Issue |
A&A
Volume 484, Number 3, June IV 2008
|
|
---|---|---|
Page(s) | 721 - 732 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361:20078748 | |
Published online | 11 March 2008 |
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
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.
Key words: surveys / Galaxy: halo / Galaxy: stellar content / Galaxy: formation / Galaxy: evolution / stars: population II
© ESO, 2008
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