A survey of HDCO and D$_{2}$CO towards Class 0/I proto-brown dwarfs

TL;DR

This survey investigates deuterium fractionation in proto-brown dwarfs using IRAM 30m observations, revealing high deuteration ratios similar to low-mass protostars, suggesting grain surface formation in early stages.

Contribution

It provides the first extensive observational data on HDCO and D$_{2}$CO in proto-brown dwarfs, highlighting their high deuteration ratios and implications for chemical formation processes.

Findings

Deuteration ratios in proto-BDs are similar to low-mass protostars.

High D$_{2}$CO/HDCO ratios suggest warm corino presence.

Observed ratios exceed current chemical model predictions.

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 $\leq$ 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.