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

Abstract

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 N${}_2$D${}^{+}$ (3-2) lines. The DCO${}^{+}$/H${}^{13}$CO${}^{+}$, DCN/H${}^{13}$CN, and DNC/HN${}^{13}$C 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 $\sim$0.02-3. Both low-temperature gas-phase ion-molecule deuteron transfer and grain surface reactions are required to explain the enhanced deuterium fractionation. The very dense and cold ($n_{H_2}$ $\ge$10${}^6$ cm${}^{-3}$, T $\le$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 $\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-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.

Accepted in MNRAS

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