c-C₃H₂ deuteration towards prestellar and starless cores in the Perseus molecular cloud

¹ RIKEN Cluster for Pioneering Research, Wako-shi, Saitama 351-0106, Japan
² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
³ Department of Astronomy, University of Virginia, 530 McCormick Rd, Charlottesville, VA 22904, USA
⁴ Department of Astronomy, University of California, Berkeley, Berkeley, CA 94720, USA
⁵ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁶ Centro de Astrobiología (CAB), CSIC-INTA, Carretera de Ajalvir, km 4, 28805 Torrejón de Ardoz, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 17 December 2025
Accepted: 19 January 2026

Abstract

Context. In cores deuterium fractionation becomes highly efficient due to low temperatures and CO freeze-out. Cyclopropenylidene (c-C₃H₂), 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-C₃H₂ deuteration ratios towards starless and prestellar cores, we present observations of c-C₃H₂ and its deuterated isotopologues on a sample of cores in the Perseus molecular cloud.

Methods. Transitions of c-C₃H₂, c-C₃HD, and c-C₃D₂ 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-C₃H₂ 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-C₃HD is detected in 94% (15/16) of the targeted cores, and its doubly deuterated form c-C₃D₂ is detected towards 56% (9/16) of the cores detected. A range of column densities towards the different cores was derived: for c-C₃H₂, (0.5–8.1)×10¹³ cm⁻²; for c-C₃HD, (0.2–2.1)×10¹² cm⁻²; and for c-C₃D₂, (0.6–1.6)×10¹¹ cm⁻². The ortho-to-para ratio of c-C₃H₂ was obtained for all except one core with a median value of 3.5 ± 0.4. The D/H and D₂/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 D₂/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-C₃H₂ and the evolutionary stage of the core, as traced by volume density n_H2. The median c-C₃H₂ D/H ratio in Perseus’ starless cores appears lower than the value for the Taurus molecular cloud and the Chamaeleon molecular cloud. The D₂/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_H2 of the cores in Perseus, strengthening the idea of D/H being a tracer of core evolution. The D₂/D ratio does not correlate with n_H2, but positively correlates with T_kin, suggesting that its formation is favoured by a slightly endothermic reaction.