Planet formation in α Centauri A revisited: not so accretion friendly after all

Abstract

We numerically explore planet formation aroundα Centauri A by focusing on the crucial planetesimals-to-embryos phase. Our approach is significa ntly improved with respect to the earlier work of Marzari & Scholl (2000), since our determini stic N-body code computing the relative velocities between test planetesimals handles bo dies with different size. Due to this step up, we can derive the accretion vs. fragmentation trend of a planetesimal population having any given size distribution. This is a critical aspec t of planet formation in binaries since the pericenter alignment of planetesimal orbits due t o the gravitational perturbations of the companion star and to gas friction strongly depends on size. Contrary to Marzari & Scholl (2000), we find that, for the nominal case of a Minimum Mass Sol ar Nebula gas disc, the region beyond∼ 0.5 AU from the primary is strongly hostile to planetesimal accretion. In this area, impact velocities between different-size bodies are increased, by the differential orbital phasing, to values too high to allow mutual accretion. For any realistic size distribution for the planetesimal population, this accretion-inhibiting effect is the dominant collision outcome and the accretion process is halted. Results are relatively robust with respect to the profile and density of the gas disc. Except for an unrealistic almost gas -free case, the inner ”accretionsafe” area never extends beyond 0.75AU. We conclude that planet formation is very diffi cult in the terrestrial region around α Centauri A, unless it started from fast-formed very large (>30km) planetesimals. Notwithstanding these unlikely initial conditions, the only possible explanation for the presence of planets around 1 AU from the star would be the hypothetical outward migration of planets formed closer to the star or a different orbital configuration in the binary’s early history. Our conclusions di ffer from those of several studies focusing on the later embryos-to-planets stage, confirming that the planet esimals-to-embryos phase is more affected by binary perturbations.