Gas and dust evolution in the circumprimary disk in binary stars

In recent years, the discovery of numerous exoplanets in multiple stellar systems has made binaries one of the main topics in planet formation research. Despite detecting approximately a dozen planets orbiting one star in close binary systems in an S-type orbit, the standard model of planet formation is unlikely to explain their formation. Investigating the evolution of disks in close binaries can provide insight into the potential mechanisms involved in the formation of planets in such systems. In this work, we aim to investigate how the presence of a secondary star in a binary system affects the dynamics of the gas and dust that surrounds the primary star. We use the γ Cephei binary star system as a reference model to simulate and study these effects. We perform two-dimensional hydrodynamical simulations using a modified version of the FARGO3D code to model disks in close binaries. In particular, we study the evolution of gas and dust particles of different sizes (100 µm, 1mm, and 1cm) over several thousand years (up to 108 kyr), specifically when the secondary star is at less perturbing configurations (the apocenter) and highly perturbing configurations (the pericenter). Our simulation confirms that when the secondary star passes through the pericenter, these perturbations result in the formation of two strong spiral arms within the disk structure. As it reaches the apocenter, the tidal forces diminish, and the disk transitions towards axisymmetric structures, damping the spiral arms. Additionally, we identified over-dense regions within the spiral arms formed during the pericenter passage, where there could be accelerated dust growth via coagulation leading to the formation of larger bodies like pebbles and planetesimals. Furthermore, we compute the mass accretion rate onto the primary star when the secondary star passes through the pericenter and when it reaches the apocenter. We found that viscous mass accretion dominates at the apocenter, leading to mass transfer onto the primary star. While at pericenter, mass is dragged away by the spiral waves. However, further detailed simulations and observations are necessary to fully comprehend the impact of these effects on circumprimary disks and planet formation in close binaries.