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Maio, Umberto (2008): Switching on the first light in the Universe. Dissertation, LMU München: Faculty of Physics
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Abstract

Star formation (SF) is still an unsolved problem in astrophysics. Gas cooling is the principal mechanism leading to the condensation of gas and consequently to star and structure formation. In a metal-free environment, the main available coolants are H, He, H$_2$ and HD; once the gas is enriched with metals, these also become important. In this work, in order to properly determine the SF in the early Universe, we compute fine-structure transition metal cooling and implement and test molecular chemistry. Moreover, we investigate its redshift ($z$) evolution and compare different modeling running very high-resolution, three-dimensional, N-body/hydrodynamic simulations including non-equilibrium, atomic and molecular chemistry, SF prescriptions and feedback effects. We also study how the primordial SF changes accordingly to different semi-analytical approaches, cosmological parameters, initial set-ups and critical metallicity ($Z_{crit}$) for the transition from a metal-free SF regime to a standard enriched one.\\ Our main findings are: the H$_2$ molecule is the most relevant coolant in early times; inclusion of HD cooling results in a $\sim 10\%-20\%$ higher gas clumping; metal cooling at low temperatures can have a significant impact on the formation and evolution of first objects; typical numerical ``sub-grid'' models fail in following the cooling of primordial gas and predict too early SF ($z\sim 30$); considering molecular cooling, we get a postponed epoch ($z\sim 15$) for the same initial conditions; rare, high-density peak can host SF even at $z\gtrsim 40$; metal-free SF regime is completely negligible with respect to the global SF rate, because of the very short first star life-times; it has some relevance only for $\Delta z\simeq 1$ (at $z\sim 16$); primordial pollution up to $\sim 10^{-3}\,Z_\odot$\footnote[1]{ The solar metallicity is $Z_\odot\simeq 0.02$. }, or higher, is extremely rapid and allows for a very fast transition to standard SF regimes; the different SF rates and metal enrichment got for different $Z_{crit}$ are well distinguishable and span about one order of magnitude for $Z_{crit}/Z_\odot \in [10^{-6}, 10^{-3}]$. Given the importance of the initial stellar mass distribution function, an analytical model describing its possible derivation from turbulent dissipation is presented.