Chemical model calculations of C2, C3, CH, CN, OH, and NH2 abundances in cometary comae

Abstract

One-dimensional radial models of the chemistry in cometary comae have been constructed using a comprehensive system of chemical reactions for a number of assumed nuclear compositions. Column abundances for C<SUB>2</SUB>, C<SUB>3</SUB>, CR, OH, CN, and NH<SUB>2</SUB> calculated from these models are compared to those calculated from observations. Models which assume a parent volatile mixture consisting of the simplest species containing carbon, oxygen, and nitrogen -- H<SUB>2</SUB>O, CO<SUB>2</SUB>, CR<SUB>4</SUB>, and NH<SUB>3</SUB> -produce column abundances of C<SUB>2</SUB>, OH, CR, and NH<SUB>2</SUB> consistent with observations but do not produce sufficient amounts of C<SUB>3</SUB> and CN. Models which assume a more complex mixture of parent volatiles, derived by assuming comets originate from a gas with composition similar to that found in interstellar molecular clouds, are successful in accounting for the observed coma column abundances of C<SUB>2</SUB>, C<SUB>3</SUB>, CN, CR, OH, and NH<SUB>2</SUB>.