The Search for Extremely Low-Metallicity Stars in Dwarf Galaxies Using the NIR Ca II Triplet

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
² Laboratoire Cassiopée UMR 6202, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, France
³ Dpto. de Astrofísica y Ciencias de la Atmósfera, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB03 0HA, UK
⁵ GEPI, Observatoire de Paris-Meudon, CNRS, Univ. de Paris Diderot, Place Jules Janssen, 92190 Meudon, France
⁶ European Organization for Astronomical Research in the Southern Hemisphere, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁷ Observatoire de Genève, Laboratoire d’Astrophysique de l’École Polytechnique Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland
⁸ University of Texas, McDonald Observatory, HC75 Box 1337-McD, Fort Davis, TX 79734, USA
⁹ Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8P 1A1, Canada

Abstract

The NIR Ca II triplet has proven to be an important tool for quantitative spectroscopy. Here we present results of synthetic spectral analysis for the Ca II triplet for low-metallicity red giant stars, combined with observational data. Our results start to deviate strongly from the widely-used and linear empirical calibrations below [Fe/H] = −2. We provide a new calibration for Ca II triplet studies which is valid down until [Fe/H] = −4 and apply this new calibration to current data sets. We suggest that the classical dwarf galaxies are not so devoid of extremely low-metallicity stars as was previously thought and discuss preliminary results and possibilities for follow-up observations of these extremely low-metallicity candidates.