Star Formation in Different Environments

Abstract

This thesis covers two aspects of the star formation process: the stellar initial mass function and the triggering of star formation by cloud-cloud collisions. In the first half of the thesis, we use radiation-magnetohydrodynamics simulations to study the environmental variation of the low mass part of the stellar initial mass function (IMF), which describes the distribution of masses in a population of newly formed stars. We carry out a series of carefully crafted simulations where we keep the dimensionless parameters constant except for those related to feedback. We vary two important parameter spaces: surface density and metallicity. We explore a range of surface densities from those found in local star-forming regions to those found in the densest super star clusters and early-type galaxies, and a range of metallicities spanning from the most metal-poor local dwarfs to the most metal-rich massive galaxies. We find that radiation feedback is more effective at higher surface densities at suppressing fragmentation of gas into lower-mass objects, but this only partially compensates for the decrease of Jeans mass with surface density. We find that protostellar outflows are more effective at suppressing formation of massive objects at higher surface densities. A combination of both these effects leads toward an IMF that has a lower characteristic mass and a narrower shape at higher surface density runs. We also find that increasing metallicity tends to produce slightly bottom-heavy IMFs, but that effect is very weak compared to that of varying surface density. We find that metallicity-induced IMF variations produce shifts in the mass-to-light ratio of old stellar populations that are too small to explain the variations found in massive galaxies, whereas surface density shifts are more comparable with the observations.

In the second half of the thesis, we perform a series of hydrodynamic simulations to investigate the role of virial ratio and cloud density profile in collisions between two turbulent molecular clouds. We explore how virial ratio and initial density profile affect structure, evolution, and star-forming activity in colliding clouds. We consider collisions where one or both of the clouds are initially bound versus unbound, and where the clouds are either uniform or centrally concentrated. We find that if both clouds are bound and begin with uniform distributions, they generate filamentary structures along the collision axis that are discernible in both position-position and position-velocity diagrams, but that such structures are absent if the clouds are unbound. Similarly, collisions between bound clouds produce almost twice as much stellar mass as collisions runs where one of the clouds is initially unbound. In both these scenarios, collisions produce much more stellar mass than control cases where the clouds do not collide. By contrast, if the clouds have a strongly centrally condensed initial density profile, the collision does not have an effect on overall SFE, because star formation begins in the dense cloud cores even before collision, and the dense cores are relatively unaffected by the collision.