Debris Disc Substructures Induced by Secular Planetary Perturbations

Abstract

Observations of debris discs have the potential to provide us with valuable information about massive planets perturbing them gravitationally. In this work, we explore the evolution of the azimuthally-averaged (or axisymmetric) surface density (ASD) -- a characteristic routinely derived from observations -- in a disc secularly perturbed by an inner planet. We develop detailed analytical understanding of ASD evolution and verify it using a novel numerical framework DebrisPy, which we make publicly available. With these tools we show that in a secularly evolving disc ASD develops a set of sharp features -- weakly discontinuous peaks at eccentricity nulls and sharp discontinuities at caustic points where particle periastra or apoastra pile up -- marching out through the disc as it ages. We probe the dependence of these features on planetary eccentricity, ratio of the free to forced particle eccentricity, and underlying radial mass distribution, showing in particular that more eccentric planets produce more prominent ASD features. Convolution with the PSF of realistic observations (as well as the non-zero random free eccentricity of debris) smooths out these features, but they can still be detectable in high-resolution observations. Radial locations of secular ASD peaks follow a particular, well-defined pattern, which should unambiguously point to their secular nature in observations. We illustrate how both detection and non-detection of such secular features in observed discs can be used (in combination with other constraints) to measure or constrain key parameters of perturbing planets (even those not yet detected) -- mass, semi-major axis and eccentricity.