2025A&A...696A.140M

Context. Deuterated molecules can be used to study the physical conditions and astro-chemical evolution of molecular clouds.
Aims. Large-sample surveys for deuterated molecules are needed to understand the enhancement of deuterated molecules from diffuse molecular gas to cold cores.
Methods. A single-pointing survey toward the 559 Planck cold clumps of the Early Cold Core Catalogue (ECC) was conducted using the Arizona Radio Observatory 12-meter telescope, focusing on the J = 1-0 transitions of DCO⁺ and DCN. The survey included observations of 309 cores for DCO⁺ and DCN 1-0 simultaneously, followed by 71 of these cores where DCO⁺ 1-0 was detected for H¹³CO⁺ and H¹³CN 1-0 simultaneously, aiming to determine the deuterated fraction (D_frac). Additionally, 250 cores were observed for DCO⁺, DCN, H¹³CO⁺, and H¹³CN 1-0 simultaneously.
Results. DCO⁺ and DCN 1-0 were detected in 79 and 11 of the 309 sources, with detection rates of 25.6% and 3.6%, respectively. In the 250 sources that were observed for all four species, DCO⁺, DCN, H¹³CO⁺, and H¹³CN 1-0 were detected in 58, 9, 57, and 13 sources, with a detection rate of 23.2%, 3.6%, 22.8%, and 5.2%, respectively. The D_frac(HCO⁺) values in 112 sources range from 0.89% to 7.4%, with a median value of 3.1%, while D_frac(HCN) values in 11 sources range from 1.5% to 5.5%, with a median value of 2.3%. The line widths of the DCO⁺ and H¹³CO⁺ 1-0 detections are mostly within 1 km s^–1.
Conclusions. The similarity in D_frac values between HCO+ and HCN indicates that the higher detection rate of DCO⁺ 1-0 compared with DCN 1-0 is due to the lower critical density of DCO⁺ 1-0. We suggest that the enhancement of DCO⁺ and DCN likely begins in the early diffuse stage of the molecular cloud and not during the cold-core formation stage.