2022MNRAS.511.6110R

We have conducted the first extensive observational survey of several deuterated species in 16 Class 0/I proto-brown dwarfs (proto-BDs) and 4 Class Flat/Class II brown dwarfs. Observations were obtained with the IRAM 30m telescope in the DCO⁺ (3-2), DCN (3-2), DNC (3-2), and N₂D⁺ (3-2) lines. The DCO⁺/H¹³CO⁺, DCN/H¹³CN, and DNC/HN¹³C ratios are comparatively higher and show a narrower range than the DCO⁺/HCO⁺, DCN/HCN, and DNC/HNC ratios, respectively. The mean D/H ratios for the proto-BDs derived from these molecules are in the range of ∼0.02-3. Both low-temperature gas-phase ion-molecule deuteron transfer and grain surface reactions are required to explain the enhanced deuterium fractionation. The very dense and cold (n_H2 ≥10⁶ cm^–3, T <=10 K) interior of the proto-BDs provide the suitable conditions for efficient deuterium fractionation in these cores. There is no correlation between the D/H ratios and the CO depletion factor, with the exception of the DCN/HCN ratios that show a strong anti-correlation possibly due to the difference in the peak emitting regions of the DCN and HCN molecules. Over a wide range in the bolometric luminosities spanning ∼0.002-40 L_☉, we find a trend of higher DCO⁺/HCO⁺ and DCN/HCN ratios, nearly constant DNC/HNC and DNC/HN¹³C ratios, and lower N₂D⁺/N₂H⁺ ratios in the proto-BDs compared to protostars. Only one Class II brown dwarf shows emission in the DCO⁺ (3-2) line. No correlation is seen between the D/H ratios and the evolutionary stage.