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

Abstract

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 $\sim 0.4^{\prime \prime }\sim 1600$ 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 $40-70{\mathrm{M}}_{\odot }$ (within a 2000 AU outer radius) for a disc inclination angle of $i=40^{\circ }$, of which $5.4-7.2{\mathrm{M}}_{\odot }$ 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 HII region, is required to explain the VLA $\sim$5 cm emission. The high enclosed mass derived for a source with a moderate bolometric luminosity ($\sim$10${}^4{\mathrm{L}}_{\odot }$) suggests that the MM1 disc may feed an unresolved high-mass binary system.

15 pages, 8 figures, 5 tables, Accepted for publication in MNRAS

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