Double Trouble: How Unresolved Spectroscopic Binary Stars Skew GALAH DR4 Stellar Parameters

Unresolved binary stars introduce systematic biases in stellar parameter estimates when analysed assuming a single star. Using about 917, 000 spectra from the Galactic Archaeology with HERMES (GALAH) Data Release 4 (DR4), we develop a binary spectral model that fits each observed spectrum as the superposition of two synthetic spectra generated from the same model architecture used by GALAH DR4. These generative spectral models take as input: effective temperature (T eff), surface gravity (g), iron abundance ([Fe/H]), projected rotation velocity (v i), microturbulence velocity (v mic), and radial velocity (v rad). In our binary model, T eff and g are not fitted directly but are instead interpolated from PARSEC isochrones based on stellar mass and age, under the assumption that both components formed contemporaneously with the same compositions. We fit for the primary and secondary masses, a common age and [Fe/H], as well as individual v i, v mic, and v rad for each component. After iterative optimization and conservative quality cuts to ensure robust detections, we identify about 12, 911 candidate binary systems. We quantify typical biases introduced by unresolved binarity: T eff shifts of-142 to +200 K, g shifts of-0.070 to +0.35 dex, and [Fe/H] shifts of-0.055 to +0.13 dex. Despite our conservative approach reducing the number of detections, we recover known trends of increasing binary fraction with higher primary mass and lower metallicity, thus reflecting the broad trends of binary populations. Accounting for unresolved binaries is essential for accurate stellar population modelling and for interpreting spectroscopic surveys--particularly in low-metallicity systems or populations with more massive stars.