Porous dust grains in debris disks

Abstract

Aims. When modeling the density and grain size distribution in debris disks, the minimum particle size is often significantly larger than the corresponding blowout size. While the dust particles are usually modeled as compact, homogenous spheres, we instead investigate the impact of porosity. The optical properties of porous particles are determined, and the influences of porosity on the blowout size and dust temperatures investigated. Methods. Using the method of discrete dipole approximation, we calculate the scattering and absorption cross sections of porous particles and derive the blowout size and the behavior of the dust temperature. Results. We determine the efficiency factors of absorption and the radiation pressure for porous particles, and investigate the influence on the β-ratio. Blowout sizes are calculated for various stellar luminosities and porosities, and an approximation equation is derived to estimate the blowout size as a function of these parameters. Furthermore, we investigate the influence of the porosity on the dust equilibrium temperature. Conclusions. The blowout size increases with the particle porosity and stellar luminosity. The dust temperature of porous particles is lower than the one of the compact spheres; in particular, the temperature of blowout grains decreases for porous particles. © 2013 ESO.