The stellar content of the Hamburg/ESO survey

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, 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 deg 2 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.

The role of binaries in the enrichment of the early Galactic halo

“…The majority of our programme stars are selected from the Hamburg/ESO survey of Christlieb and collaborators (HES;Christlieb et al 2008) with the addition of one star from the HK survey, CS 30301−015. The CEMP star LP 625−44 was added since it is well studied, and previous authors have suggested that it might be a CEMP-r/s star.…”

“…Carbon-enhanced metal-poor (CEMP) stars were originally identified among the VMP and EMP stars discovered in the HK survey of Beers, Preston, & Shectman (Beers et al 1985, 1992 and the Hamburg/ESO survey of Christlieb and collaborators (Christlieb et al 2008), and supplemented by a number of surveys since. The fraction of CEMP stars rises with decreasing metallicity (conventionally tracked by the iron abundance, [Fe/H]); hence they are of particular importance for studies of the early chemical evolution of the Galactic halo.…”

TOPoS

“…Until recently, the deepest survey for searching metal-poor stars was the Hamburg/ESO Survey (HES), which reached V ≈ 16 (Christlieb et al 2008), which increased the number of known metal-poor stars ([Fe/H] < −3.0) by about a factor of two with respect to all previous surveys. In terms of overall metallicity Z, or [M/H], the most metal-poor stars known were the giants CD −38 • 245 ([Fe/H] = −4.1), CS 22885-096, BS 16467-062, and CS 22172-002 with [Fe/H] ≈ −3.8 (François et al 2003;Cayrel et al 2004), and also the turn-off binary star CS 22876-32 (Norris et al 2000;González Hernández et al 2008) with [Fe/H] ≈ −3.7.…”

The Hamburg/ESOR-process enhanced star survey (HERES)

“…We also investigate the reliability of multiple Th/X chronometers for HE 2327−5642, where X is an element in the Ba−Hf range. HE 2327−5642 was identified as a candidate metal-poor star in the Hamburg/ESO Survey (HES; see Christlieb et al (2008) for details of the candidate selection procedures). Moderateresolution (Δλ = 2 Å) spectroscopy obtained at the Siding Spring Observatory (SSO) 2.3 m-telescope with the Double Beam Spectrograph (DBS) confirmed its metal-poor nature.…”