Unveiling a multi-scale view of massive star and cluster formation

Doctoral thesis, 2024

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. However, there are many open questions about how these systems form from diffuse interstellar gas. For example it is not yet known whether magnetic fields, turbulence or feedback are the most important actors in regulating gravitational collapse. It is also unclear to what extent potential protostellar crowding within a protocluster may affect massive star formation. Thus it is important to measure levels of turbulence and magnetic fields in star-forming clouds to test theoretical formation models. 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. Studying the variation of star formation properties with galactic environment, e.g., metallicity, is also an important goal for helping to develop the most general theoretical understanding of star formation.

We first present 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 (40Msun) 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 describe the protostar well. Follow-up studies have measured the astrochemical content of the protostellar envelope and will help guide future chemodynamical models of massive star formation.

We next present the first results of an observational survey Polarised Light from Massive Protoclusters (POLIMAP), which studies the magneto-kinematic properties of a sample of infrared dark clouds (IRDCs) using SOFIA-HAWC+ observations, complemented by GBT-Argus observations of 13CO and C18O line emission. We present POLIMAP results for the massive IRDC G28.37+0.07. We show that magnetic fields are playing an important role in this IRDC, i.e., during the early stages of massive star and star cluster formation.

Finally, we present a study of the Galactic carbon isotope abundance gradient derived from observations of the species H2CO and HC3N from a large sample of 100 massive star-forming clumps across the Galaxy, observed as part of the ALMAGAL survey. We find an average ratio of 12C to 13C of 30, which is relatively low compared to previous studies. Our measurements also show a large scatter of 1.7 dex. We propose that these results can be explained by the effects of optical depth in the 12C lines of HC3N and H2CO leading to systematic biases in the measurements. We quantify the impact of optical depth with a combination of single dish and interferometric data, which for a handful of sources show that the ratio of of12C to13C could increase by a factor of 2-3. Hence, we argue that multi-transition analysis is required to accurately correct for optical depth effects to obtain accurate isotopic ratios.