Understanding the early stages of massive star formation with ALMA : 1.3 mm continuum observations of young stellar protoclusters

Massive stars dominate the energetics of the interstellar medium, yet the mechanism of their formation has not been established. Constraining proposed scenarios requires the characterisation of massive star-forming regions at early stages, with (sub)millimetre-λ interferometry. This thesis is primarily an analysis of 1.3 mm continuum observations from the ALMA EGO-10 survey. The targets are ten Extended Green Objects - active outflows driven by young massive protostars. Chapter 2 describes the observations and continuum imaging. The resulting images reveal protoclusters in all targets. These data are compared with previous SMA observations in a search for accretion bursts. Chapter 3 presents the 1.3 mm source identification, yielding 568 dense cores - between 13 and 135 per field. Core properties are reported in a catalogue. Protocluster structure is then quantified, including with the 𝒬-parameter, revealing structural diversity within the sample - 0.53 < 𝒬 < 0.93. Chapter 4 investigates the association of cores with cm-λ star formation tracers. This reveals 39 protostellar cores, 11 of which host 6.7 GHz masers and thus, massive protostars. Massive protostars reside near protocluster centres (offset <0.02 pc in 10/11 cases), and all in regions at d < 3 kpc inhabit highly clustered locales; >10 cores within 10,000 AU. Chapter 5 quantifies the relationship between protocluster structure and evolutionary state. A new evolutionary indicator is presented - the ratio of protocluster cm-λ continuum luminosity to the mass of the associated clump - which positively correlates with 𝒬. The relationship is driven by the massive protostars. These results suggest dynamic protocluster structure, even in the early stages of massive star formation; and that the evolution of massive protostars is coupled with that of their host protocluster. This is consistent with the predictions of clump-fed models of massive star formation, in which global collapse is traced by the core spatial distribution. An analysis of time-resolved synthetic 1.3 mm ALMA observations supports this interpretation