2014A&A...569A..19T

The massive star-forming region Monoceros R2 (Mon R2) hosts the closest ultra-compact HII region, where the photon-dominated region (PDR) between the ionized and molecular gas can be spatially resolved with current single-dish telescopes. We aim at studying the chemistry of deuterated molecules toward Mon R2 to determine the deuterium fractions around a high-UV irradiated PDR and investigate the chemistry of these species. We used the IRAM-30m telescope to carry out an unbiased spectral survey toward two important positions (namely IF and MP2) in Mon R2 at 1, 2, and 3mm. This spectral survey is the observational basis of our study of the deuteration in this massive star-forming region. Our high spectral resolution observations (∼0.25-0.65km/s) allowed us to resolve the line profiles of the different species detected. We found a rich chemistry of deuterated species at both positions of Mon R2, with detections of C₂D, DCN, DNC, DCO⁺, D₂CO, HDCO, NH₂D, and N₂D⁺ and their corresponding hydrogenated species and rarer isotopologs. The high spectral resolution of our observations allowed us to resolve three velocity components: the component at 10km/s is detected at both positions and seems associated with the layer most exposed to the UV radiation from IRS 1; the component at 12km/s is found toward the IF position and seems related to the foreground molecular gas; finally, a component at 8.5km/s is only detected toward the MP2 position, most likely related to a low-UV irradiated PDR. We derived the column density of the deuterated species (together with their hydrogenated counterparts), and determined the deuterium fractions as D_frac=[XD]/[XH]. The values of D_frac are around 0.01 for all the observed species, except for HCO⁺ and N₂H⁺, which have values 10 times lower. The values found in Mon R2 are similar to those measured in the Orion Bar, and are well explained with a pseudo-time-dependent gas-phase model in which deuteration occurs mainly via ion-molecule reactions with H₂D⁺, CH₂D⁺ and C₂HD⁺. Finally, the H¹³CN/HN¹³C ratio is very high (∼11) for the 10km/s component, which also agree with our model predictions for an age of ∼0.01 to a few 0.1Myr. The deuterium chemistry is a good tool for studying the low-mass and high-mass star-forming regions. However, while low-mass star-forming regions seem well characterized with D_frac(N₂H⁺) or D_frac(HCO⁺), a more complete chemical modeling is required to date massive star-forming regions. This is due to the higher gas temperature together with the rapid evolution of massive protostars.