ALMA observations of the Extended Green Object G19.01−0.03 – I. A Keplerian disc in a massive protostellar system

Using the Atacama Large Millimetre/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA), we observed the Extended Green Object (EGO) G19.01-0.03 with sub-arcsec resolution from 1.05 mm to 5.01 cm wavelengths. Our 0.4 arcsec 1600 au angular resolution ALMA observations reveal a velocity gradient across the millimetre core MM1, oriented perpendicular to the previously known bipolar molecular outflow, which is consistently traced by 20 lines of 8 molecular species with a range of excitation temperatures, including complex organic molecules (COMs). Kinematic modelling shows the data are well described by models that include a disc in Keplerian rotation and infall, with an enclosed mass of 40-70 M (within a 2000 au outer radius) for a disc inclination angle of i = 40, of which 5.4-7.2 M is attributed to the disc. Our new VLA observations show that the 6.7 GHz Class II methanol masers associated with MM1 fo a partial ellipse, consistent with an inclined ring, with a velocity gradient consistent with that of the theal gas. The disc-to-star mass ratio suggests the disc is likely to be unstable and may be fragmenting into as-yet-undetected low-mass stellar companions. Modelling the centimetre-millimetre spectral energy distribution of MM1 shows the ALMA 1.05 mm continuum emission is dominated by dust, whilst a free-free component, interpreted as a hypercompact H ii region, is required to explain the VLA 5 cm emission. The high enclosed mass derived for a source with a moderate bolometric luminosity (104L) suggests that the MM1 disc may feed an unresolved high-mass binary system.