2019MNRAS.483.1139R

We present a study of the CO isotopologues and the high-density tracers H₂CO, HCO⁺, and CS in Class 0/I proto-brown dwarfs (proto-BDs). We have used the IRAM 30m telescope to observe the ¹²CO (2-1), ¹³CO (2-1), C¹⁸O (2-1), C¹⁷O (2-1), H₂CO (3-2), HCO⁺ (3-2), and CS (5-4) lines in seven proto-BDs. The hydrogen column density for the proto-BDs derived from the CO gas emission is ∼2-15 times lower than that derived from the dust continuum emission, indicating CO depletion from the gas-phase. The mean H₂CO ortho-to-para ratio is ∼3 for the proto-BDs and indicates gas-phase formation for H₂CO. We have investigated the correlations in the molecular abundances between the proto-BDs and protostars. Proto-BDs on average show a factor of ∼2 higher ortho-to-para H₂CO ratio than the protostars. Possible explanations include a difference in the H₂CO formation mechanism, spin-selective photodissociation, self-shielding effects, or different emitting regions for the ortho and para species. There is a tentative trend of a decline in the HCO⁺ and H₂CO abundances with decreasing bolometric luminosity, while the CS and CO abundances show no particular difference between the proto-BDs and protostars. These trends reflect the scaled-down physical structures for the proto-BDs compared to protostars and differences in the peak emitting regions for these species. The C¹⁷O isotopologue is detected in all of the proto-BDs as well as the more evolved Class Flat/Class II BDs in our sample, and can probe the quiescent gas at both early and late evolutionary stages.