Dust size and spatial distributions in debris discs: predictions for exozodiacal dust dragged in from an exo-Kuiper belt

The spectral energy distributions of some nearby stars show mid-infrared (IR) excesses from warm habitable zone dust, known as exozodiacal dust. This dust may originate in collisions in a planetesimal belt before being dragged inwards. This paper presents an analytical model for the size distribution of particles at different radial locations in such a scenario, considering evolution due to destructive collisions and Poynting-Robertson (P-R) drag. Results from more accurate but computationally expensive numerical simulations of this process are used to validate the model and fit its free parameters. The model predicts 11 mu m excesses (R-11) for discs with a range of dust masses and planetesimal belt radii using realistic grain properties. We show that P-R drag should produce exozodiacal dust levels detectable with the Large Binocular Telescope Interferometer (LBTI) (R-11 > 0.1 per cent) in systems with known outer belts; non-detection may indicate dust depletion, e.g. by an intervening planet. We also find that LBTI could detect exozodiacal dust dragged in from a belt too faint to detect at far-IR wavelengths, with fractional luminosity f similar to 10(-7) and radius similar to 10-80 au. Application to systems observed with LBTI shows that P-R drag can likely explain most (5/9) of the exozodiacal dust detections in systems with known outer belts; two systems (beta Uma and eta Corvi) with bright exozodi may be due to exocomets. We suggest that the three systems with exozodiacal dust detections but no known belt may have cold planetesimal belts too faint to be detectable in the far-IR. Even systems without outer belt detections could have exozodiacal dust levels R-11 > 0.04 per cent which are problematic for exo-Earth imaging.