Other
Scientific paper
Feb 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt.........2s&link_type=abstract
PhD Thesis, Naturwissenschaftlich-Mathematische Gesamtfakultät der Universität Heidelberg, Germany. VIII + 103 p. (2002)
Other
3
Population Iii Stars, Stellar Evolution, Nuclear Reactions
Scientific paper
The first generation of stars in the universe, the hypothetical Population III stars, are believed to have formed out of pure hydrogen-helium gas with negligible amounts of heavy elements. The (quasi-)hydrostatic evolution of these stars in the mass range 15 Msolar - 150 Msolar is studied starting from the pre-main sequence until the end of the main-sequence phase. Compared to "normal" stars, both the initial absence of CNO elements and the small temperature dependence of pp burning lead to more compact and hotter objects on the main sequence for all massive Population III stars. Above 30 Msolar, Population III stars contract to central temperatures of 108K, produce carbon via the 3α process and settle on the main sequence in the mode of hot CNO burning. In the range of 15 Msolar - 30 Msolar the main sequence is reached with pp burning but even in these cases CNO-burning takes over during the main-sequence phase. The hot temperature of these stars leads to the coupling of the convective mixing timescale and the timescales of proton capture and β-decay reactions of the CNO-cycle. The assumption of instantaneous mixing across convective regions is not fulfilled. For the first time, a newly developed stellar evolution code utilizing the software package LIMEX integrates the stellar structure equations together with a time-dependent nuclear reaction network, time-dependent mixing and a time-dependent theory of convection fully coupled and implicitly. The improved modeling of nuclear reactions and mixing only changes the spatial distribution of CNO elements that deviates 10%-20% from homogeneity. In all models the employed time-dependent theory of convection leads to larger convection zones compared to other work. This results in longer main-sequence lifetimes and larger helium cores with possible consequences for the final stages of these stars and the metal feedback for successive generations of stars.
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