2018ApJ...863..126C

We use the deuteration of c-C₃H₂ to probe the physical parameters of starless and protostellar cores, related to their evolutionary states, and compare it to the N₂H⁺-deuteration in order to study possible differences between the deuteration of C- and N-bearing species. We observed the main species c-C₃H₂, the singly and doubly deuterated species c-C₃HD and c-C₃D₂, as well as the isotopologue c-H¹³CC₂H toward 10 starless cores and five protostars in the Taurus and Perseus complexes. We examined the correlation between the N(c-C₃HD)/N(c-C₃H₂) ratio and the dust temperature along with the H₂ column density and the CO depletion factor. The resulting N(c-C₃HD)/N(c-C₃H₂) ratio is, within error bars, consistent with 10% in all starless cores with detected c-C₃HD. This also accounts for the protostars except for the source HH211, where we measure a high deuteration level of 23%. The deuteration of N₂H⁺ follows the same trend but is considerably higher in the dynamically evolved core L1544. We find no significant correlation between the deuteration of c-C₃H₂ and the CO depletion factor among the starless and protostellar cores. Toward the latter the coolest objects show the largest deuterium fraction in c-C₃H₂. We show that the deuteration of c-C₃H₂ can trace the early phases of star formation and is comparable to that of N₂H⁺. However, the largest c-C₃H₂ deuteration level is found toward protostellar cores, suggesting that while c-C₃H₂ is mainly frozen onto dust grains in the central regions of starless cores, active deuteration is taking place on ice.