Large Molecules in the Interstellar Medium

Abstract

The synthesis of large molecules in dense interstellar clouds and carbon-rich circumstellar envelopes is discussed. Gas phase reactions can produce observed abundances of molecules with as many as 11 atoms in dense clouds at socalled “early times” of 10⁵ — 10⁶ yr despite the oxygen-rich nature of the medium. Extensions of current models to incorporate much larger molecules can only show that efficient synthesis of such species is possible, given the large uncertainties in relevant rate coefficients. Alternately, the molecules produced in dense clouds may serve as seeds for the gas phase production of larger species in diffuse clouds. The synthesis of large molecules on the surfaces of dust particles in interstellar clouds is possible if hydrogenation of reactive radicals on grain surfaces to saturated stable forms does not dominate. In carbon-rich circumstellar envelopes, the condensation of carbonaceous grains occurs near the stellar photosphere, the production of PAH’s proceeds somewhat farther out at temperatures near 1000 K, and the synthesis of carbon clusters and unsaturated hydrocarbons occurs in the outer envelope.