Eccentric debris disc morphologies I: exploring the origin of apocentre and pericentre glows in face-on debris discs

TL;DR

This paper investigates how the surface brightness features of eccentric debris discs depend on their eccentricity profiles and observational wavelengths, challenging traditional assumptions about dust distribution and glow effects.

Contribution

It provides theoretical and modeling insights into the dependence of debris disc brightness morphology on eccentricity profiles and wavelength, clarifying misconceptions about apocentre and pericentre glow.

Findings

Surface brightness maxima depend on eccentricity profile and wavelength.

At short wavelengths, pericentre glow is dominant; at long wavelengths, either glow can appear.

Resolution affects the observed glow, complicating traditional interpretations.

Abstract

The location of surface brightness maxima (e.g. apocentre and pericentre glow) in eccentric debris discs are often used to infer the underlying orbits of the dust and planetesimals that comprise the disc. However, there is a misconception that eccentric discs have higher surface densities at apocentre and thus necessarily exhibit apocentre glow at long wavelengths. This arises from the expectation that the slower velocities at apocentre lead to a "pile up'" of dust, which fails to account for the greater area over which dust is spread at apocentre. Instead we show with theory and by modelling three different regimes that the morphology and surface brightness distributions of face-on debris discs are strongly dependent on their eccentricity profile (i.e. whether this is constant, rising or falling with distance). We demonstrate that at shorter wavelengths the classical pericentre glow effect remains true, whereas at longer wavelengths discs can either demonstrate apocentre glow or pericentre glow. We additionally show that at long wavelengths the same disc morphology can produce either apocentre glow or pericentre glow depending on the observational resolution. Finally, we show that the classical approach of interpreting eccentric debris discs using line densities is only valid under an extremely limited set of circumstances, which are unlikely to be met as debris disc observations become increasingly better resolved.