Inhomogeneous chemical evolution of the galactic halo

I present a stochastic chemical evolution model to investigate the enrichment of the interstellar medium (ISM) during Galaxy formation. Contrary to classical chemical evolution models, it is able to resolve local chemical inhomogeneities in the ISM caused by single core-collapse supernovae. These inhomogeneities lead to different element abundance patterns in very metal-poor stars which can be seen as scatter in the abundances of halo stars with metallicities [Fe/H] . The early chemical evolution of the halo proceeds in different enrichment phases: At [Fe/H] , the halo ISM is unmixed and dominated by local inhomogeneities caused by individual core-collapse supernova events. For metallicities [Fe/H] the halo ISM is well mixed, showing an element abundance pattern integrated over the initial mass function. In the range [Fe/H] a continuous transition from the unmixed to the well mixed ISM occurs. For some elements (Si, Ca), the scatter in element-to-iron ratios of metal-poor halo stars can be reproduced. Stellar yields of other elements, however, predict a scatter which, compared to observations, is too large (O, Mg) or too small (Ni). This shows, that inhomogeneous chemical evolution models are heavily dependent on theoretical nucleosynthesis yields of core-collapse supernovae. Hence inhomogeneous chemical evolution models present themselves as a test for stellar nucleosynthesis calculations. One problem revealed by the model is the predicted scatter in [O/Fe] and [Mg/Fe] which is too large compared to the one observed in metal-poor halo stars. This can be either due to the oxygen or magnesium yields or due to the iron yields (or both). However, oxygen and magnesium are -elements that are produced mainly during hydrostatic burning and thus are not affected by the theoretical uncertainties afflicting the collapse and explosion of a massive star. Stellar iron yields, on the other hand, depend heavily on the choice of the mass-cut between ejecta and proto-neutron star and are therefore very uncertain. In this work, iron yield distributions as function of progenitor mass are derived which are consistent with the abundance distribution of metal-poor halo stars and are in agreement with observed yields of core-collapse supernovae with known progenitor masses. The iron yields of lower-mass Type II supernovae (in ) are well constrained by these observations. Present observations, however, do not allow the range us to determine a unique solution for higher-mass Type II supernovae. Nevertheless, the main dependence of the stellar iron yields as function of progenitor mass can be derived and may be used as a constraint for future core-collapse supernova/hypernova models. A prediction of hypernova models which can be tested by future observations is the existence of ultra -element enhanced stars at metallicities [Fe/H] . The results are of importance for the earliest stages of galaxy formation when the ISM is dominated by local chemical inhomogeneities and the instantaneous mixing approximation is not valid. The astrophysical nature of r-process sites is a long standing mystery and many probable sources were suggested in the past, among them lower-mass core-collapse supernovae (in the range ), higher-mass core-collapse supernovae (with masses ) and neutron star mergers. In this work, I present a detailed inhomogeneous chemical evolution study that considers for the first time neutron star mergers as major rprocess sources, and compare this scenario to the ones in which core-collapse supernovae act as dominant r-process sites. Furthermore, the enrichment of the interstellar medium with neutron-capture elements during Galaxy formation by r- and s-process sources is investigated. I conclude that, due to the lack of reliable iron and r-process yields as function of progenitor mass, it is not possible to date to distinguish between the lower-mass and higher-mass supernovae scenario within the framework of inhomogeneous chemical evolution. However, neutron-star mergers seem to be ruled out as dominant r-process source, since their low coalescence rates are not consistent with observations of r-process elements at very low metallicities. Furthermore, the considerable injection of r-process material by a single neutron-star merger leads to a scatter in r-process abundances at later times which is much too large compared to observations. Finally, a low star-formation efficiency is required during halo formation to be consistent with the appearance of s-process elements at very low metallicities.