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Thébault Philippe, Marzari Francesco, Scholl Hans

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

Monthly notices of the Royal Astronomical Society, 2008, vol. 388, pp. 1528-1536

Référence DOI : 10.1111/j.1365-2966.2008.13536.x
Référence ADS : 2008MNRAS.388.1528T

Résumé :

We numerically explore planet formation around alpha Centauri A by focusing on the crucial planetesimals-to-embryos phase. Our approach is significantly improved with respect to the earlier work of Marzari & Scholl, since our deterministic N-body code computing the relative velocities between test planetesimals handles bodies with different size. Due to this step-up, we can derive the accretion versus fragmentation trend of a planetesimal population having any given size distribution. This is a critical aspect of planet formation in binaries since the pericenter alignment of planetesimal orbits due to the gravitational perturbations of the companion star and to gas friction strongly depends on size. Contrary to Marzari & Scholl, we find that, for the nominal case of a Minimum-Mass Solar Nebula gas disc, the region beyond ~0.5au from the primary is strongly hostile to planetesimal accretion. In this area, impact velocities between different-sized 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 `accretion-safe' area never extends beyond 0.75au. We conclude that planet formation is very difficult in the terrestrial region around alpha 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 1au 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 differ from those of several studies focusing on the later embryos-to-planets stage, confirming that the planetesimals-to-embryos phase is more affected by binary perturbations.

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