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Many nearby stars show mid-infrared excesses which are indicative of the presence of warm habitable zone dust, known as exozodiacal dust, in analogy to the zodiacal cloud. The presence of even low levels of this dust will be problematic for future attempts at detection and characterisation of Earth-like planets. Therefore, the study of exozodiacal dust allows us to better target planet detection missions and learn more about the inner regions of planetary systems. Such dust should disappear rapidly due to its proximity to the star via collisions and radiation pressure, and several mechanisms have been suggested to explain the presence of exozodiacal dust. Given the observed correlation between the presence of warm exozodiacal dust and cold planetesimal belts, we present an analytical model for exozodiacal dust dragged inwards by Poynting-Robertson drag from an exo-Kuiper belt undergoing a collisional cascade. We show that Poynting-Robertson drag should produce detectable levels of exozodiacal dust in systems which have a previously detected outer planetesimal belt, such that non-detections could imply the presence of intervening planets. Our model is applied to the HOSTS survey of exozodiacal dust for systems with known planetesimal belts, and we find that some detections can be explained by our Poynting-Robertson drag model. In particular, our model is applied in more detail to beta Leo, which hosts both warm and cold dust. We also discuss the likelihood of drag producing exozodiacal dust levels which are problematic for direct imaging of exo-Earths.

Predictions for exozodiacal dust dragged in from an exo-Kuiper belt