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

N. Christlieb, T. Schörck+4 more|ArXiv.org|Apr 9, 2008

Stellar, planetary, and galactic studiesPhysics and Astronomy51 references83 citations

TL;DR

This paper presents a quantitative, multi-wavelength selection method for identifying metal-poor star candidates in the Hamburg/ESO Survey using Ca II K line strength (KP index), HES-derived B-V colors, and 2MASS J-K colors. The method achieves >97% rejection of stars with [Fe/H] > -2.0 and near-100% completeness for [Fe/H] < -3.5, yielding a sample of 20,271 candidates over 8,853 deg² with a survey volume 10× larger than the HK survey due to a fainter magnitude limit (B ≈ 17.2).

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 Angstrom, and a calibration of the HES B-V colors, using CCD photometry, yields a 1-sigma uncertainty of 0.07 mag for stars in the color range 0.3 &lt; B-V &lt; 1.4. These accuracies make it possible to reliably reject stars with [Fe/H] &gt; -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] &lt; -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] &gt; -2.5 and 97% of all stars with [Fe/H] &gt; -2.0 are rejected. The selection was applied to 379 HES fields, covering a nominal area of 8853 square degrees of the southern high Galactic latitude sky. The candidate sample consists of 20,271 stars in the magnitude range 10 &lt; B &lt; 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.

Motivation & Objective

• To develop a reliable, quantitative method for selecting metal-poor star candidates from the Hamburg/ESO Survey (HES) digital spectra.
• To improve upon the HK survey's selection by extending the survey volume to fainter magnitudes and higher Galactic latitudes.
• To minimize contamination from non-metal-poor stars while maintaining high completeness for the most metal-poor stars.
• To enable statistical studies of the Milky Way's metal-poor halo population by defining a well-understood selection function.
• To identify rare, extremely metal-poor stars (e.g., [Fe/H] < -5.0) previously undetected by earlier surveys.

Proposed method

• Measuring the Ca II K line strength (KP index) directly from digital HES spectra with a precision of ±1.0 Å.
• Calibrating HES-derived B-V colors using CCD photometry, achieving a 1-σ uncertainty of 0.07 mag for 0.3 < B-V < 1.4.
• Combining KP index and B-V with J-K colors from the 2MASS survey to define a color–index selection window in parameter space.
• Using the selection criteria derived from Beers et al. (1999), which relate KP, B-V, and J-K to [Fe/H], to identify candidates with [Fe/H] < -2.5.
• Applying visual inspection to reject false positives and refine the final candidate list, focusing on the most promising objects.
• Validating the selection using a test sample of 1,121 HK survey stars present on HES plates to assess completeness and contamination rates.

Experimental results

Research questions

• RQ1 What is the completeness of the HES selection method for stars with [Fe/H] < -3.5?
• RQ2 How effectively does the method reject stars with [Fe/H] > -2.0 and [Fe/H] > -2.5?
• RQ3 To what extent does the HES survey volume exceed that of the HK survey due to its fainter magnitude limit?
• RQ4 How accurate are the HES-derived photometric indices (KP, B-V) in predicting metallicity?
• RQ5 Can the method reliably identify extremely metal-poor stars (e.g., [Fe/H] < -5.0) and rare objects?

Key findings

• The HES selection method achieves near-100% completeness for stars with [Fe/H] < -3.5, based on a test sample of 1,121 HK survey stars.
• The method rejects 97% of stars with [Fe/H] > -2.0 and 50% of stars with [Fe/H] > -2.5, indicating low contamination in the candidate sample.
• The HES covers 8,853 deg² of the southern high Galactic latitude sky, with an effective survey area of 6,726 deg² after accounting for overlaps.
• The magnitude limit of the HES sample (B ≈ 17.2) is approximately 2 mag fainter than the HK survey (B ≈ 15.2), increasing the survey volume for metal-poor stars by a factor of ~10.
• The method successfully identified several extremely rare objects, including the three most heavy-element-deficient stars known to date.
• The selection function is well-defined and quantifiable, enabling future statistical studies of the Milky Way's metal-poor halo, including the low-metallicity tail of the metallicity distribution function (MDF).