ALMA View of the Infalling Envelope around a Massive Protostar in S255IR
  SMA1
  The massive young stellar object S255IR NIRS3 embedded in the star forming
core SMA1 has been recently observed with a luminosity burst, which is
conjectured as a disc-mediated variable accretion event. In this context, it is
imperative to characterize the gas properties around the massive young stellar
object. With this in mind, we carried out high angular resolution observations
with the Atacama Large Millimeter and submillimeter Array and imaged the 900
$\mu m$ dust continuum and the CH$_3$CN $J$=19$-$18 $K$=0$-$10 transitions of
S255IR SMA1. The integrated CH$_3$CN emission exhibits an elongated feature
with an extent of 1800 au in the northwest-southeast direction at a position
angle of 165 degree, which is nearly perpendicular to the bipolar outflow. We
confirm the presence of dense (a few $\times 10^{9}$ cm$^{-3}$) and hot ($\sim$
400 K) gas immediately surrounding the central protostar. The CH$_3$CN emission
features a velocity gradient along the elongated ridge and by modelling the gas
kinematics based on features in the position-velocity diagram, we infer that
the gas is best described by a flattened rotating infalling envelope (or
pseudo-disc). A mass infall rate of a few $\times$ 10$^{-4}$ solar-mass per
year is derived. If there exists a putative Keplerian disc directly involved in
the mass accretion onto the star and jet/outflow launching, it is likely
smaller than 125 au and unresolved by our observations. We show qualitative
resemblances between the gas properties (such as density and kinematics) in
255IR SMA1 inferred from our observations and those in a numerical simulation
particularly tailored for studying the burst mode of massive star formation.
