The Cygnus Allscale Survey of Chemistry and Dynamical Environments: CASCADE. III. The large scale distribution of DCO+, DNC, and DCN in the DR21 filament

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 H II 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 (H 2 ). Conclusions . The strongest anticorrelation is found with R D (DCO + ) and N (H 2 ), with a Pearson correlation coefficient of ρ = −0.74. We analyzed the SiO emission as a tracer for shocks and the N (HCO)/ N (H 13 CO + ) as a tracer for increased photodissociation by ultraviolet radiation. It is suggested that the anticorrelation of R D (DCO + ) and N (H 2 ) 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 13 C-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.