First observations of warm and cold methanol in Class 0/I proto-brown dwarfs

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 similar to 7-49 K, and critical densities, n(crit) of 10(5)-10(9) 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 similar to 21-49 K and n(crit) similar to 10(5) to 10(8) cm(-3). In comparison with the brown dwarf formation models, the high excitation lines likely probe the warm (similar to 25-50 K) corino region at similar to 10-50 au in the proto-BDs, while the low-excitation lines trace the cold (<20 K) gas at similar to 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 similar to 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 similar to 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 similar to 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.