G11.92-0.61 MM1: a Keplerian disc around a massive young proto-O star

The formation process of massive stars is not well understood, and advancement in our understanding benefits from high-resolution observations and modelling of the gas and dust surrounding individual high-mass (proto)stars. Here, we report subarcsecond (≲1550 au) resolution observations of the young massive star G11.92-0.61 MM1 with the Submillimeter Array (SMA) and Very Large Array (VLA). Our 1.3 mm SMA observations reveal consistent velocity gradients in compact molecular line emission from species such as CH₃CN, CH₃OH, OCS, HNCO, H₂CO, DCN and CH₃CH₂CN, oriented perpendicular to the previously reported bipolar molecular outflow from MM1. Modelling of the compact gas kinematics suggests a structure undergoing rotation around the peak of the dust continuum emission. The rotational profile can be well fitted by a model of a Keplerian disc, including infall, surrounding an enclosed mass of ∼30-60 M_⊙, of which 2-3 M_⊙ is attributed to the disc. From modelling the CH₃CN emission, we determine that two temperature components, of ∼150 K and 230 K, are required to adequately reproduce the spectra. Our 0.9 and 3.0 cm VLA continuum data exhibit an excess above the level expected from dust emission; the full centimetre-submillimetre wavelength spectral energy distribution of MM1 is well reproduced by a model including dust emission, an unresolved hypercompact H II region, and a compact ionized jet. In combination, our results suggest that MM1 is an example of a massive proto-O star forming via disc accretion, in a similar way to that of lower mass stars.