Infrared colours and inferred masses of metal-poor giant stars in the
  Kepler field
  Intrinsically luminous giant stars in the Milky Way are the only potential
volume-complete tracers of the distant disk, bulge, and halo. The chemical
abundances of metal-poor giants also reflect the compositions of the earliest
star-forming regions, providing the initial conditions for the chemical
evolution of the Galaxy. However, the intrinsic rarity of metal-poor giants
combined with the difficulty of efficiently identifying them with broad-band
optical photometry has made it difficult to exploit them for studies of the
Milky Way. We re-derive a giant star photometric selection using existing
public g-band and narrow-band DDO51 photometry obtained in the Kepler field.
Our selection is simple and yields a contamination rate of main-sequence stars
of $\lesssim$1% and a completeness of about 80% for giant stars with $T_{\rm
eff} \lesssim 5250$ K - subject to the selection function of the spectroscopic
surveys used to estimate these rates, and the magnitude range considered
($11\lesssim g\lesssim 15$). While the DDO51 filter is known to be sensitive to
stellar surface gravity, we further show that the mid-infrared colours of
DDO51-selected giants are strongly correlated with spectroscopic metallicity.
This extends the infrared selection developed by Schlaufman & Casey,
demonstrating that the principal contaminants in their selection can be
efficiently removed by the photometric separation of dwarfs and giants. This
implies that any similarly efficient dwarf/giant discriminant (e.g., Gaia) can
be used with WISE colours to select samples of giant stars with high
completeness and low contamination. We employ our photometric selection to
identify three metal-poor giant candidates in the Kepler field with global
asteroseismic parameters and find that masses inferred for these three stars
using standard asteroseismic scaling relations are systematically
over-estimated by 20-175%. [abridged]
