Signatures of unresolved binaries in stellar spectra: implications for
  spectral fitting
  The observable spectrum of an unresolved binary star system is a
superposition of two single-star spectra. Even without a detectable velocity
offset between the two stellar components, the combined spectrum of a binary
system is in general different from that of either component, and fitting it
with single-star models may yield inaccurate stellar parameters and abundances.
We perform simple experiments with synthetic spectra to investigate the effect
of unresolved main-sequence binaries on spectral fitting, modeling spectra
similar to those collected by the APOGEE, GALAH, and LAMOST surveys. We find
that fitting unresolved binaries with single-star models introduces systematic
biases in the derived stellar parameters and abundances that are modest but
certainly not negligible, with typical systematic errors of $300\,\rm K$ in
$T_{\rm eff}$, 0.1 dex in $\log g$, and 0.1 dex in $[\rm Fe/H]$ for APOGEE-like
spectra of solar-type stars. These biases are smaller for spectra at optical
wavelengths than in the near-infrared. We show that biases can be corrected by
fitting spectra with a binary model, which adds only two labels to the fit and
includes single-star models as a special case. Our model provides a promising
new method to constrain the Galactic binary population, including systems with
single-epoch spectra and no detectable velocity offset between the two stars.
