Gaia FGK benchmark stars: new candidates at low metallicities ^⋆,⋆⋆

¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
e-mail: khawkins@ast.cam.ac.uk
² Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
³ Université de Bordeaux – CNRS, LAB – UMR 5804, BP 89, 33270 Floirac, France
⁴ Observatoire de Genève, Université de Genève, 1290 Versoix, Switzerland
⁵ Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, The Australian National University, ACT 2611, Australia
⁶ Max-Planck Institute for Astronomy Konigstuhl 17, 69117 Heidelberg, Germany
⁷ INAF/Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁸ INAF/Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁹ ASI Science Data Center, via del Politecnico snc, 00133 Roma, Italy

Received: 7 February 2016
Accepted: 26 May 2016

Abstract

Context. We have entered an era of large spectroscopic surveys in which we can measure, through automated pipelines, the atmospheric parameters and chemical abundances for large numbers of stars. Calibrating these survey pipelines using a set of “benchmark stars” in order to evaluate the accuracy and precision of the provided parameters and abundances is of utmost importance. The recent proposed set of Gaia FGK benchmark stars has up to five metal-poor stars but no recommended stars within −2.0 < [Fe/H] < −1.0 dex. However, this metallicity regime is critical to calibrate properly.

Aims. In this paper, we aim to add candidate Gaia benchmark stars inside of this metal-poor gap. We began with a sample of 21 metal-poor stars which was reduced to 10 stars by requiring accurate photometry and parallaxes, and high-resolution archival spectra.

Methods. The procedure used to determine the stellar parameters was similar to the previous works in this series for consistency. The difference was to homogeneously determine the angular diameter and effective temperature (T_eff) of all of our stars using the Infrared Flux Method utilizing multi-band photometry. The surface gravity (log g) was determined through fitting stellar evolutionary tracks. The [Fe/H] was determined using four different spectroscopic methods fixing the T_eff and log g from the values determined independent of spectroscopy.

Results. We discuss, star-by-star, the quality of each parameter including how it compares to literature, how it compares to a spectroscopic run where all parameters are free, and whether Fe i ionisation-excitation balance is achieved.

Conclusions. From the 10 stars, we recommend a sample of five new metal-poor benchmark candidate stars which have consistent T_eff, log g, and [Fe/H] determined through several means. These stars, which are within −1.3 < [Fe/H] < −1.0, can be used for calibration and validation purpose of stellar parameter and abundance pipelines and should be of highest priority for future interferometric studies.