First CH${}_3$D detection in Class 0/I proto-brown dwarfs: constraints on CH${}_4$ abundances

Abstract

We report the first detection in the $J_K$ = 1${}_0$ - 0${}_0$ rotational transition line of CH${}_3$D towards three Class 0/I proto-brown dwarfs (proto-BDs) from IRAM 30 m observations. Assuming a rotational temperature of 25 K, the CH${}_3$D abundances (relative to H${}_2$) are in the range of (2.3--14.5) $\times$ 10${}^{-7}$. The CH${}_4$ abundances derived from the CH${}_3$D abundances and assuming the DCO${}^{+}$/HCO${}^{+}$ ratios are in the range of (0.05--4.8) $\times$10${}^{-5}$. The gas-phase formation of CH${}_3$D via CH${}_2$D${}^{+}$ is enhanced at high densities of 10${}^8$--10${}^{10}$ cm${}^{-3}$ and our observations are likely probing the innermost dense and warm regions in proto-BDs. Thermal and/or non-thermal desorption can return the CH${}_3$D and CH${}_4$ molecules formed at an early stage on grain surfaces to the gas-phase. The gas phase abundances are indicative of warm carbon-chain chemistry in proto-BDs where carbon-chain molecules are synthesized in a lukewarm ($\sim$20-30 K) region close to the central source.

arXiv commentAccepted in MNRAS Letters

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Cited by in corpus (2)

• First observations of warm and cold methanol in Class 0/I proto-brown dwarfs
• A survey of HDCO and D${}_2$CO towards Class 0/I proto-brown dwarfs