Dark cloud modeling for the ortho-to-para abundance ratio of the cyclic C 3 H 2
We present the first attempt to model the ortho-to-para abundance ratio (hereafter o/p ) of c-C 3 H 2 so as to reproduce its observed ratio in cold dense clouds. According to observations for TMC-1 (a quiescent, starless core), TMC-1C (a prestellar core) and L1527 (a protostellar core), the o/p -C 3 H 2 ratio is 2.4 - 2.7, a value which is only slightly lower than the statistical ratio of 3 pertaining to high temperature equilibrium. In order to model the ortho-to-para abundance ratio, we used a large network of chemical reactions augmented by reactions that specifically consider the formation and depletion of ortho and para forms of the molecules c-C 3 H 2 and c-C 3 H 3 ^+ . The reaction branchfractions were determined by a variety of considerations. We then investigated the sensitivity of the calculated ortho-to-para ratio for c-C 3 H 2 to a number of factors such as time, density, elemental C/O ratio, the depletion of metals from the gas, and the cosmic ray ionization rate. While these parameters can affect the calculated ratio, the major factor appears to be the choice of branching fractions for the neutral products of the C 3 H 3 ^+ + e - dissociative recombination reaction. In particular, in order to reproduce the o/p -C 3 H 2 ratio, it is necessary that the product channel C 3 H 2 + H be the dominant one.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:chemical modeling molecular abundances processes
Date of Publication:01/01/2005