The Origin of the Initial Mass Function: The Role of Gravity, Turbulence, Jets, and Radiative Feedback

Abstract

Understanding the origin of stellar masses is one of the most compelling and important challenges in modern astrophysics. This is because the evolution and lifetime of a star, which are influenced by its mass, shape the structure and evolution of the interstellar medium of galaxies. Understanding the origin of the mass of a star is directly linked to understanding the mass distribution of stars since stars generally form in clusters. The stellar mass distribution is referred to as the initial mass function (IMF). The IMF is often thought to be relatively universal, and explaining the IMF characteristics like the existence of a peak mass and the power-law nature at the high mass end lies at the forefront of star formation research. In this thesis, I perform a comprehensive parameter study of the IMF using state-of-the-art star cluster formation simulations that incorporate physically important mechanisms such as gravity, turbulence, magnetic fields, jets, and radiative heating.

I show that the inclusion of outflow feedback in numerical simulations is essential to accurately model star formation and the IMF, as its effects significantly influence other key physical mechanisms such as turbulence and stellar radiative heating. I find that protostellar outflows shift the IMF to lower masses by roughly a factor of 2. Furthermore, I find that variations in the mode of turbulence driving play a significant role in shaping the form of the IMF, where a purely compressive (curl-free) turbulence driving produces a higher fraction of low-mass stars as compared to a purely solenoidal (divergence-free) driving. Changing the turbulence driving mode in simulations from purely solenoidal to purely compressive decreases the median stellar mass by a factor of 1.5. My study suggests that the dominance of solenoidal turbulence driving modes and relatively high cloud virial parameter values detected in the vicinity of the Galactic centre can partly explain the top-heavy nature of the IMFs in the associated clouds I propose that the scatter in the IMF observed in different regions of the Milky Way is due to the difference in the environmental conditions.

I also use the results from my simulation suite to study the binary properties of stars, which are closely tied to understanding the IMF. I verify that my simulations reproduce the observed trends like the increasing multiplicity fraction with mass and the dependence of orbital eccentricity on binary separation. I find that the turbulence driving mode has a considerable influence on the binary eccentricity, with simulations having compressively driven turbulence producing a higher fraction of high-eccentricity binaries as compared to simulations with solenoidal or a mixture of driving modes. This implies that the universal eccentricity distribution often used as an initial condition in N-body simulations of stellar cluster dynamics needs revision and further shows the broad impact of my work. I conclude that the IMF and the binary properties are controlled by the combined effect of gravity, magnetic fields, turbulence and protostellar feedback. The inferences from my studies can be used to calibrate sub-grid models in large-scale simulations of galaxy evolution, which often assume a universal IMF independent of the local physical conditions. This would enable more accurate modelling of energy output from star clusters, which are crucial drivers of galaxy evolution. My study advocates for future observational surveys to quantify the associated cloud conditions, such as the turbulence properties, together with measuring the IMF of star clusters. Creating distinct ensembles of observational IMFs for various cloud conditions would enable isolating the individual effects of different physical mechanisms and facilitate one-to-one comparisons with my theoretical work. Meticulous comparisons between observations and theory are quintessential to deepening our understanding of the IMF and star formation.