Component masses in stellar and sub-stellar binaries from Gaia astrometry and photometry

The masses of stars and planets can be measured dynamically in binary systems. For an unresolved binary, time series astrometry reveals the orbital parameters, but it cannot provide the component masses, because we observe only the motion of the system's photocentre. However, as a star's luminosity is related to its mass, the observable photometry of both components together provides information on the system's mass. Here we describe a method to determine the individual component masses of an unresolved binary using the astrometric orbit together with three-band photometry from Gaia. We used a mass-flux relation fitted from stellar isochrone models for each Gaia band to infer the unknown flux ratio. This enables our method to identify near-equal-mass stellar binaries, which are expected to be the dominant source of false-positive exoplanet candidates, without the need for additional follow-up. Using a likelihood approach, we sampled the posterior probability distribution over the stellar parameters, marginalising over system age and metallicity. We applied this to 20 000 systems with a main-sequence primary within 300 pc of the Sun using data from the Gaia Data Release 3 non-single star catalogue. Primary masses can be determined with a precision (1σ posterior width) of 10–20% in 90% of cases. Secondary masses, which extend down to planetary-mass objects, are less precise, although half are more than 25% precise. Interestingly, adding either infrared photometry or spectroscopic orbits from Gaia does not change the mass estimates significantly (less than 4% and 1%, respectively). Interstellar extinction likewise has little impact for this sample. We provide a catalogue of our mass estimates. This work shows that reasonably precise masses can be obtained for many stars and sub-stellar objects using just the Gaia astrometry and photometry.