Inorganic Theories of Grain Formation

Abstract

The idea of nucleation of grains from low density interstellar gas clouds owes its origin to the classic work of B. Lindblad (1935). Lindblad’s appreciation of the need for interstellar condensation stemmed mainly from the observation that parameters characterising the gas and the dust correlated strongly one with another. This has become evident to an even greater degree in recent times, as can be seen from the correlation of the hydrogen column density to E_B-V (Fig. 2.1). Furthermore, we have seen in Chapter 5 that the depletions of heavy elements from the gas phase increase with E_B-V indicating an incorporation of gas atoms into grains. The present evidence does not, however, support the old ideas entirely in demanding that solid particles nucleated and condensed from the gas phase. The minimum requirement is that interstellar atoms become somehow incorporated into grains, possibly through a cycle involving star formation, or simply by atoms and molecules sticking onto pre-existing grains. The latter possibility has a measure of support in view of the high values of A_V/E_B-V that are found to be associated with dense clouds and star-forming regions.