Gwen Williams

High-mass stars (>8 solar masses) are key constituents of galaxies, yet much remains unknown about their formation and mass growth. It is known that low-mass stars accrete material through rotationally supported circumstellar discs, and that shedding of excess angular momentum is achieved through molecular outflows emanating from the poles. A cartoon schematic of this is shown on slide 2. Models in theory suggest it is favourable for high-mass stars to accrete in a scaled-up version of this process. This paradigm of mass accretion is routinely observed for low-mass stars, however these disc-outflow systems remain elusive in the high-mass regime, with only a few examples observed around the most extreme proto-O type stars. This may be somewhat attributed to high-mass star forming regions being trickier to observe than their low-mass counterparts, as they are (i) several thousand parsecs more distant, (ii) are by nature clustered regions, and (iii) are deeply embedded in their natal cloud during their formation. Recent advances in high-angular resolution studies in the sub-millimetre wavelength regime with the Atacama Large Millimetre/submillimetre Array (ALMA) now allow us to resolve and disentangle these regions, aiding us in the understanding of how high-mass stars gain their mass.

We here present a study of the G19.01-0.03 MM1 proto-O star (hereafter called MM1) observed with ALMA. Previous works that studied MM1 with the Submillimetre Array (SMA) showed MM1 to be driving a high-velocity bipolar outflow, making MM1 an excellent candidate for also harbouring a hidden accompanying circumstellar accretion disc. A figure of this outflow in relation to MM1, traced by 12CO(2-1) emission with the SMA, is shown in the “More details” section on slide 3. With our new ALMA data, we have 6 times better angular resolution and 16 times better sensitivity in comparison to the previously published SMA data, meaning our new ALMA data is well-placed to search MM1 for a candidate circumstellar accretion disc.

One diagnostic of such discs in molecular line data involves the velocity of the gas. As depicted in the cartoon schematic on the right hand side of slide 3, the gas in the rotating circumstellar disc that moves towards us is blue shifted with respect to the source velocity, and the gas moving away from us is red shifted, all thanks to the doppler effect. Seeing such a such a velocity gradient across the target in our data would represent good evidence of a candidate disc towards MM1.

To trace the gas velocity, we identify 43 molecular line transitions in our ALMA spectral bands, and create maps of the gas velocity traced by some of these molecules using a moment analysis. The moment one velocity map for 8 of the molecular transitions is shown in the figure on slide 4, where the colour bar represents the gas velocity, and the black contours show the ALMA dust emission at 1.05mm. A velocity pattern similar to that depicted in the cartoon at the top of slide 4 is observed, with blue shifted gas to the West (on-sky direction) and red shifted gas to the East. This represents the first evidence ever reported of a circumstellar accretion disc towards MM1, and we therefore add MM1 to the short list of high-mass proto-O star disc-outflow candidates in the literature.

Further kinematic analysis not shown here reveal the disc to enclose a massive 50 solar masses of material. This is quite large in comparison to the known bolometric luminosity of MM1 of 10,000 solar luminosities. We speculate that MM1 may therefore have this large mass distributed in a high-mass binary system, and further high angular resolution studies are needed to inspect this.