Deuterium chemistry and D/H ratios in Class 0/I proto-brown dwarfs. Issue 4 (27th January 2022)

• Record Type: Journal Article
• Authors: Riaz, B; Thi, W-F
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 511:Issue 4(2022)
• Page Start: 6110
• Page End: 6125
• Publication Date: 2022-01-27
• Subjects: astrochemistry -- brown dwarfs -- stars: evolution -- stars: formation -- ISM: abundances -- ISM: molecules; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac156
• Languages: English
• ISSNs: 0035-8711

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 N2 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 ∼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/HN 13 C ratios, and lower N2 D + /N2 H + 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.