The Extended IRTF Spectral Library: Expanded coverage in metallicity,
  temperature, and surface gravity
  We present a $0.7-2.5\mu m$ spectral library of 284 stars observed with the
medium-resolution infrared spectrograph, SpeX, at the 3.0 meter NASA Infrared
Telescope Facility (IRTF) on Maunakea, Hawaii. This library extends the
metallicity range of the IRTF Cool Star library beyond solar metallicity to
$-1.7 <$ [Fe/H] $< 0.6$. All of the observed stars are also in the MILES
optical stellar library, providing continuous spectral coverage for each star
from $0.35-2.5\mu m$. The spectra are absolute flux calibrated using Two Micron
All Sky Survey photometry and the continuum shape of the spectra is preserved
during the data reduction process. Synthesized $JHK_S$ colors agree with
observed colors at the $1-2\%$ level, on average. We also present a spectral
interpolator that uses the library to create a data-driven model of spectra as
a function of $teff$, $logg$, and [Fe/H]. We use the library and interpolator
to compare empirical trends with theoretical predictions of spectral feature
behavior as a function of stellar parameters. These comparisons extend to the
previously difficult to access low-metallicity and cool dwarf regimes, as well
as the previously poorly sampled super-solar metallicity regime. The library
and interpolator are publicly available.
