The Pristine survey III: Spectroscopic confirmation of an efficient search for extremely metal-poor stars

Abstract

The Pristine survey is a narrow-band, photometric survey focused around the wavelength region of the Ca II H & K absorption lines, designed to efficiently search for extremely metal-poor stars. In this work, we use the first results of a medium-resolution spectroscopic follow-up to refine the selection criteria for finding extremely metal-poor stars ($\text{[Fe/H]}\le -3.0$) in the Pristine survey. We consider methods by which stars can be selected from available broad-band and infrared photometry plus the additional Pristine narrow-band photometry. The spectroscopic sample presented in this paper consists of 205 stars in the magnitude range $14<V<18$. Applying the photometric selection criteria cuts the sample down to 149 stars, and from these we report a success rate of 70% for finding stars with $\text{[Fe/H]}\le -2.5$ and 22% for finding stars with $\text{[Fe/H]}\le -3.0$. These statistics compare favourably with other surveys that search for extremely metal-poor stars, namely an improvement by a factor of $\sim 4-5$ for recovering stars with $\text{[Fe/H]}\le -3.0$. In addition, Pristine covers a fainter magnitude range than its predecessors, and can thus probe deeper into the Galactic halo.

arXiv comment14 pages, 6 figures, 4 tables, full version of Table 1 available on-line only, accepted for publication in MNRAS

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