A PR drag origin for the Fomalhaut disc’s pervasive inner dust: constraints on collisional strengths, icy composition, and embedded planets

ABSTRACT Recent JWST observations of the Fomalhaut debris disc have revealed a significant abundance of dust interior to the outer planetesimal belt, raising questions about its origin and maintenance. In this study, we apply an analytical model to the Fomalhaut system, which simulates the dust distribution interior to a planetesimal belt, as collisional fragments across a range of sizes are dragged inward under Poynting–Robertson (PR) drag. We generate spectral energy distributions and synthetic JWST/MIRI images of the model discs, and perform an extensive grid search for particle parameters – pertaining to composition and collisional strength – that best match the observations. We find that a sound fit can be found for particle properties that involve a substantial water ice component, around 50–80 per cent by total volume, and a catastrophic disruption threshold, $Q_\mathrm{D}^\star$, at a particle size of $D\!\approx \!30\, \mu {\rm m}$ of (2–4)$\, \times 10^6\, {\rm erg\, g}^{-1}$. Based on the expected dynamical depletion of migrating dust by an intervening planet, we discount planets with masses $>1\, M_{\rm Saturn}$ beyond $\sim$50 au in the extended disc, though a planet shepherding the inner edge of the outer belt of up to $\sim 2\, M_{\rm Saturn}$ is reconcilable with the PR-drag-maintained disc scenario, contingent upon higher collisional strengths. These results indicate that PR drag transport from the outer belt alone can account for the high interior dust contents seen in the Fomalhaut system, which may thus constitute a common phenomenon in other belt-bearing systems. This establishes a framework for interpreting mid-planetary system dust around other stars, with our results for Fomalhaut providing a valuable calibration of the models.