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Register a new userStochastic chemical enrichment in metal-poor systems I. Theory
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T. Karlsson
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A stochastic model of the chemical enrichment of metal-poor systems by core-collapse (Type II) supernovae is presented, allowing for large-scale mixing of the enriched material by turbulent motions and cloud collisions in the interstellar medium. Infall of pristine material is taken into account by following the evolution of the gas density in the medium. Analytical expressions were derived for the number of stars enriched by a given number of supernovae, as well as for the amount of mass with which the ejected material from a supernova is mixed before being locked up in a subsequently formed star. It is shown that for reasonable values of the gas density (~0.1 cm-3) and of the supernova rate (~0.25 kpc-3 Myr-1) of the Galactic halo, the resulting metallicity distributions of the extreme Population II stars show a distinct cut-off at [Fe/H] ~= -4. In fact, by assuming no low-mass Population III stars were able to form out of the primordial interstellar medium, the derived fraction of stars below [Fe/H] = -4 is in agreement with observations. Moreover, the probability is high that even the most metal-poor stars observed to date have been enriched by several contributing supernovae. This partly explains the relatively small star-to-star scatter in many chemical-abundance ratios for stars down to [Fe/H] = -4, as recently found in several observational studies. Contribution from the thermonuclear (Type Ia) supernovae is found to be negligible over almost the entire extremely metal-poor regime. (***abridged***)
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A stochastic model of the chemical enrichment of metal-poor systems by core-collapse supernovae is used to study the scatter in stellar abundance ratios. The resulting scatter in abundance ratios, e.g. as functions of the overall metallicity, is demonstrated to be crucially dependent on the as yet uncertain supernovae yields. The observed abundance ratios and their scatters therefore have diagnostic power as regards the yields. The relatively small star-to-star scatter observed in many chemical abundance ratios, e.g. by Cayrel et al. (2004) for stars down to [Fe/H] = -4, is tentatively explained by the averaging of a large number of contributing supernovae and by the cosmic selection effects favoring contributions from supernovae in a certain mass range for the most metal-poor stars. The scatter in observed abundances of alpha-elements is understood in terms of observational errors only, while additional spread in yields or sites of nucleosynthesis may affect the odd-even elements Na and Al. For the iron-group elements we find systematically too high predicted Cr/Fe and Cr/Mg ratios, as well as differences between the different sets of yields, both in terms of predicted abundance ratios and scatter. The semi-empirical yields recently suggested by Francois et al. (2004) are found to lead to scatter in abundance ratios significantly greater than observed, when applied in the inhomogeneous models. Spurs, very narrow sequences in abundance-ratio diagrams, may disclose a single-supernova origin of the elements of the stars on the sequence. Verification of the existence of such features, called single supernova sequences (SSSs), is challenging. This will require samples of several hundred stars with abundance ratios observed to accuracies of 0.05 dex or better.
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