The scattered disc of HR 8799

Abstract

HR 8799 is a young F0-type star with four directly imaged giant planets and two debris belts, one located exterior and another one interior to the region occupied by the planetary orbits. Having an architecture similar to that of our Solar System, but also revealing dissimilarities such as high masses of planets, a huge radial extent and a high mass of the outer debris belt, HR 8799 is considered to be a benchmark to test formation and evolution models of planetary systems. Here we focus on the outer debris ring and its relation to the planets. We demonstrate that the models of the outer disc, proposed previously to reproduce Herschel observations, are inconsistent with the ALMA data, and vice versa. In an attempt to find a physically motivated model that would agree with both observational sets, we perform collisional simulations. We show that a narrow planetesimal belt and a radiation pressure induced dust halo cannot account for the observed radial brightness profiles. A single, wide planetesimal disc does not reproduce the data either. Instead, we propose a two-population model, comprising a Kuiper-Belt-like structure of a low-eccentricity planetesimal population ("the classical Kuiper Belt") and a high-eccentricity population of comets ("scattered disc"). We argue that such a structure of the exo-Kuiper belt of HR 8799 could be explained with planet migration scenarios analogous to those proposed for the Kuiper Belt of the Solar System.

Dust spreading in debris discs: do small grains cling on to their birth environment?

“…We also note that the radially extended belts, such as those listed in Matrà et al (2018), can be viably attributed to the "scattered discs", similar to the scattered disc in our solar system (Wyatt et al 2017;Geiler et al 2019). Such a scattered disc could be composed of planetesimals in orbits with a narrow range of semimajor axes, but large eccentricities.…”

Spatially resolved spectroscopy of the debris disk HD 32297

“…The presence of millimeter grains in the halo complicates the understanding of the large bow, which is expected to be populated with loosely bound grains placed on very eccentric orbits by stellar radiation pressure (Strubbe & Chiang 2006;Thébault & Wu 2008) that should be sensitive to interactions with the ISM. We note however that there are some other alternatives and the ALMA halo may be explained as the presence of a scattered disk (Geiler et al 2019).…”

Extrasolar Kuiper belts

“…Indeed the favoured interpretation of the radial structure of the HR8799 disk is that it has a low eccentricity disk (i.e., a classical Kuiper belt analogue) as well as a high eccentricity population of comets in a scattered disk analogue (see Fig. 6, Geiler et al 2019), an interpretation strengthened by the fact that multiple planets interior to the belt are known in this system (Marois et al 2010).…”

“…(Right) Radial structure in the HR8799 disk (see Fig. 5) includes a halo of mm-sized grains that is best modelled with a high eccentricity population potentially analogous to the scattered disk (Geiler et al 2019).…”