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

Abstract

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 ALMA 12CO (2-1) line and 1.37 mm continuum observations at an angular resolution of ∼0.411. The pseudo-disc is a bright feature in the CO position-velocity diagram (PVD), elongated in a direction perpendicular to the jet axis, with a total (gas+dust) mass of ∼0.02 Mo, 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, circum-binary 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% of the CO is depleted from the gas phase.