Dhanoa, H; (2014) The pressure-driven fragmentation of clouds at high redshift. Doctoral thesis , UCL (University College London).

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Abstract

Understanding the role of star formation and its feedback effects at high redshift is extremely important, as this greatly influences the nature of the first galaxies. This knowledge will also allow us to resolve the formation conditions of hyper-metal poor stars such as SDSS J102915+172927 (Caffau et al. 2011). This star is thought to be the first ‘truly’ low metallicity star, as it possesses a total metallicity between 10..5 .. 10..7 Z (Caffau et al. 2012). Hence it’s formation was probably triggered by a single primordial supernova (SN) event. Fragmentation studies that only include metal-line cooling, cannot reproduce the conditions in which such a star could form (Klessen et al. 2012). Therefore it is critical to simulate the physical processes that occur on the small scale as reliably as possible, as they impact large scale dynamics. At present early universe supernova shock models only include non-equilibrium chemistry and its associated cooling for temperatures below 104 K (Machida et al. 2005; Kitayama & Yoshida 2005; Nagakura et al. 2009; Chiaki et al. 2013). The metal-line cooling is often calculated separately assuming the collisional equilibrium. If we want to obtain realistic results, it is important to incorporate a complete chemistry (which includes metals, molecules and dust) and therefore evaluate the non-equilibrium cooling that occurs at all temperatures. In Chapter 2, we first try to understand the chemistry that would occur in a low metallicity gas. We investigate the chemical evolution of a metal-free cloud that has been mixed with ejecta from a single supernova. The very first stars would have been massive and simulations predict a range of masses (Bromm & Yoshida 2011). The initial mass of the star dictates the type of supernova explosion it will undergo. As each type of SN produces a different elemental yield, we would like to ascertain if it is possible to constrain molecular tracers of progenitor mass from a primordial cloud that interacted with that particular SN ejecta. A metal-free chemical network with its associated cooling is coupled to a hydrodynamics code in Chapter 3. Previous studies (Machida et al. 2005; Kitayama & Yoshida 2005; Nagakura et al. 2009; Chiaki et al. 2013) have focused on the fragmentation of the shell that forms as early universe supernova remnant interacts with an interstellar medium of a uniform density. Our model has improved upon these studies by modelling a multiphase medium with multidimensional simulations, with the goal to investigate the shock-driven fragmentation of a metal-free clump. We also investigate the effect of cosmic rays, CMB ionisation and deuterium chemistry on clumping and fragmentation of a neutral clump. Vasiliev et al. (2008) highlighted an important link between the formation of extremely metal poor stars and the radial distribution of primordial gas within a first galaxy, prior to the supernova explosion. This distribution is heavily dependent on the size of the metal-free star and its HII region prior the explosion. Hence in Chapter 4 we extend our metal-free model by simulating the formation of a HII region around a 40 M star in a number of gas clouds with differing density profiles. As the explosion mechanism for this star is not well understood, we explore a range of explosion energies and their impact on compression and fragmentation of the clump. The impact of metal and dust cooling on the fragmentation of low metallicity gas has been highlighted by a number os studies (e.g. Bromm & Loeb 2003; Santoro & Shull 2006; Omukai et al. 2005; Schneider et al. 2012). In Chapter 5 we consider the effect of cooling from metals, metal bearing molecules and dust. The nature and production of high redshift dust is not well constrained. Assuming dust-to-gas ratio can be scaled with metallicity, a simple dust model is implemented which includes cooling induced by gas-grain collisions is evaluated at all temperatures. The observational properties of dust and the physical consequences of its presence in the interstellar medium are extremely well-known and well-documented (Draine 2003). However its composition, structure and size-distribution are still subjects of much discussion. In Chapter 6 we have carried out an investigation of the chemical evolution of gas in different carbon-rich circumstellar environments. We pay careful attention to the accurate calculation of the molecular photoreaction rate coefficients to ascertain whether there is a universal formation mechanism for carbon dust in strongly irradiated astrophysical environments.

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