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 disk-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/submillimeter Array and imaged the 900 mu m dust continuum and the CH3CN J.=.19-18 K.=.0-10 transitions of S255IR.SMA1. The integrated CH3CN emission exhibits an elongated feature with an extent of 1800 au in the northwest-southeast direction at a position angle of 165 degrees, which is nearly perpendicular to the bipolar outflow. We confirm the presence of dense (a few x10(9) cm(-3)) and hot (similar to 400 K) gas immediately surrounding the central protostar. The CH3CN emission features a velocity gradient along the elongated ridge, and by modeling 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-disk). A mass infall rate of a few x 10(-4) M-circle dot yr(-1) is derived. If there exists a putative Keplerian disk 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.