2017ApJ...849...23M

We report on interferometric observations of a face-on accretion system around the high-mass young stellar object, G353.273+0.641. The innermost accretion system of 100 au radius was resolved in a 45 GHz continuum image taken with the Jansky-Very Large Array. Our spectral energy distribution analysis indicated that the continuum could be explained by optically thick dust emission. The total mass of the dusty system is ∼0.2 M_☉ at minimum and up to a few M_☉ depending on the dust parameters. 6.7 GHz CH₃OH masers associated with the same system were also observed with the Australia Telescope Compact Array. The masers showed a spiral-like, non-axisymmetric distribution with a systematic velocity gradient. The line-of-sight velocity field is explained by an infall motion along a parabolic streamline that falls onto the equatorial plane of the face-on system. The streamline is quasi-radial and reaches the equatorial plane at a radius of 16 au. This is clearly smaller than that of typical accretion disks in high-mass star formation, indicating that the initial angular momentum was very small, or the CH₃OH masers selectively trace accreting material that has small angular momentum. In the former case, the initial specific angular momentum is estimated to be 8 x 10²⁰ (M_*/10M_☉)^0.5 cm² s^–1, or a significant fraction of the initial angular momentum was removed outside of 100 au. The physical origin of such a streamline is still an open question and will be constrained by the higher-resolution (∼10 mas) thermal continuum and line observations with ALMA long baselines.