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We have conducted the first extensive observational survey of severaldeuterated species in 16 Class 0/I proto-brown dwarfs (proto-BDs) and 4 ClassFlat/Class II brown dwarfs. Observations were obtained with the IRAM 30mtelescope in the DCO$^{+}$ (3-2), DCN (3-2), DNC (3-2), and N$_{2}$D$^{+}$(3-2) lines. The DCO$^{+}$/H$^{13}$CO$^{+}$, DCN/H$^{13}$CN, and DNC/HN$^{13}$Cratios are comparatively higher and show a narrower range than theDCO$^{+}$/HCO$^{+}$, DCN/HCN, and DNC/HNC ratios, respectively. The mean D/Hratios for the proto-BDs derived from these molecules are in the range of$\sim$0.02-3. Both low-temperature gas-phase ion-molecule deuteron transfer andgrain surface reactions are required to explain the enhanced deuteriumfractionation. The very dense and cold ($n_{H_{2}}$ $\geq$10$^{6}$ cm$^{-3}$, T$\leq$10 K) interior of the proto-BDs provide the suitable conditions forefficient deuterium fractionation in these cores. There is no correlationbetween the D/H ratios and the CO depletion factor, with the exception of theDCN/HCN ratios that show a strong anti-correlation possibly due to thedifference in the peak emitting regions of the DCN and HCN molecules. Over awide range in the bolometric luminosities spanning $\sim$0.002--40 L$_{\odot}$,we find a trend of higher DCO$^{+}$/HCO$^{+}$ (r = -0.7) and DCN/HCN (r = -0.6)ratios, nearly constant DNC/HNC (r = -0.4) and DNC/HN$^{13}$C (r = -0.1)ratios, lower N$_{2}$D$^{+}$/N$_{2}$H$^{+}$ ratios (r = 0.6) in the proto-BDscompared to protostars. Only one Class II brown dwarf shows emission in theDCO$^{+}$ (3-2) line. No correlation is seen between the D/H ratios and theevolutionary stage.

Deuterium chemistry and D/H ratios in Class 0/I proto-brown dwarfs