Debris discs in binaries: morphology and photometric signatures^★

¹ LESIA-Observatoire de Paris, UPMC Univ. Paris 06, Univ. Paris-Diderot, France
e-mail: philippe.thebault@obspm.fr
² Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
³ Núcleo Milenio Formación Planetaria - NPF, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile

Received: 2 October 2020
Accepted: 18 December 2020

Abstract

Context. Since about half of all main-sequence stars reside in multiple star systems, it is important to consider the effect of binarity on the evolution of planetesimal belts in these complex systems.

Aims. We aim to see whether debris belts evolving between two stars may be impacted by the presence of the companion and whether this leaves any detectable signature that could be observed with current or future instruments.

Methods. We consider a circumprimary parent body (PB) planetesimal belt that is placed just inside the stability limit between the two stars and we use the state-of-the-art DyCoSS code to follow the coupled dynamical and collisional evolution of the dust produced by this PB belt. We explore several free parameters, such as the belt’s mass and the binary’s mass ratio as well as its orbital eccentricity. We use the GraTeR package to produce 2D luminosity maps and system-integrated spectral energy distributions (SEDs).

Results. We confirm a preliminary result obtained by previous DyCoSS studies, which is that the coupled effect of collisional activity, binary perturbations, and stellar radiation pressure is able to place and maintain a halo of small grains in the dynamically unstable region between the two stars. In addition, we identify several prominent spatial structures, notably, a single spiral arm stretching all the way from the PB belt to the companion star. We also identify a fainter and more compact disc around the secondary star, which is non-native and feeds off small grains from the unstable halo. The halo, spiral arm, and secondary disc should all be detectable on resolved images by instruments with capacities on the level of SPHERE. The system as a whole is depleted of small grains when compared to a companion-free case. This depletion leaves an imprint on the system’s integrated SED, which appears colder than for the same parent body belt around a single star. This new finding could explain why the SED-derived location, r_disc, of some unresolved discs-in-binaries places their primary belt in the dynamically ’forbidden’ region between the two stars: indeed, this apparent paradox could be due to an overestimation of r_disc when using empirical prescriptions that are valid for the case of a single star.