A survey of HDCO and D2CO towards Class 0/I proto-brown dwarfs
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY(2022)
摘要
Deuterium fractionation can constrain the physical and chemical conditions at the early stage of brown dwarf formation. We present IRAM 30-m observations over a wide frequency range of 213-279 GHz of singly and doubly deuterated species of formaldehyde (HDCO and D2CO) towards Class 0/I proto-brown dwarfs (proto-BDs). Multiple low-excitation HDCO and D2CO transition lines with upper energy level <= 40 K are detected. The D2CO/HDCO, HDCO/H2CO, and D2CO/H2CO 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 D2CO/HDCO similar to 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 D2CO/HDCO, D2CO/H2CO, and HDCO/H2CO ratios for the proto-BDs are comparatively higher than the range predicted by the current gas-grain chemical models, indicating that HDCO and D2CO are formed via grain surface reactions in the dense and cold interiors of the proto-BDs at an early formation stage.
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关键词
astrochemistry,brown dwarfs,stars: evolution,stars: formation,ISM: abundances,ISM: molecules
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