthermal speed distribution and encounter other grains, coagulating or disrupting each other into smaller fragments according to simple assumed laws. Grains encounter gas-phase atoms of refractory elements (i.e., those normally found to be heavily depleted from the gas phase) that stick to them. In the WNM, stellar winds inject refractory element atoms into the gas and interstellar shocks sputter and disrupt grains. We vary the speed distributions for grains and the coagulation and disruption laws over wide values of parameter space. For each set of parameters we determine the size distribution of grains and gas phase abundances of refractory atoms in both the clouds and WNM. With only interchange between diffuse clouds and the WNM, we can explain either the gas-phase depletion of the most heavily depleted element, Ti, by having large numbers of very small particles, or else provide reasonable extinction laws by having appreciable numbers of relatively large grains, but not both in the same model. We simulate material exchange with molecular clouds (or other dense regions), assuming that refractory elements are completely depleted in the gas returned to the diffuse ISM. A rapid interchange, if the rate of grain destruction by shocks provides a mean grain lifetime of approximate to 4 x 10(8) yr, can produce the observed depletions of refractory elements and the extinction law. However, the details of the processes within molecular clouds are not considered here.