21,864 Unresolved, Low-mass Binaries Identified via Their Overluminosity in Gaia Data Release 3 and a Catalog of 347,440 Systems within 100 pc of the Sun

Abstract The fundamental parameters of a low-mass star can potentially be determined from its photometry and astrometry. This is complicated by the fact that 10%–20% of low-mass stars are predicted to be equal-mass binaries. These unresolved systems appear more luminous compared to single stars with the same fundamental parameters. We present a method to differentiate binary stars from single-star main-sequence K and M dwarfs using their Gaia DR3 XP spectra. We assemble a training set of stars that have pristine astrometry and photometry, are located within 100 pc of the Sun, and exclude stars with Gaia DR3 flags suggesting they may be unequal mass systems, thereby leaving stars that are predominantly either single- or equal-mass binaries. We then iteratively train random forest regression models to predict absolute magnitude and color given the RP spectral coefficients of a star. After each model, we remove the stars that have absolute magnitudes significantly brighter than their predicted values. This method converges on a model trained only on single stars. We then use this model to identify the “overluminous” K and M stars in Gaia DR3 within 100 pc, with some quality cuts. We find that ∼13% of the sample is significantly overluminous and assume these to be unresolved binaries. We aggregate several multiplicity surveys across different projected separations and incorporate our overluminous binaries to create a general Catalog of Systems within 100 pc. We use this Catalog to provide lower limits on the multiplicity fraction for stars between 0.1 and 0.7 M ⊙ .