Eccentricity of radiative disks in close binary-star systems

¹ Dipartimento di Fisica, University of Padova, via Marzolo 8, 35131 Padova, Italy
e-mail: francesco.marzari@pd.infn.it
² DAMTP, University of Cambridge, Wilberforce Road, Cambridge CB30 WA, UK
³ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁴ Laboratoire Cassiopée, Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex, France
⁵ Observatoire de Paris, Section de Meudon, 92195 Meudon Principal Cedex, France

Received: 12 September 2011
Accepted: 10 January 2012

Abstract

Context. Disks in binaries have a complex behavior because of the perturbations of the companion star. Planetesimal growth and planet formation in binary-star systems both depend on the companion star parameters and the properties of the circumstellar disk. An eccentric disk may significantly increase the impact velocity of planetesimals and therefore jeopardize the accumulation process.

Aims. We model the evolution of disks in close binaries including the effects of self-gravity and adopting different prescriptions to model the disk radiative properties. We focus on the dynamical properties and evolutionary tracks of the disks.

Methods. We use the hydrodynamical code FARGO and include in its energy equation both heating and cooling effects.

Results. Radiative disks have a lower disk eccentricity than locally isothermal disks with the same temperature profile. Their average eccentricity is about 0.05, and is almost independent of the eccentricity of the binary orbit, in contrast to locally isothermal disk models. As a consequence, we do not observe the formation of an internal elliptical low density region as in locally isothermal disk models. However, the disk eccentricity depends on the disk mass in terms of the opacities. Akin to locally isothermal disk models, self-gravity forces the disk’s longitude of pericenter to librate about a fixed orientation with respect to the binary apsidal line (π).

Conclusions. The disk radiative properties play an important role in the evolution of disks in binaries. A radiative disk has an overall shape and internal structure that differ significantly from those of a locally isothermal disk with a similar temperature profile. This is an important finding for both describing the evolutionary track of the disk during its progressive mass loss, and for planet formation because the internal structure of the disk is relevant to planetesimal growth in binary systems. The non-symmetrical distribution of mass in these disks causes high eccentricities for planetesimals, whose growth may be affected.