Dynamics of the β Pictoris planetary system and its falling evaporating bodies

Abstract

Context. For decades, the spectral variations of β Pictoris have been modelled as the result of the evaporation of exocomets close to the star, termed falling evaporating bodies (FEBs). Resonant perturbations by a hypothetical giant planet have been proposed to explain the dynamical origin of these stargrazers. The disk is now known to harbour two giant planets, β Pic b and c, orbiting the star at 9.9 and 2.7 au. While the former almost matches the planet formerly suspected, the recent discovery of the latter complicates the picture. Aims. We first question the stability of the two-planet system. Then we investigate the dynamics of a disk of planetesimals orbiting the star together with both planets to check the validity of the FEB generation mechanism. Methods. Symplectic N-body simulations are used to first determine which regions of the planetesimal disk are dynamically stable and which are not. Then we focus on regions where disk particles are able to reach high eccentricities, mainly thanks to resonant mechanisms. Results. The first result is that the system is dynamically stable. Both planets may temporarily fall in 7:1 mean-motion resonance (MMR). Then, simulations with a disk of particles reveal that the whole region extending between ~l.5 au and ~25 au is unstable to planetary perturbations. However, a disk below 1.5 au survives, which appears to constitute an active source of FEBs via high-order MMRs with β Pic c. In this new picture, β Pic b acts as a distant perturber that helps sustain the whole process. Conclusions. Our new simulations rule out the preceding FEB generation mechanism model, which placed their origin at around 4–5 au. Conversely, FEBs are likely to originate from a region much further in and related to MMRs with β Pic c. That mechanism also appears to last longer, as new planetesimals are able to continuously enter the MMRs and evolve towards the FEB state. Subsequently, the physical nature of the FEBs may differ from that previously thought, and presumably may not be icy.