Collisional Evolution around A-type Stars with Strong Silicate Features

Abstract

Around luminous A-type stars, retaining micrometer-sized dust grains is difficult owing to the high radiation pressure that repels particles with sizes below the blowout limit. Yet such particles have been evidenced in the terrestrial zone around those stars, via pronounced silicate spectral features. An in situ planetesimal belt that collisionally replenishes the dust is expected to deplete quickly, limiting the lifetime of observable features. However, modeling of such systems often relies on a power-law description of the grain size distribution that has a lower size limit owing to blowout. Using more complex, collisional simulations instead, we investigated the temporal evolution of hypothetical planetesimal belts in the terrestrial zone around five A-type stars showing strong spectral silicate features. A collisional cascade that is modified by radiation pressure results in dust distributions that replicate observed spectral features without requiring other physical processes. While features can be altered by particles below the blowout limit, their magnitude is dominated by the emission of larger grains. Using the systems' ages, we investigated the origin of the planetesimal belt. For the three younger systems of our sample, a planetesimal belt that emerged from the protoplanetary disk phase can explain the observations. This is even possible for two older systems if the planetesimals are located at slightly larger distances than the dust, e.g., if the dust drifted inward owing to the Poynting-Robertson drag. Alternatively, a recent production of the planetesimals, for instance, in a giant collision event, could have happened only several million years ago.