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 singlestar 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, modelling spectra similar to those collected by the APOGEE, GALAHandLAMOSTsurveys. We find that fitting unresolved binarieswith singlestar models introduces systematic biases in the derived stellar parameters and abundances that are modest but certainly not negligible, with typical systematic errors of 300K in T_eff, 0.1dex in log g and 0.1 dex in [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.