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

TL;DR

This study presents the first extensive observational survey of deuterated molecules in proto-brown dwarfs, revealing high deuterium fractionation levels influenced by cold, dense conditions and providing insights into chemical processes in early substellar objects.

Contribution

It provides new observational data on deuterium chemistry in proto-brown dwarfs, highlighting the conditions for deuterium fractionation and comparing molecular ratios across different evolutionary stages.

Findings

Higher D/H ratios in proto-BDs compared to protostars.

No correlation between D/H ratios and CO depletion factor.

Trend of decreasing DCO+/HCO+ and DCN/HCN ratios with increasing luminosity.

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}}$ $\geq$10$^{6}$ cm$^{-3}$, T $\leq$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.