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Metal-poor halo stars as tracers of ISM mixing processes during halo formation
D. Argast 1,2,
M. Samland 1,
O.E. Gerhard 1 and
F.-K. Thielemann 2
Received 15 November 1999 / Accepted 8 February 2000
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] .
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 (SN) 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, Eu), the scatter in the element-to-iron ratio [El/Fe] 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 SN yields.
Finally, we discuss the metallicity distribution in the model. Compared to the 28 very metal-poor stars observed with metallicities in the range [Fe/H] , no star is known with confirmed metallicity [Fe/H] , while our model predicts stars with [Fe/H] . These should be present if the halo ISM started at primordial metallicities and no pre-enrichment by population III stars occurred.
Key words: nuclear reactions, nucleosynthesis, abundances ISM: abundances Galaxy: abundances Galaxy: halo
Send offprint requests to: D. Argast
Online publication: April 17, 2000