ALMA reveals a pseudo-disc in a proto-brown dwarf

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 (CO)-C-12 (2-1) line and 1.37 mm continuum observations at an angular resolution of similar to 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 similar to 0.02M(circle dot), 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 COspectrum 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 similar to 30 000-40 000 yr. A comparison of the H-2 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.