Understanding the $\rm C_3H_2$ cyclic-to-linear ratio in L1544

TL;DR

This study combines chemical and physical models to explain the unusually high cyclic-to-linear $ m C_3H_2$ ratio in L1544, suggesting a rate coefficient ratio of about 20 is needed to match observations.

Contribution

It identifies the key reaction rate ratio affecting the cyclic-to-linear $ m C_3H_2$ ratio, challenging the common 1:1 assumption in astrochemical networks.

Findings

A rate coefficient ratio of ~20 reproduces observed ratios.

Current models assume a 1:1 formation ratio, which may be inaccurate.

Laboratory or theoretical studies are needed for confirmation.

Abstract

Aims. We aim to understand the high cyclic-to-linear $\rm C_3H_2$ ratio ($32 \pm 4$) observed toward L1544 by Spezzano et al. (2016). Methods. We combine a gas-grain chemical model with a physical model for L1544 to simulate the column densities of cyclic and linear $\rm C_3H_2$ observed toward L1544. The most important reactions for the formation and destruction of both forms of $\rm C_3H_2$ are identified, and their relative rate coefficients are varied to find the best match to the observations. Results. We find that the ratio of the rate coefficients of $\rm C_3H_3^+ + e^- \longrightarrow C_3H_2 + H$ for cyclic and linear $\rm C_3H_2$ must be $\sim 20$ in order to reproduce the observations, depending on the branching ratios assumed for the $\rm C_3H_3^+ + e^- \longrightarrow C_3H + H_2$ reaction. In current astrochemical networks it is assumed that cyclic and linear $\rm C_3H_2$ 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 based on observational constraints.