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Published November 2009 | Published + Accepted Version
Journal Article Open

The stellar content of the Hamburg/ESO survey. V. The metallicity distribution function of the Galactic halo


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 > 20000) 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 Z_(cr) = 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] < −3.6 are known, the evidence for the existence of a tail of the halo MDF extending to [Fe/H] ~ −5.5 is weak from the sample considered in this paper, because it only includes two stars [Fe/H] < −3.6. Therefore, a comparison with theoretical models has to await larger statistically complete and unbiased samples. A comparison of the MDF of Galactic globular clusters and of dSph satellites to the Galaxy shows qualitative agreement with the halo MDF, derived from the HES, once the selection function of the latter is included. However, statistical tests show that the differences between these are still highly significant.

Additional Information

© 2009 ESO. Received 6 September 2008; accepted 7 September 2009. We thank T. Karlsson, N. Prantzos, and S. Salvadori for providing us with electronic versions of published theoretical MDFs, and for enlightening discussions. Valuable comments on an earlier version of this paper by S. Ryan are gratefully acknowledged. We thank the DART collaboration for providing us with unpublished metallicities of stars in Carina, Sextans and Sculptor. N.C. and D.R. acknowledge financial support from Deutsche Forschungsgemeinschaft through grants Ch 214/3 and Re 353/44. N.C. is also supported by the Knut and Alice Wallenberg Foundation. J.G.C. is grateful to NSF grant AST-0507219 for partial support. T.C.B. acknowledges partial funding for this work from grants AST 04-06784, AST 06-07154, AST 07-07776, PHY 02-16873, and PHY 08-226498: Physics Frontier Center/Joint Institute for Nuclear Astrophysics (JINA), all awarded by the US National Science Foundation. M.S.B. and J.E.N. acknowledge support from the Australian Research Council under grants DP0342613 and DP0663562. A.F. acknowledges support from the W. J. McDonald Fellowship of the McDonald Observatory. P.S.B. is a Royal Swedish Academy of Sciences Research Fellow supported by a grant from the Knut and Alice Wallenberg Foundation. P.S.B. also acknowledges the support of the Swedish Research Council.

Attached Files

Published - Schoerck2009p6528Astron_Astrophys.pdf

Accepted Version - 0809.1172.pdf


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