Dense Core Formation Simulations in Turbulent Molecular Clouds with Large Scale Anisotropy

Abstract

<p> In this thesis, we study star formation in clustered environment within molecular clouds using Smooth Particle Hydrodynamics (SPH) simulations. Our first approach was to use "sink particles" to replace the dense gas particles where stars are forming. We implemented this type of particle in GASOLINE, and ran a simulation with a similar set of parameters to Bate et al. (2003). We found a good general agreement with this study. However, this work raised increasing concerns about some of the approximations used to follow the fragmentation process over many orders of magnitude in density. Our first issue was with the polytropic equation of state used to simulate gas of high density, that we believe would require some form of radiative transfer to be reliable. We also had concerns about the sink particles themselves, potentially overestimating the accretion rates.</p> <p> This guided our following work, where we choose to avoid both sinks and polytropic assumptions; allowing us to concentrate on the role of turbulence in forming prestellar cores. Supersonic turbulence is known to decay rapidly even when considering magnetic fields and gravity. However these studies are based on grid codes for periodic boxes. Our simulations are not periodic, they have open boundaries. Therefore the gravitational collapse can occur for the whole molecular cloud, not only for small portions of it. Hence the picture we observe in our self-gravitating turbulent molecular clouds is different. We found that under gravitational collapse turbulence is naturally developed and maintained with properties in good agreement with the current observational and theoretical picture.</p> <p> We also compared the cores we formed with observations. We looked at several observable properties of cores: density profiles, velocity dispersion and rotation of the cores, core-core velocity dispersion, core-envelope velocity dispersion, velocity dispersion vs. core size relation and the core mass function. We found a good general agreement between our simulated and observed cores, which indicates that extra physics like magnetic fields, outflows, proper equation of state or radiative transfer would have only secondary effects at this formation stages, or would tend to cancel each other.</p>