Metal-poor halo stars as tracers of ISM mixing processes during halo formation

We introduce a stochastic halo formation model to compute the early chemical enrichment of the interstellar medium (ISM) of the halo. Contrary to 1-zone chemical evolution models, we are able to resolve local 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] < 2.0. The early chemical evolution of the halo proceeds in different enrichment phases: At [Fe/H] < -3.0, the halo ISM is unmixed and dominated by local inhomogeneities caused by single core-collapse supernova (SN) events. For metallicities [Fe/H] >-2.0 the halo ISM is well mixed, showing an element abundance pattern integrated over the initial mass function. In the range -3.0 < [Fe/H] < -2.0 a continuous transition from the unmixed to the well mixed ISM occurs. For some elements (Si, Ca, Eu), the scatter in the element-to-iron ratio [El/Fe] seen in the observations of metal-poor halo stars can be reproduced. Stellar yields of other elements predict a scatter which, compared to the observations, is too large (O, Mg) or too small (Ni). Cr and Mn show a decreasing trend for lower metallicities, which can not be explained by metallicity independent yields, provided that the mixing of the ejecta with the interstellar medium does not depend on progenitor mass. This demonstrates the need for revised, self-consistent core-collapse SN yields.