Constraints on the structure of hot exozodiacal dust belts

Abstract

Recent interferometric surveys of nearby main-sequence stars show a faint but significant near-infrared excess in roughly two dozen systems, i.$\,$e. around $10\,\%$ to $30\,\%$ of stars surveyed. This excess is attributed to dust located in the immediate vicinity of the star, the origin of which is highly debated. We used previously published interferometric observations to constrain the properties and distribution of this hot dust. Considering both scattered radiation and thermal reemission, we modelled the observed excess in nine of these systems. We find that grains have to be sufficiently absorbing to be consistent with the observed excess, while dielectric grains with pure silicate compositions fail to reproduce the observations. The dust should be located within $\sim\,0.01-1\,au$ from the star depending on its luminosity. Furthermore, we find a significant trend for the disc radius to increase with the stellar luminosity. The dust grains are determined to be below $0.2-0.5\,\mu\textrm{m}$, but above $0.02-0.15\,\mu\textrm{m}$ in radius. The dust masses amount to $(0.2-3.5)\times10^{-9}\,M_\oplus$. The near-infrared excess is probably dominated by thermal reemission, though a contribution of scattered light up to $35\,\%$ cannot be completely excluded. The polarisation degree predicted by our models is always below $5\,\%$, and for grains smaller than $\sim0.2\,\mu\textrm{m}$ even below $1\,\%$. We also modelled the observed near-infrared excess of another ten systems with poorer data in the mid-infrared. The basic results for these systems appear qualitatively similar, yet the constraints on the dust location and the grain sizes are weaker.