The stellar content of the Hamburg/ESO survey

Abstract

We determine the metallicity distribution function (MDF) of the Galactic halo by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO objective-prism survey (HES). The sample was corrected for minor biases introduced by the strategy for spectroscopic follow-up observations of the metal-poor candidates, namely best and brightest stars first. Comparison of the metallicities [Fe/H] of the stars determined from moderate-resolution (i.e., R ∼ 2000) follow-up spectra with results derived from abundance analyses based on high-resolution spectra (i.e., R > 20 000) shows that the [Fe/H] estimates used for the determination of the halo MDF are accurate to within 0.3 dex, once highly C-rich stars are eliminated. We determined the selection function of the HES, which must be taken into account for a proper comparison between the HES MDF with MDFs of other stellar populations or those predicted by models of Galactic chemical evolution. The latter show a reasonable agreement with the overall shape of the HES MDF for [Fe/H] >-3.6, but only a model of Salvadori et al. (2007) with a critical metallicity for low-mass star formation of Zcr = 10-3.4 Z reproduces the sharp drop at [Fe/H] ∼-3.6 present in the HES MDF. Although currently about ten stars at [Fe/H]