2022A&A...664A.119G

Context. In the centre of pre-stellar cores, the deuterium fractionation is enhanced due to the cold temperatures and high densities. Therefore, the chemistry of deuterated molecules can be used to probe the evolution and the kinematics in the earliest stages of star formation.
Aims. We analyse emission maps of cyclopropenylidene, c-C₃H₂, to study the distribution of the deuteration throughout the prototypical pre-stellar core L1544.
Methods. We used single-dish observations of c-C₃H₂, c-H¹³CC₂H, c-C₃HD, and c-C₃D₂ towards the pre-stellar core L1544, performed at the IRAM 30 m telescope. We derived the column density and deuterium fraction maps, and compared these observations with non-local thermodynamic equilibrium radiative transfer simulations.
Results. The highest deuterium fractions are found close to the dust peak at the centre of L1544, where the increased abundance of H₂D⁺ ions drives the deuteration process. The peak values are N(c-C₃HD)/N(c-C₃H₂) = 0.17 ± 0.01, N(c-C₃D₂)/N(c-C₃H₂) = 0.025 ± 0.003, and N(c-C₃D₂)/N(c-C₃HD) = 0.16 ± 0.03, which is consistent with previous single-pointing observations. The distributions of c-C₃HD and c-C₃D₂ indicate that the deuterated forms of c-C₃H₂ in fact trace the dust peak and not the c-C₃H₂ peak.
Conclusions. The N(c-C₃D₂)/N(c-C₃HD) map confirms that the process of deuteration is more efficient towards the centre of the core and demonstrates that carbon-chain molecules are still present at high densities. This is likely caused by an increased abundance of He⁺ ions destroying CO, which increases the number of carbon atoms in the gas phase.