A Very Metal-poor RR Lyrae Star with a Disk Orbit Found in the Solar
  Neighborhood
  Metal-deficient stars are important tracers for understanding the early
formation of the Galaxy. Recent large-scale surveys with both photometric and
spectroscopic data have reported an increasing number of metal-deficient stars
whose kinematic features are consistent with those of the disk stellar
populations. We report the discovery of an RR~Lyrae variable (hereafter RRL)
that is located within the thick disk and has an orbit consistent with the
thick-disk kinematics. Our target RRL (HD 331986) is located at around 1 kpc
from the Sun and, with V=11.3, is among the 130 brightest RRLs known so far.
However, this object was scarcely studied because it is in the midplane of the
Galaxy, the Galactic latitude around -1 deg. Its near-infrared spectrum
(0.91-1.32 micron) shows no absorption line except hydrogen lines of the
Paschen series, suggesting [Fe/H] less than -2.5. It is the most
metal-deficient RRL, at least, among the RRLs whose orbits are consistent with
the disk kinematics, although we cannot determine to which of the disk and the
halo it belongs. This unique RRL would provide us with essential clues for
studying the early formation of stars in the inner Galaxy with further
investigations, including high-resolution optical spectroscopy.
