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

Abstract

Excess near-infrared emission is detected around one fifth of main-sequence stars, but its nature is a mystery. These excesses are interpreted as thermal emission from populations of small, hot dust very close to their stars (‘hot exozodis’), but such grains should rapidly sublimate or be blown out of the system. To date, no model has fully explained this phenomenon. One mechanism commonly suggested in the literature is cometary supply, where star-grazing comets deposit dust close to the star, replenishing losses from grain sublimation and blowout. However, we show that this mechanism alone is very unlikely to be responsible for hot exozodis. We model the trajectory and size evolution of dust grains released by star-grazing comets, to establish the dust and comet properties required to reproduce hot-exozodi observations. We find that cometary supply alone can only reproduce observations if dust ejecta has an extremely steep size distribution upon release, and the dust-deposition rate is extraordinarily high. These requirements strongly contradict our current understanding of cometary dust and planetary systems. Cometary supply is therefore unlikely to be solely responsible for hot exozodis, so may need to be combined with some dust-trapping mechanism (such as gas or magnetic trapping) if it is to reproduce observations.

The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope. IV. Aperture Masking Interferometry

“…The persistence of this hot dust despite its short dynamical lifetime has led Marino et al (2017) and Marino et al (2018) to suggest cometary material is passed by a "conveyor-belt" of one or more planets from the outer disk to the inner disk. This interpretation is sufficiently dynamically favored that it may explain hot dust populations in many other systems, but remains controversial (Pearce et al 2022). AMI interferometry of η Crv in GO 1242 (simulated in Figure 3) will probe the spatial scales in between the under-resolved LBTI and over-resolved VLTI to make the first map of the hot inner disk.…”

The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope -- IV. Aperture Masking Interferometry

“…The persistence of this hot dust despite its short dynamical lifetime has led Marino et al (2017) and Marino et al (2018) to suggest cometary material is passed by a 'conveyor-belt' of one or more planets from the outer disk to the inner disk. This interpreta-tion is sufficiently dynamically favoured that it may explain hot dust populations in many other systems, but remains controversial (Pearce et al 2022). AMI interferometry of η Crv in GO 1242 (simulated in Figure 3) will probe the spatial scales in between the under-resolved LBTI and over-resolved VLTI to make the first map of the hot inner disk.…”