Unveiling a multiscale view of massive star and cluster formation

Licentiatavhandling, 2022

Massive stars regulate the physical and chemical evolution of galaxies. Most stars within these galaxies, including massive ones, appear to be born in star clusters and associations. However, many questions remain unanswered about how massive stars and clusters form from the diffuse gas of interstellar space. For example, it is not yet known whether magnetic fields, turbulence or feedback are the most important actors in regulating gravitational collapse to give birth to these systems. It is also unclear to what extent potential protostellar crowding within a protocluster may affect massive star formation. Overdense clumps within giant molecular clouds (GMCs), often appearing in their earliest phases as infrared dark clouds (IRDCs), are the nurseries of massive stars and clusters. Properties of turbulence and magnetic fields in IRDCs are thus important to measure to give inputs for theoretical models of the formation processes. On the smaller scales of individual massive star formation, various theories, including core accretion, competitive accretion and protostellar collisions, may be viable depending on environmental conditions. Hence, studying how massive stars are forming in environments with relatively extreme conditions, e.g., in terms of crowding or isolation, may yield the most stringent constraints on these models.

This licentiate thesis first presents a study of a massive protostar (G28.2-0.05) that appears to be forming in relative isolation. Observational data, especially from the Atacama Large Millimeter/Submillimeter Array (ALMA), are used to investigate the nature of the system, including its dense and ionized gas structures, small-scale kinematics and dynamics and large-scale outflows. Mid to Far Infrared observations and archival data are used to measure the spectral energy distribution (SED) to further constrain protostellar properties. We conclude the system is a massive (∼ 24 M ) protostar that has an accretion powered wide angle bipolar molecular outflow and is also in the first stages of producing significant ionizing feedback. An examination of the mm dust continuum emission in the surroundings finds a near complete dearth of other sources, which is evidence for the system’s isolation and a strong constraint on competitive accretion models. Overall, core accretion models appear to give a good description of the protostar.

We also present first results of an observational program that attempts to use molecular gas kinematics and dust polarized continuum emission to measure properties of turbulence and magnetic fields across a range of scales from GMCs to IRDCs to individual massive protostars. We discuss the methods to be used and present the first data collected for the project. The overall work of this thesis leads in a direction of characterization of both large-scale molecular cloud environments and individual massive protostars forming within them to reveal a holistic, multi-scale view of the birth of massive stars and clusters