Planet formation in binary star systems is a complex issue due to thegravitational perturbations of the companion star. One of the crucialsteps of the core-accretion model is planetesimal accretion into large protoplanets which finally coalesce into planets.In a planetesimal swarm surrounding theprimary star, the average mutual impact velocitydetermines if larger bodies form or if thepopulation is grinded down to dust, halting the planetformation process. This velocity is stronglyinfluenced by the companion gravitational pull and by gas drag.The combined effect of these two forces may act in favour of or against planet formation, setting a lower or equal probability ofthe existence of extrasolar planets around single or binary stars. Planetesimal accretion in binaries has been studied so far withtwo different approaches. N-body codes based on the assumptionthat the disk is axisymmetric are very cost-effective sincethey allow the study of the mutual relative velocity with limitedCPU usage. A large amount of planetesimal trajectories can becomputed making it possible to outline the regions around the star whereplanet formation is possible. The main limitation of the N-body codes is the axisymmetric assumption.The companion perturbations affect not only the planetesimal orbits,but also the gaseous disk, by forcing spiral density waves. In addition, the overallshape of the disk changes from circular to elliptic. Hybrid codes have been recently developed which solvethe equations for the diskwith a hydrodynamical grid codeand use the computed gas densityand velocity vector to calculate an accurate value of the gas dragforce on the planetesimals. These codes are more complex andmay compute the trajectories of only a limited number of planetesimals.
Planet formation: is it good or bad to have a stellar companion?
MARZARI, FRANCESCO;
2010
Abstract
Planet formation in binary star systems is a complex issue due to thegravitational perturbations of the companion star. One of the crucialsteps of the core-accretion model is planetesimal accretion into large protoplanets which finally coalesce into planets.In a planetesimal swarm surrounding theprimary star, the average mutual impact velocitydetermines if larger bodies form or if thepopulation is grinded down to dust, halting the planetformation process. This velocity is stronglyinfluenced by the companion gravitational pull and by gas drag.The combined effect of these two forces may act in favour of or against planet formation, setting a lower or equal probability ofthe existence of extrasolar planets around single or binary stars. Planetesimal accretion in binaries has been studied so far withtwo different approaches. N-body codes based on the assumptionthat the disk is axisymmetric are very cost-effective sincethey allow the study of the mutual relative velocity with limitedCPU usage. A large amount of planetesimal trajectories can becomputed making it possible to outline the regions around the star whereplanet formation is possible. The main limitation of the N-body codes is the axisymmetric assumption.The companion perturbations affect not only the planetesimal orbits,but also the gaseous disk, by forcing spiral density waves. In addition, the overallshape of the disk changes from circular to elliptic. Hybrid codes have been recently developed which solvethe equations for the diskwith a hydrodynamical grid codeand use the computed gas densityand velocity vector to calculate an accurate value of the gas dragforce on the planetesimals. These codes are more complex andmay compute the trajectories of only a limited number of planetesimals.Pubblicazioni consigliate
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