c-C ₃ H ₂ deuteration towards prestellar and starless cores in the Perseus molecular cloud

Context . In cores deuterium fractionation becomes highly efficient due to low temperatures and CO freeze-out. Cyclopropenylidene (c-C 3 H 2 ), a small cyclic molecule formed early in chemical evolution, and its deuterated forms serve as valuable tracers of gas-phase deuteration in these environments. Aims . In order to statistically explore the c-C 3 H 2 deuteration ratios towards starless and prestellar cores, we present observations of c-C 3 H 2 and its deuterated isotopologues on a sample of cores in the Perseus molecular cloud. Methods . Transitions of c-C 3 H 2 , c-C 3 HD, and c-C 3 D 2 were observed with the Yebes 40m, the Arizona Radio Observatory (ARO) 12m and the Institut de Radioastronomie Millimétrique (IRAM) 30 metre telescopes towards a total of 16 starless and prestellar cores in the Perseus molecular cloud. The lines were fitted with Gaussian profiles and their column densities were computed using the non-local thermodynamic equilibrium (non-LTE) software RADEX. Results . The main isotopologue c-C 3 H 2 is detected in 93% (14/15) of the targeted cores (for one of the 16 cores, none of its transitions were covered), its singly deuterated form c-C 3 HD is detected in 94% (15/16) of the targeted cores, and its doubly deuterated form c-C 3 D 2 is detected towards 56% (9/16) of the cores detected. A range of column densities towards the different cores was derived: for c-C 3 H 2 , (0.5–8.1)×10 13 cm −2 ; for c-C 3 HD, (0.2–2.1)×10 12 cm −2 ; and for c-C 3 D 2 , (0.6–1.6)×10 11 cm −2 . The ortho-to-para ratio of c-C 3 H 2 was obtained for all except one core with a median value of 3.5 ± 0.4. The D/H and D 2 /D ratios were obtained for the cores with detections, yielding a statistically corrected D/H range of 0.5–9.2% with a median value of 1.5 ± 0.2% and a statistically corrected D 2 /D range 9.0–55.2% with a median value of 25.9 ± 4.3%. Conclusions . No apparent trend is seen with the ortho-to-para ratio of c-C 3 H 2 and the evolutionary stage of the core, as traced by volume density n H 2 . The median c-C 3 H 2 D/H ratio in Perseus’ starless cores appears lower than the value for the Taurus molecular cloud and the Chamaeleon molecular cloud. The D 2 /D ratio is equivalent between the Perseus and Taurus molecular clouds within the uncertainties. There is a correlation with the D/H ratio and the n H 2 of the cores in Perseus, strengthening the idea of D/H being a tracer of core evolution. The D 2 /D ratio does not correlate with n H 2 , but positively correlates with T kin , suggesting that its formation is favoured by a slightly endothermic reaction.