Dynamics and Planet Formation in/Around Binaries

Abstract

The extent to which planetesimal accretion is affected by the perturbing presence of a companion star is an important issue in the formation of planets in and around binary systems. In this chapter, we review this issue by concentrating on one crucial parameter: the distribution of encounter velocities within the planetesimal swarm. The evolution of this parameter is numerically explored accounting for the secular perturbations of the binary and the friction due to the very likely presence of gas in the disk. Maps of the average encounter velocity ⟨Δv⟩ between different size planetesimals are presented for a total of 120 stellar dynamical configurations obtained by different combinations of the binary semimajor axis a b and eccentricity e b . According to the different values of ⟨Δv⟩, 3 different planetesimal accumulation modes are identified: 1) in regions where ⟨Δv⟩ is comparable to that derived for planetesimal swarms around single-stars, “standard” accretion is likely, eventually via runaway growth, 2) in regions where ⟨Δv⟩ is larger than v ero , the threshold velocity above which all impacts are eroding, no accretion is possible and planet growth is stopped, 3) in between these two extremes, there is a large fraction of binary configurations where the increase in ⟨Δv⟩ is still below the erosion threshold. Planetesimal accumulation can still occur but it possibly proceeds at a slower rate than in the single-star case, following the so-called type II runaway growth mode.