Dust Growth in Binary Systems: Inhibition of dust settling and growth in circumbinary discs

Abstract

Stellar multiplicity alters the density structure of protoplanetary discs and thereby the initial conditions for planet formation. Yet, the interplay between companion-disc interactions and dust growth remains poorly understood. The goal of this work is to investigate to what extent the density structure of a disc undergoing tidal interactions with a companion star promotes or inhibits the growth of dust grains. We perform a set of hydrodynamical simulations of protoplanetary discs orbiting one or both stars of a binary, including dust growth and fragmentation. We explore a range of companion orbits and compare the results with a single-star reference case. We find that dust growth is mainly driven by local accumulations of dust. In circumbinary discs, the maximum grain size is up to five times smaller than in isolated discs. This result likely originates from the perturbations caused by the inner binary, which prevent dust grains from properly settling and drifting. As a consequence, the conditions required to trigger strong clumping driven by the streaming instability are difficult to achieve. In contrast, circumstellar discs in binary systems exhibit grain sizes similar to those in isolated discs, leading to comparable conditions for strong clumping by the streaming instability. Planet formation through core accretion seems challenging in circumbinary discs harbouring binaries larger than a few au, suggesting that circumbinary planets observed near the dynamical stability limit did not form in situ. Conversely, perturbations from external companions only marginally affect density-driven dust growth compared to isolated systems.