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 ${}^{12}$CO (2-1) line and 1.37 mm continuum observations at an angular resolution of $\sim$0.4${}^{\prime \prime }$. 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 $\sim$0.02 M${}_{\odot }$, size of 165-192 AU, and a velocity spread of $\pm$2 km s${}^{-1}$. The large velocity gradient is a combination of infalling and rotational motions, indicating a contribution from a pseudo-disc and an unresolved inner Keplerian disc. There is weak emission detected in the H${}_2$CO (3-2) and N${}_2$D${}^{+}$ (3-2) lines. H${}_2$CO emission likely probes the inner Keplerian disc where CO is expected to be frozen, while N${}_2$D${}^{+}$ 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 $\sim$30,000-40,000 yr. A comparison of the H${}_2$ column density derived from the CO line and 1.37 mm continuum emission indicates that only about 2% of the CO is depleted from the gas phase.

arXiv commentAccepted in MNRAS

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