Unveiling the dynamics and physical property of asteroid systems based on Gaia astrometric data

Gaia is a space mission from the European Space Agency (ESA) that was launched in 2013. The mission opens a window to explore the unprecedented high precision astrometric data for a large population of solar system objects. Its latest data release – the Focused Product Release (Gaia FPR) published in October 2023 – contains 66 months of data, for about 160,000 asteroids. By covering a main-belt asteroid’s typical orbital period, it has been shown that the Gaia data alone can provide very precise heliocentric orbits [1]. Thanks to this unprecedented precision, Gaia data is able to reveal the astrometric signature of binary asteroids. This is the case for the recently discovered binary (4337) Arecibo system, analyzed with the Gaia DR3 data [2]; where an astrometric wobble was clearly detected in a time window of several days covering successive transits. In this study, we continue the research on the Arecibo system, taking into account all the Gaia FPR observations. We begin by fitting the heliocentric orbit. These residuals contain the binary signal, which is proportional to the relative orbit with a scaling factor related to the flux ratio and the mass ratio of the components (see [3], [4] for the analytical formula). We then fit the relative orbit to derive the relevant parameters. We obtain an estimate of the component masses, their density ratio, and flux ratio. With an  estimation of the volume, a bulk density of ρ1 ≈ 1.2 and ρ2 ≈ 1.6, for the primary and secondary, is derived. The results are consistent with an ice-rich body in the outer main belt. The high accuracy of Gaia's astrometric solutions enables us, for the first time, to estimate the individual masses and therefore the density of each component of a Small Solar System binary, which generally offer valuable insights into the formation of the Solar System, as well as its collisions and dynamic evolution. Moreover, with the orbital parameters, we are able to predict future mutual events and stellar occultations that will provide additional constraints on the individual density.  [1] Gaia Collaboration, David, P., Mignard, F., et al. 2023, A&A, 680, A37[2] Tanga, P., Pauwels, T., Mignard, F., et al. 2023, A&A, 674, A12[3] Pravec, P. & Scheirich, P. 2012, Planet. Space Sci., 73, 56[4] Lindegren, L. 2022