The unexpected narrowness of eccentric debris rings: a sign of eccentricity during the protoplanetary disc phase

Abstract

This paper shows that the eccentric debris rings seen around the stars Fomalhaut and HD 202628 are narrower than expected in the standard eccentric planet perturbation scenario (sometimes referred to as ‘pericentre glow’). The standard scenario posits an initially circular and narrow belt of planetesimals at semi-major axis a , whose eccentricity is increased to e f after the gas disc has dispersed by secular perturbations from an eccentric planet, resulting in a belt of width 2 ae f . In a minor modification of this scenario, narrower belts can arise if the planetesimals are initially eccentric, which could result from earlier planet perturbations during the gas-rich protoplanetary disc phase. However, a primordial eccentricity could alternatively be caused by instabilities that increase the disc eccentricity, without the need for any planets. Whether these scenarios produce detectable eccentric rings within protoplanetary discs is unclear, but they nevertheless predict that narrow eccentric planetesimal rings should exist before the gas in protoplanetary discs is dispersed. PDS 70 is noted as a system hosting an asymmetric protoplanetary disc that may be a progenitor of eccentric debris ring systems.

High-resolution ALMA and HST images of q1 Eri: an asymmetric debris disc with an eccentric Jupiter

“…A more complicated clump profile may better model this system with more success given that this distribution may be too simplistic to interpret the physical origin of the extra emission (i.e., this may be azimuthally extended around an orbit if the emission is due to a collision within the belt, which we consider in §5.2.2). On the other hand, whilst the disc models without a clump (either fully symmetrical or having an underlying constant eccentricity distribution) were poorer fits to the data, an eccentric model with both free and forced eccentricity terms, or one with a outwardly falling eccentricity (e.g., if due to the forced eccentricity of a planet sculpting the inner edge) may provide a better fit than the constant eccentricity modelled here (for example, similar to the models of MacGregor et al 2017;Kennedy 2020). We suggest future work may wish to conduct detailed modelling of this scenario to explore the origin of the asymmetric emission of this debris disc, which we further discuss in §5.2.…”

Fomalhaut b could be massive and sculpting the narrow, eccentric debris disc, if in mean-motion resonance with it

“…An alternative hypothesis is that Fomalhaut b does not have significant mass, but is in fact a dispersing dust cloud (Janson et al 2012;Galicher et al 2013;Lawler, Greenstreet & Gladman 2015;Janson et al 2020); in this case, the disc eccentricity would have to be driven by a different mechanism (e.g. Shannon, Clarke & Wyatt 2014;Faramaz et al 2015;Kaib, White & Izidoro 2018;Kennedy 2020). We have shown that Fomalhaut b is not precluded from having planetary mass by dynamical arguments, so a distinction between the planet and dust cloud hypotheses must come from observations.…”

ALMA's view of the M-dwarf GSC 07396-00759's edge-on debris disc: AU Mic's coeval twin

“…All subsequent reported offsets have had that Gaia to phase-centre offset subtracted such that the offset measurements are relative to the Gaia location at the time of observation. We also include a parameter for scaling the weightings of the u-v data points as their absolute uncertainty can be offset as described in Matrà et al (2019b) and Kennedy (2020).…”