Planet formation in highly inclined binaries

¹ Dipartimento di Fisica, University of Padova, via Marzolo 8, 35131 Padova, Italy e-mail: marzari@pd.infn.it
² LESIA, Observatoire de Paris, Section de Meudon, 92195 Meudon Principal Cedex, France e-mail: philippe.thebault@obspm.fr
³ Laboratoire Cassiopée, Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex, France e-mail: Hans.Scholl@oca.eu

Received: 23 April 2009
Accepted: 20 July 2009

Abstract

Aims. We explore planet formation in binary systems around the central star where the protoplanetary disk plane is highly inclined with respect to the companion star orbit. This might be the most frequent scenario for binary separations larger than 40 AU. We focus on planetesimal accretion and compute average impact velocities in the habitable region and up to 6 AU from the primary.

Methods. Planetesimal trajectories are computed within the frame of the restricted 3-body problem determined by the central star, the companion star and massless planetesimals. Relative velocities are computed and interpreted in terms of accreting or eroding impacts.

Results. We first show that, for binary inclinations higher than 10 degrees, planetesimals evolve, to a first approximation, in a gas-free environment. Planetesimal accretion is confined around the central star in a region determined by two main parameters, firstly by the mutual inclination between the binary plane and the disk, and, secondly, by the binary eccentricity.

Conclusions. The onset of large mutual inclinations between planetesimals due to the nodal randomization causes an increase in the relative velocity. The chances for a successful planet accumulation process depend on the balance between the timescale for node randomization and that of planetesimal accretion. When the binary semimajor axis is larger than 70 AU, planet formation appears possible even for eccentric binaries (up to 0.4). For lower binary separations the region where planetesimals accumulate into protoplanets shrinks consistently. When the mutual inclination between the binary plane and that of the planetesimal disk is larger than , the Kozai mechanism strongly inhibits planetesimal accumulation.