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-arcsecond resolution from 1.05 mm to 5.01 cm wavelengths. Our ~0.4″ ~ 1600 AU angular resolution ALMA observations reveal a velocity gradient across the millimetre core MM1, oriented perpendicular to the previously known bipolar molecular outflow, that 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 form a partial ellipse, consistent with an inclined ring, with a velocity gradient consistent with that of the thermal 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 (~10⁴ L_⊙) suggests that the MM1 disc may feed an unresolved high-mass binary system.