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 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.
