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

K. Youakim, E. Starkenburg, D. S. Aguado, N. F. Martin, M. Fouesneau, J. I. González Hernández, C. Allende Prieto, P. Bonifacio, M. Gentile, C. Kielty, P. Côté, P. Jablonka, A. McConnachie, R. Sánchez Janssen, E. Tolstoy, K. Venn

Published 2017-08-03Version 1

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 ($\textrm{[Fe/H]} \leq -3.0$) 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 $\textrm{[Fe/H]} \leq -2.5$ and 22% for finding stars with $\textrm{[Fe/H]} \leq -3.0$. 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 $\textrm{[Fe/H]} \leq -3.0$. In addition, Pristine covers a fainter magnitude range than its predecessors, and can thus probe deeper into the Galactic halo.