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

Abstract

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 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 ($n_{H_{2}} \ge 10^{6}$ 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.