The stellar content of the Hamburg/ESO survey IV. Selection of candidate metal-poor stars

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, B − V colors (both measured directly from the digital HES spectra), as well as J − K 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 B − V colors, using CCD photometry, yields a 1-σ uncertainty of 0.07 mag for stars in the color range 0.3 < B − V < 1.4. 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.