A survey of HDCO and D${}_2$CO towards Class 0/I proto-brown dwarfs

Abstract

Deuterium fractionation can constrain the physical and chemical conditions at the early stage of brown dwarf formation. We present IRAM 30m observations over a wide frequency range of 213-279 GHz of singly and doubly deuterated species of formaldehyde (HDCO and D${}_2$CO) towards Class 0/I proto-brown dwarfs (proto-BDs). Multiple low-excitation HDCO and D${}_2$CO transition lines with upper energy level $\le$ 40 K are detected. The D${}_2$CO/HDCO, HDCO/H${}_2$CO, D${}_2$CO/H${}_2$CO abundance ratios range between 0.01 and 2.5 for the proto-BDs, similar to the range seen in low-mass protostars. The highest ratios of D${}_2$CO/HDCO $\sim$1.3-2.5 are measured for two Stage 0 proto-BDs. These objects could possess a warm corino, similar to the few hot corino cases reported among Class 0 protostars. The mean D${}_2$CO/HDCO, D${}_2$CO/H${}_2$CO, and HDCO/H${}_2$CO ratios for the proto-BDs are comparatively higher than the range predicted by the current gas-grain chemical models, indicating that HDCO and D${}_2$CO are formed via grain surface reactions in the dense and cold interiors of the proto-BDs at an early formation stage.

arXiv commentAccepted in MNRAS

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Cited by in corpus (1)

• First observations of warm and cold methanol in Class 0/I proto-brown dwarfs