The stellar content of the Hamburg/ESO survey^*

VI. Metallicity distribution of main-sequence turnoff stars in the Galactic halo

¹ Key Lab of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences A20 Datun Road, Chaoyang, Beijing 100012, PR China e-mail: [lhn;gzhao]@nao.cas.cn
² Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany e-mail: N.Christlieb@lsw.uni-heidelberg.de
³ Graduate University of Chinese Academy of Sciences, Beijing 100080, PR China
⁴ Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston, ACT 2611, Australia e-mail: [jen;bessell;yong]@mso.anu.edu.au
⁵ Department of Physics and Astronomy, and JINA: Joint Institute for Nuclear Astrophysics, Michigan State University, E. Lansing, MI 48824, USA e-mail: [beers;lee]@pa.msu.edu
⁶ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA e-mail: afrebel@cfa.harvard.edu

Received: 15 April 2010
Accepted: 10 June 2010

Abstract

We determine the metallicity distribution function (MDF) of the Galactic halo based on metal-poor main-sequence turnoff-stars (MSTO) which were selected from the Hamburg/ESO objective-prism survey (HES) database. Corresponding follow-up moderate-resolution observations (R ∽ 2000) of 682 stars (among which 617 were accepted program stars) were carried out with the 2.3 m telescope at the Siding Spring Observatory (SSO). Corrections for the survey volume covered by the sample stars were quantitatively estimated and applied to the observed MDF. The corrections are quite small, when compared with those for a previously studied sample of metal-poor giants. The corrected observational MDF of the turnoff sample was then compared with that of the giants, as well as with a number of theoretical predictions of Galactic chemical evolution, including the mass-loss modified simple model. Although the survey-volume corrected MDFs of the metal-poor turnoff and the halo giants notably differ in the region of [Fe/H] > -2.0, below [Fe/H] ~ -2.0, (the region we scientifically focus on most) both MDFs show a sharp drop at [Fe/H] ~ -3.6 and present rather similar distributions in the low-metallicity tail. Theoretical models can fit some parts of the observed MDF, but none is found to simultaneously reproduce the peak as well as the features in the metal-poor region with [Fe/H] between -2.0 to -3.6. Among the tested models only the GAMETE model, when normalized to the tail of the observed MDF below [Fe/H] ~ -3.0, and with Z_cr = 10^-3.4 , is able to predict the sharp drop at [Fe/H]~-3.6.