First CH$_{3}$D detection in Class 0/I proto-brown dwarfs: constraints on CH$_{4}$ abundances

We report the first detection in the ${\it J}_{{\it K}}$ = 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.