2022MNRAS.511L..50R

We report the first detection in the J_K = 1₀-0₀ rotational transition line of CH₃D towards three Class 0/I proto-brown dwarfs (proto-BDs) from IRAM 30-m observations. Assuming a rotational temperature of 25 K, the CH₃D abundances (relative to H₂) are in the range of (2.3-14.5) x 10^–7. The CH₄ abundances derived from the CH₃D abundances and assuming the DCO⁺/HCO⁺ ratios are in the range of (0.05-4.8) x 10^–5. The gas-phase formation of CH₃D via CH₂D⁺ is enhanced at high densities of 10⁸-10¹⁰ 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 CH₃D and CH₄ 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 (∼20-30 K) region close to the central source.