Vega's hot dust from icy planetesimals scattered inwards by an outward-migrating planetary system

Vega has been shown to host multiple dust populations, including both hot exozodiacal dust at sub-au radii and a cold debris disc extending beyond 100 au. We use dynamical simulations to show how Vega's hot dust can be created by long-range gravitational scattering of planetesimals from its cold outer regions. Planetesimals are scattered progressively inwards by a system of 5-7 planets from 30 to 60 au to very close-in. In successful simulations, the outermost planets are typically Neptune mass. The back-reaction of planetesimal scattering causes these planets to migrate outwards and continually interact with fresh planetesimals, replenishing the source of scattered bodies. The most favourable cases for producing Vega's exozodi have negative radial mass gradients, with sub-Saturn-to Jupiter-mass inner planets at 5-10 au and outer planets of 2.5 - 20M(circle plus). The mechanism fails if a Jupiter-sized planet exists beyond similar to 15 au because the planet preferentially ejects planetesimals before they can reach the inner system. Direct-imaging planet searches can therefore directly test this mechanism.