2019MNRAS.486.4114R

We present the observational evidence of a pseudo-disc around the proto-brown dwarf Mayrit 1701117, the driving source of the large-scale HH 1165 jet. Our analysis is based on Atacama Large Millimeter/submillimeter Array ¹²CO (2-1) line and 1.37 mm continuum observations at an angular resolution of ∼0.4 arcsec. The pseudo-disc is a bright feature in the CO position-velocity diagram, elongated in a direction perpendicular to the jet axis, with a total (gas+dust) mass of ∼0.02 M_☉, size of 165-192 au, and a velocity spread of ±2 km s^–1. The large velocity gradient is a combination of infalling and rotational motions, indicating a contribution from a pseudo-disc and an unresolved inner Keplerian disc. There is weak emission detected in the H₂CO (3-2) and N₂D⁺ (3-2) lines. H₂CO emission likely probes the inner Keplerian disc where CO is expected to be frozen, while N₂D⁺ possibly originates from an enhanced clump at the outer edge of the pseudo-disc. We have considered various models (core collapse, disc fragmentation, circumbinary disc) that can fit both the observed CO spectrum and the position-velocity offsets. The observed morphology, velocity structure, and the physical dimensions of the pseudo-disc are consistent with the predictions from the core collapse simulations for brown dwarf formation. From the best model fit, we can constrain the age of the proto-brown dwarf system to be ∼30 000-40 000 yr. A comparison of the H₂ column density derived from the CO line and 1.37 mm continuum emission indicates that only about 2 per cent of the CO is depleted from the gas phase.