2024A&A...688A.215C

Context. Deuterated molecules and their molecular D/H-ratios (R_D (D)) are important diagnostic tools with which to study the physical conditions of star-forming regions. The degree of deuteration, R_D (D), can be significantly enhanced over the elemental D/H-ratio depending on physical parameters such as temperature, density, and the ionization fraction.
Aims. Within the Cygnus Allscale Survey of Chemistry and Dynamical Environments (CASCADE), we aim to explore the large-scale distribution of deuterated molecules in the nearby (d ∼ 1.5 kpc) Cygnus-X region, a giant molecular cloud complex that hosts multiple sites of high-mass star formation. We focus on the analysis of large-scale structures of deuterated molecules in the filamentary region hosting the prominent HII region DR21 and DR21(OH), a molecular hot core that is in an earlier evolutionary state.
Methods. The DR21 filament has been imaged using the IRAM 30-m telescope in a variety of deuterated molecules and transitions. Here, we discuss the HCO⁺, HNC, and HCN molecules and their deuterated isotopologs DCO⁺, DNC, and DCN, and their observed line emissions at 3.6, 2, and 1.3 mm.
Results. The spatial distributions of integrated line emissions from DCO⁺, DNC, and DCN reveal morphological differences. Notably, DCO⁺ displays the most extended emission, characterized by several prominent peaks. Likewise, DNC exhibits multiple peaks, although its emission appears less extended compared to DCO⁺. In contrast to the extended emission of DCO⁺ and DNC, DCN appears the least extended, with distinct peaks. Focusing only on the regions where all three molecules are observed, the mean deuteration ratios for each species, R_D, are 0.01 for both DNC and DCN, and = 0.005 for DCO⁺, respectively. Anticorrelations are found with deuterated molecules and dust temperature or N(H₂).
Conclusions. The strongest anticorrelation is found with R_D(DCO⁺) and N(H₂), with a Pearson correlation coefficient of ρ = -0.74. We analyzed the SiO emission as a tracer for shocks and the N (HCO)/N (H¹³CO⁺) as a tracer for increased photodissociation by ultraviolet radiation. It is suggested that the anticorrelation of R_D (DCO⁺) and N(H₂) is a result of a combination of an increased photodissociation degree and shocks. A strong positive correlation between the ratio of integrated intensities of DCN and DNC with their ¹³C-isotopologs is found in high-column-density regions. The positive relationship between the ratios implies that the D-isotopolog of the isomers could potentially serve as a tracer for the kinetic gas temperature.