Massive Star Formation Across Evolution and Environment

Massive stars, i.e., those with more than 8 solar masses, play a key role in the regulation of galactic environments via their radiative, mechanical and chemical feedback. However, there is little consensus on the basic formation mechanism of these stars. Theories range from Core Accretion, e.g., the Turbulent Core Model (TCM), which is a scaled-up version of low-mass star formation from relatively ordered self-gravitating gas cores, to Competitive Accretion in which massive stars form more chaotically along with a cluster of lower-mass stars, and perhaps even involving Protostellar Collisions in the densest regions.

This thesis involves obtaining and analyzing multi-wavelength data of massive star-forming regions, especially early-stage Infrared Dark Clouds (IRDCs) and more evolved examples of high-mass and intermediate-mass protostars. The science goals include testing the Core Accretion paradigm by measuring properties of dense cores and early-stage protostars in IRDCs and the later-stage massive protostars. Thus a broad range of the evolutionary sequence of massive star formation is studied. The influence of environmental conditions on the star formation process is also investigated.

First, results studying star formation in IRDC environments are presented. The Atacama Large Mm/sub-mm Array (ALMA) was used to study 1.3~mm continuum emission tracing dusty, dense cores in 32 IRDC clumps. More than 100 cores were identified and a global core mass function (CMF) measured that has a high-end power law distribution of the form $d N / d\:{\rm log} M \propto M^{-\alpha}$ with $\alpha\simeq0.86\pm0.11$ for $M \geq 0.79\:M_\odot$, which is a significantly more top-heavy distribution than the Salpeter stellar initial mass function (IMF) that has an equivalent index of 1.35. Next the protostellar properties of these cores were investigated, including their outflow activity traced by SiO line emission, the presence of cm continuum radio jets (as observed by the VLA) and their mid-infrared (MIR) to far-infrared (FIR) spectral energy distributions (SEDs) (via archival Spitzer and Herschel telescope data). This study enables an estimate of the conditions needed for the onset of SiO emission (when $L\gtrsim 100\:L_\odot$) and cm continuum emission (at somewhat later stages) as diagnostic tracers of protostars. SiO outflows, like those previously studied via CO, tend to be collimated as expected in Core Accretion models, although one prominent example of more complex morphology is found, either indicating the presence of multiple sources and/or a more disordered outflow geometry.

The second part of the thesis concerns a study of a sample of about 40 high- and intermediate-mass protostars that make up the bulk of the SOFIA Massive (SOMA) Star Formation Survey observed with the FORCAST instrument from $\sim$ 10 to 40~$\rm \mu$m. These are selected to be MIR-bright sources, but are still expected to cover a range of evolutionary states and environments, i.e., from relatively early phase protostars in IRDCs to later phase ultracompact ionized regions, and from sources that are relatively isolated to those that are highly clustered. Multi-wavelength images of the protostars are presented. Core Accretion models predict that MIR morphologies are elongated along the direction of lower density outflow cavities, especially on the near-facing, blueshifted side. This signature is seen clearly in most of the largest, well-resolved sources, but is harder to detect in smaller, generally intermediate-mass, protostars. The source SEDs are fit with radiative transfer (RT) models, especially those developed for the TCM that have only a few key physical parameters of initial core mass ($M_c$), environmental clump mass surface density ($\Sigma_{\rm cl}$) and current protostellar mass ($m_*$), along with viewing angle to outflow axis and amount of foreground extinction. Almost all the protostellar SEDs can be well fit with these models, although extensive degeneracies can be present in the allowed parameters. The only exception is a source of extreme luminosity and distance, which is not well fit by the models and is likely to be a cluster of several (proto)stars. Overall, based on averages of best fitting models, the SOMA sources span luminosities from $\sim10^{2}-10^{6}\:L_{\odot}$, current protostellar masses from $\sim0.5-44\:M_{\odot}$ and ambient clump mass surface densities, $\Sigma_{\rm cl}$ from $0.1-3\:{\rm{g\:cm}^{-2}}$. We find no evidence that a threshold value of clump mass surface density is needed to form protostars up to $\sim25\:M_\odot$. However, there is tentative evidence that $\Sigma_{\rm{cl}}$ needs to be $\gtrsim1\:{\rm{g\,cm}}^{-2}$ to form more massive protostars. We argue that this result is best explained by the effect of $\Sigma_{\rm cl}$ on the efficiency of star formation of the core that is set by outflow and radiative feedback, as predicted by the TCM. The SOMA protostars are being used for further tests of the Core Accretion theory, especially utilizing further multiwavelength follow-up observations that are now underway.