We study the chemical evolution and formation of the Galactic halo through
the analysis of its stellar metallicity distribution function and some key
elemental abundance patterns. Starting from the two-infall model for the
Galaxy, which predicts too few low-metallicity stars, we add a gas outflow
during the halo phase with a rate proportional to the star formation rate
through a free parameter, lambda. In addition, we consider a first generation
of massive zero-metal stars in this two-infall + outflow model adopting two
different top-heavy initial mass functions and specific population III yields.
The metallicity distribution function of halo stars, as predicted by the
two-infall + outflow model shows a good agreement with observations, when the
parameter lambda=14 and the time scale for the first infall, out of which the
halo formed, is not longer than 0.2 Gyr, a lower value than suggested
previously. Moreover, the abundance patterns [X/Fe] vs. [Fe/H] for C, N and
alpha-elements O, Mg, Si, S, Ca show a good agreement with the observational
data. If population III stars are included, under the assumption of different
initial mass functions, the overall agreement of the predicted stellar
metallicity distribution function with observational data is poorer than in the
case without population III. We conclude that it is fundamental to include both
a gas infall and outflow during the halo formation to explain the observed halo
metallicity distribution function, in the framework of a model assuming that
the stars in the inner halo formed mostly in situ. Moreover, we find that it
does not exist a satisfactory initial mass function for population III stars
which reproduces the observed halo metallicity distribution function. As a
consequence, there is no need for a first generation of only massive stars to
explain the evolution of the Galactic halo.Comment: Accepted for publication in A&A. 11 pages, 5 figure
