Gaia FGK Benchmark Stars: New Candidates At Low-Metallicities
  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 of Heiter et al. (2015) has no
recommended stars within the critical metallicity range of $-2.0 <$ [Fe/H] $<
-1.0$ dex. 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. The procedure used to determine the stellar
parameters was similar to Heiter et al. (2015) and Jofre et al. (2014) for
consistency. The effective temperature (T$_{\mathrm{eff}}$) of all candidate
stars was determined 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$_{\mathrm{eff}}$ and log g from the values
determined independent of spectroscopy. 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. From the 10 stars, we recommend a
sample of five new metal-poor benchmark candidate stars which have consistent
T$_{\mathrm{eff}}$ , log g, and [Fe/H] determined through several means. These
stars can be used for calibration and validation purpose of stellar parameter
and abundance pipelines and should be of highest priority for future
interferometric studies.
