Light scattering experiments with micron-sized dust aggregates: results on ensembles of SiO2 monospheres and of irregularly shaped graphite particles

Abstract

We present two series of measurements of angular-resolved light scattering on ensembles of large dust aggregates grown via cluster-cluster-aggregation. As first non-absorbing sample we use spherical monodisperse amorphous SiO2 particles with 750nm radius. The aggregates measured consist of up to approximately 100 individual particles. As the second absorbing material we use graphite. This sample is rather polydisperse with a broad size distribution. Individual graphite particles are plate like with typical sizes of 1–2μm as plate diameter and a few hundred nm as thickness. The wavelength of the incident radiation in the experiments is 680nm corresponding to a size parameter of x=6.93 for the individual SiO2 spheres and X≈100 for the SiO2 aggregates. The largest graphite aggregates were somewhat smaller than the largest SiO2 aggregates. For the SiO2 particles, we observe a change in the scattering function and degree of linear polarization over the whole range of aggregate growth. We could not observe a limit to mutual interaction within an aggregate within our range of parameters, i.e. up to aggregates consisting of 100 monomers. Scattering gets more diffuse. This is in agreement with calculations that show that the asymmetry parameter is slightly decreasing as large aggregates grow. The graphite particles show almost no change in the scattering function and polarization. We apply the results to dusty astrophysical environments.