Abstract
Recent observations show that planet can reside in close binary systems with stellar separation of only about 20 AU. However, planet formation in such close binary systems is a challenge to current theory. One of the major theoretical problems occurs in the intermediate stage-planetesimals accretion into planetary embryos-during which the companion's perturbations can stir up the relative velocites(dV) of planetesimals and thus slow down or even cease their growth. However, all previous studies assumed a 2-dimentional (2D) disk and a coplanar binary orbit. Extending previous studies by including a 3D gas disk and an inclined binary orbit with small relative inclination of i_B=0.1-5 deg, we numerically investigate the conditions for planetesimal accretion at 1-2 AU, an extension of the habitable zone(1-1.3 AU), around alpha Centauri A in this paper. Inclusion of the binary inclination leads to: (1) differential orbital phasing is realized in the 3D space, and thus different-sized bodies are separated from each other; (2) total impact rate becomes lower, and impacts mainly occur between similar-sized bodies; (3) accretion is more favored, but the balance between accretion and erosion remains uncertain, and the "possible accretion region" extends up to 2AU when assuming an optimistic Q*(critical specific energy that leads to catastrophic fragmentation); and (4) impact velocities (dV) are significantly reduced but still much larger than their escape velocities, which infers that planetesimals grow by means of type II runaway mode. As a conclusion, inclusion of a small binary inclination is a promising mechanism that favors accretion, opening a possibility that planet formation in close binary systems can go through the difficult stage of planetesimals accretion into planetary embryos.
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