Radical Formation, Chemical Processing, and Explosion of Interstellar Grains

Abstract

The ultraviolet radiation in interstellar space is shown to create a sufficient steady state density of free radicals in the grain mantle material consisting of oxygen, carbon, nitrogen, and hydrogen to satisfy the critical condition for initiation of chain reactions. The criterion for minimum critical particle size for maintaining the chain reaction is of the order of the larger grain sizes in a distribution satisfying the average extinction and polarization measures. The triggering of the explosion of interstellar grains leading to the ejection of complex interstellar molecules is shown to be most probable where the grains are largest and where radiation is suddenly introduced; i.e. in regions of new star formation. Similar conditions prevail at the boundaries between very dark clouds and H II regions. When the energy released by the chemical activity of the free radicals is inadequate to explode the grain, the resulting mantle material must consist of extremely large organic molecules which are much more resistant to the hostile environment of H II regions than the classical dirty ice mantles made up of water, methane, and ammonia.

Work supported in part by grants from the Research Foundation of the State University of New York and by grant #NGR-33-011-043 from the National Aeronautics and Space Administration.

This paper was begun when the author was a visiting professor at the University of Tokyo under the auspices of the Japan Society for the Promotion of Science.