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

Abstract

ABSTRACT 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, Eupper ∼7–49 K, and critical densities, ncrit of 105–109 cm−3. The most commonly detected lines are at Eupper < 20 K, while 11 proto-BDs also show emission in the higher excitation lines at Eupper ∼21–49 K and ncrit ∼ 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 (Eupper > 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.