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Excess near-infrared emission is detected around one fifth of main-sequencestars, but its nature is a mystery. These excesses are interpreted as thermalemission from populations of small, hot dust very close to their stars (`hotexozodis'), but such grains should rapidly sublimate or be blown out of thesystem. To date, no model has fully explained this phenomenon. One mechanismcommonly suggested in the literature is cometary supply, where star-grazingcomets deposit dust close to the star, replenishing losses from grainsublimation and blowout. However, we show that this mechanism alone is veryunlikely to be responsible for hot exozodis. We model the trajectory and sizeevolution of dust grains released by star-grazing comets, to establish the dustand comet properties required to reproduce hot-exozodi observations. We findthat cometary supply alone can only reproduce observations if dust ejecta hasan extremely steep size distribution upon release, and the dust-deposition rateis extraordinarily high. These requirements strongly contradict our currentunderstanding of cometary dust and planetary systems. Cometary supply istherefore unlikely to be solely responsible for hot exozodis, so may need to becombined with some dust-trapping mechanism (such as gas or magnetic trapping)if it is to reproduce observations.

Hot exozodis: cometary supply without trapping is unlikely to be the mechanism