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