A Study of the c-C3HD/c-C3H2 Ratio in Low-mass Star-forming Regions

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-(HCC2H)-C-13 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 H-2 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.