The stellar content of the Hamburg/ESO survey ^*,**

IV. Selection of candidate metal-poor stars

¹ Department of Astronomy and Space Physics, Uppsala University, Box 515, 75120 Uppsala, Sweden e-mail: norbert@astro.uu.se
² Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: nchristlieb/dreimers@hs.uni-hamburg.de
³ McDonald Observatory, The University of Texas at Austin, 1 University Station, C1400, Austin, TX 78712-0259, USA e-mail: anna@astro.as.utexas.edu
⁴ Department of Physics and Astronomy, CSCE: Center for the Study of Cosmic Evolution, and JINA: Joint Institute for Nuclear Astrophysics, Michigan State University, E. Lansing, MI 48824, USA e-mail: beers@pa.msu.edu
⁵ Astrophysical Institute Postsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: lutz@aip.de

Received: 27 September 2007
Accepted: 4 March 2008

Abstract

We present the quantitative methods used for selecting candidate metal-poor stars in the Hamburg/ESO objective-prism survey (HES). The selection is based on the strength of the Ca II K line, colors (both measured directly from the digital HES spectra), as well as colors from the 2 Micron All Sky Survey. The KP index for Ca II K can be measured from the HES spectra with an accuracy of 1.0 Å, and a calibration of the HES colors, using CCD photometry, yields a 1-σ uncertainty of 0.07 mag for stars in the color range . These accuracies make it possible to reliably reject stars with [Fe/H] > -2.0 without sacrificing completeness at the lowest metallicities. A test of the selection using 1121 stars of the HK survey of Beers, Preston, and Shectman present on HES plates suggests that the completeness at [Fe/H] < -3.5 is close to 100% and that, at the same time, the contamination of the candidate sample with false positives is low: 50% of all stars with [Fe/H] > -2.5 and 97% of all stars with [Fe/H] > -2.0 are rejected. The selection was applied to 379 HES fields, covering a nominal area of 8853 deg² of the southern high Galactic latitude sky. The candidate sample consists of 20 271 stars in the magnitude range 10 ≲ B ≲ 18. A comparison of the magnitude distribution with that of the HK survey shows that the magnitude limit of the HES sample is about 2 mag fainter. Taking the overlap of the sky areas covered by both surveys into account, it follows that the survey volume for metal-poor stars has been increased by the HES by about a factor of 10 with respect to the HK survey. We have already identified several very rare objects with the HES, including, e.g., the three most heavy-element deficient stars currently known.