Increasing planet-stirring efficiency of debris discs by 'projectile stirring' and 'resonant stirring'

Extrasolar debris discs are detected by observing dust, which is thought to be released during planetesimal collisions. This implies that planetesimals are dynamically excited ('stirred'), such that collisions are sufficiently common and violent. The most frequently considered stirring mechanisms are self-stirring by disc self-gravity, and planet-stirring via secular interactions. However, these models face problems when considering disc mass, self-gravity, and planet eccentricity, leading to the possibility that other, unexplored mechanisms instead stir debris. We hypothesize that planet-stirring could be more efficient than the traditional secular model implies, due to two additional mechanisms. First, a planet at the inner edge of a debris disc can scatter massive bodies on to eccentric, disc-crossing orbits, which then excite debris ('projectile stirring'). Second, a planet can stir debris over a wide region via broad mean-motion resonances, both at and between nominal resonance locations ('resonant stirring'). Both mechanisms can be effective even for low-eccentricity planets, unlike secular-planet-stirring. We run N-body simulations across a broad parameter space, to determine the viability of these new stirring mechanisms. We quantify stirring levels using a bespoke program for assessing REBOUND debris simulations, which we make publicly available. We find that even low-mass projectiles can stir discs, and verify this with a simple analytic criterion. We also show that resonant stirring is effective for planets above similar to 0.5 M-Jup. By proving that these mechanisms can increase planet-stirring efficiency, we demonstrate that planets could still be stirring debris discs even in cases where conventional (secular) planet-stirring is insufficient.