How much large dust could be present in hot exozodiacal dust systems?

Abstract

Context. An infrared excess over the stellar photospheric emission of main-sequence stars has been found in interferometric surveys, commonly attributed to the presence of hot exozodiacal dust (HEZD). While submicrometer-sized grains in close vicinity to their host star have been inferred to be responsible for the found near-infrared excesses, the presence and amount of larger grains as part of the dust distributions are weakly constrained. Aims. We quantify how many larger grains (above-micrometer-sized) could be present in addition to submicrometer-sized grains, while being consistent with observational constraints. This is important in order to distinguish between various scenarios for the origin of HEZD and to better estimate its observational appearance when observed with future instruments. Methods. We extended a model suitable to reproduce current observations of HEZD to investigate a bimodal size distribution. By deriving the characteristics of dust distributions whose observables are consistent with observational limits from interferometric measurements in the K and N bands we constrained the radii of sub- and above-micrometer-sized grains as well as their mass, number, and flux density ratios. Results. In the most extreme cases of some of the investigated systems, large grains ≳10 µm might dominate the mass budget of HEZD while contributing up to 25 % of the total flux density originating from the dust at a wavelength of 2.13 µm and up to 50 % at a wavelength of 4.1 µm; at a wavelength of 11.1 µm their emission might clearly dominate over the emission of small grains. While it is not possible to detect such hot-dust distributions using ALMA, the ngVLA might allow us to detect HEZD at millimeter wavelengths. Conclusions. Large dust grains (above-micrometer-sized) might have a more important impact on the observational appearance of HEZD than previously assumed, especially at longer wavelengths.