A key open question in modern astrophysics is whether a “scaled-up” version of low-mass star formation theory can also explain the formation of high-mass stars. A major uncertainty is whether/how disc accretion can operate quickly enough to form a high-mass star. In 2017, an unexpected discovery transformed our view of this question: the first reported observational evidence for “accretion bursts”— sudden, sharp increases in the rate at which material is gained by a forming star—in young high-mass stars. These events, long known in low mass stars, were observed as sudden increases in the emission from the dust surrounding the forming stars (observed at infrared and submillimetre wavelengths). Importantly, both bursts were also associated with flares in maser emission—stimulated, non-thermal emission in specific molecular lines. This fact led to a coordinated worldwide campaign to search for such maser flares, leading to the discovery, in 2019, of the third known accretion burst in a young high-mass star, G358.93-0.03 (in the following G358).
So far, the analysis of accretion burst events in young high-mass stars has focused primarily on the spatial distribution of detected masers and the changes in the dust emission. The aim of this doctoral project is to make progress in two important areas that have not yet been studied, using the world’s most powerful telescopes:
- Methanol maser lines at submillimetre wavelengths. Masers are important astrophysical tools because they require specific physical conditions to exist, and so have the potential to unlock the physical conditions associated with accretion bursts. This, however, requires a detailed understanding of which maser lines are and which are not present in normal (non-outbursting) young high-mass stars. For submillimetre-wavelength methanol masers, such as those seen in G358, no observations of such “normal” sources exist. The student will use the Atacama Pathfinder 12 meter submillimetre telescope, APEX, to carry out a comprehensive search, with the goal of understanding the physics of the maser pumping mechanism and its connection to accretion bursts. Together with radiative transfer modelling, these new data will elucidate the maser excitation process and, in turn, yield unique information on the closest environments of young high-mass stars where the masers reside.
- The outflow-outburst connection in accretion burst sources. Forming stars drive outflows (consisting of a fraction of the accreted material), which persist for 100s–1000s of years and so preserve a history of the star’s accretion activity. Using in-hand data from the Atacama Large Millimeter/ submillimeter Array (ALMA), the student will conduct the first analysis of the accretion history of the recently-discovered high-mass outburst sources. By quantifying outflow properties in multi-epoch ALMA observations, the student will characterise the dynamical impact of protostellar accretion bursts.
The scholarship will support a co-tutelle doctoral degree programme between the School of Physics and Astronomy at St Andrews and the University of Bonn in Germany. The student will be supervised by Dr Claudia Cyganowski (University of St Andrews) and Prof Dr Karl Menten (MPIfR, University of Bonn).
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