Quantifying the Contamination from nearby Stellar Companions in Gaia DR3 Photometry

Identifying and removing binary stars from stellar samples is a crucial but complicated task. Regardless of how carefully a sample is selected, some binaries will remain and complicate interpretation of results, especially via flux contamination of survey photometry. One such sample is the data from the Gaia spacecraft, which is collecting photometry and astrometry of more than 10⁹ stars. To quantify the impact of binaries on Gaia photometry, we assembled a sample of known binary stars observed with adaptive optics and with accurately measured parameters, which we used to predict Gaia photometry for each stellar component. We compared the predicted photometry to the actual Gaia photometry for each system, and found that the contamination of Gaia photometry because of multiplicity decreases nonlinearly from near-complete contamination (ρ ≤ ) to no contamination (binary projected separation, or ρ > ). We provide an analytic relation to analytically correct photometric bias in a sample of Gaia stars using the binary separation. This correction is necessary because the Gaia PSF photometry extraction does not fully remove the secondary star flux for binaries with separations with ρ ≲ . We also evaluated the utility of various Gaia quality-of-fit metrics for identifying binary stars and found that Renormalized Unit Weight Error (RUWE) remains the best indicator for unresolved binaries, but multipeak image fraction probes a separation regime not currently accessible to RUWE.