A study of the $c$-${\mathrm{C}}_3\mathrm{HD}$/$c$-${\mathrm{C}}_3{\mathrm{H}}_2$ ratio in low-mass star forming regions

Abstract

We use the deuteration of $c$-${\mathrm{C}}_3{\mathrm{H}}_2$ to probe the physical parameters of starless and protostellar cores, related to their evolutionary states, and compare it to the ${\mathrm{N}}_2{\mathrm{H}}^{+}$-deuteration in order to study possible differences between the deuteration of C- and N-bearing species. We observed the main species $c$-${\mathrm{C}}_3{\mathrm{H}}_2$, the singly and doubly deuterated species $c$-${\mathrm{C}}_3\mathrm{HD}$ and $c$-${\mathrm{C}}_3{\mathrm{D}}_2$, as well as the isotopologue $c$-${\mathrm{H}}^{13}\mathrm{C}{\mathrm{C}}_2\mathrm{H}$ toward 10 starless cores and 5 protostars in the Taurus and Perseus Complexes. We examined the correlation between the $N$($c$-${\mathrm{C}}_3\mathrm{HD}$)/$N$($c$-${\mathrm{C}}_3{\mathrm{H}}_2$) ratio and the dust temperature along with the ${\mathrm{H}}_2$ column density and the CO depletion factor. The resulting $N$($c$-${\mathrm{C}}_3\mathrm{HD}$)/$N$($c$-${\mathrm{C}}_3{\mathrm{H}}_2$) ratio is within the error bars consistent with $10\%$ in all starless cores with detected $c$-${\mathrm{C}}_3\mathrm{HD}$. This also accounts for the protostars except for the source HH211, where we measure a high deuteration level of $23\%$. The deuteration of ${\mathrm{N}}_2{\mathrm{H}}^{+}$ follows the same trend but is considerably higher in the dynamically evolved core L1544. Toward the protostellar cores the coolest objects show the largest deuterium fraction in $c$-${\mathrm{C}}_3{\mathrm{H}}_2$. We show that the deuteration of $c$-${\mathrm{C}}_3{\mathrm{H}}_2$ can trace the early phases of star formation and is comparable to that of ${\mathrm{N}}_2{\mathrm{H}}^{+}$. However, the largest $c$-${\mathrm{C}}_3{\mathrm{H}}_2$ deuteration level is found toward protostellar cores, suggesting that while $c$-${\mathrm{C}}_3{\mathrm{H}}_2$ is mainly frozen onto dust grains in the central regions of starless cores, active deuteration is taking place on ice.