Understanding the C₃H₂ cyclic-to-linear ratio in L1544

Max-Planck-Institute for Extraterrestrial Physics (MPE), Giessenbachstr. 1, 85748 Garching, Germany
e-mail: osipila@mpe.mpg.de

Received: 11 April 2016
Accepted: 23 May 2016

Abstract

Aims. We aim to understand the high cyclic-to-linear C₃H₂ ratio (32 ± 4) that has been observed toward L1544.

Methods. We combined a gas-grain chemical model with a physical model for L1544 to simulate the column densities of cyclic and linear C₃H₂ observed toward L1544. The most important reactions for the formation and destruction of both forms of C₃H₂ were identified, and their relative rate coefficients were varied to find the best match to the observations.

Results. We find that the ratio of the rate coefficients of C₃H₃⁺ + e⁻ ➝ C₃H₂ + H for cyclic and linear C₃H₂ must be ~ 20 to reproduce the observations, depending on the branching ratios assumed for the C₃H₃⁺ + e⁻ ➝ C₃H + H₂ reaction. In current astrochemical networks it is assumed that cyclic and linear C₃H₂ are formed in a 1:1 ratio in the aforementioned reactions. Laboratory studies and/or theoretical calculations are needed to confirm the results of our chemical modeling, which is based on observational constraints.