2023MNRAS.522.4934R

We present results from the first molecular line survey to search for the fundamental complex organic molecule, methanol (CH₃OH), in 14 Class 0/I proto-brown dwarfs (proto-BDs). IRAM 30-m observations over the frequency range of 92-116 and 213-280 GHz have revealed emission in 14 CH₃OH transition lines, at upper state energy level, E_upper ∼7-49 K, and critical densities, n_crit of 10⁵-10⁹ cm^–3. The most commonly detected lines are at E_upper < 20 K, while 11 proto-BDs also show emission in the higher excitation lines at E_upper ∼21-49 K and n_crit ∼ 10⁵ to 10⁸ cm^–3. In comparison with the brown dwarf formation models, the high excitation lines likely probe the warm (∼25-50 K) corino region at ∼10-50 au in the proto-BDs, while the low-excitation lines trace the cold (<20 K) gas at ∼50-150 au. The column density for the cold component is an order of magnitude higher than the warm component. The CH₃OH ortho-to-para ratios range between ∼0.3 and 2.3. The volume-averaged CH₃OH column densities show a rise with decreasing bolometric luminosity among the proto-BDs, with the median column density higher by a factor of ∼3 compared to low-mass protostars. Emission in high-excitation (E_upper > 25 K) CH₃OH lines together with the model predictions suggest that a warm corino is present in ∼78 per cent of the proto-BDs in our sample. The remaining shows evidence of only the cold component, possibly due to the absence of a strong, high-velocity jet that can stir up the warm gas around it.