sMILES: A Library of Semi-Empirical MILES Stellar Spectra with Variable
  [$\alpha$/Fe] Abundances
  We present a new library of semi-empirical stellar spectra that is based on
the empirical MILES library. A new, high resolution library of theoretical
stellar spectra is generated that is specifically designed for use in stellar
population studies. We test these models across their full wavelength range
against other model libraries and find reasonable agreement in their
predictions of spectral changes due to atmospheric $\alpha$-element variations,
known as differential corrections. We also test the models against the MILES
and MaStar libraries of empirical stellar spectra and also find reasonable
agreements, as expected from previous work. We then use the abundance pattern
predictions of the new theoretical stellar spectra to differentially correct
MILES spectra to create semi-empirical MILES (sMILES) star spectra with
abundance patterns that differ from those present in the Milky Way. The final
result is 5 families of 801 sMILES stars with [$\alpha$/Fe] abundances ranging
from $-$0.20 to 0.60 dex at MILES resolution (FWHM=$2.5\,${\AA}) and wavelength
coverage ($3540.5-7409.6\,${\AA}). We make the sMILES library publicly
available.
