EXPERIMENTAL STUDY OF LIGHT SCATTERING BY LARGE DUST AGGREGATES CONSISTING OF MICRON-SIZED SiO~2 MONOSPHERES

We present measurements of angular resolved light scattering on ensembles of large dust aggregates grown via a cluster-cluster aggregation (CCA). The aggregates consist of up to 100 spherical 0.75 mum radius SiO2 particles. The wavelength of incident radiation is 680 nm, corresponding to a size parameter of x = 6.93 for the individual spheres. Interaction between neighboring particles changes the intensity and polarization of scattered light significantly, although the individual spheres are large. In addition to these short-range interactions, there is also a long-range interaction. This can best be described in terms of multiple scattering within individual aggregates, and it partly counteracts the effects of the short-range interactions. These results have a significant effect on the analysis of astronomical observations, for example of protoplanetary disks. The data show that large aggregates even as fluffy as CCA aggregates scatter much more diffusely than might be assumed. Light scattering by large aggregates is not just the sum of individual spheres or small interacting portions of the aggregates as independent scattering centers. We apply the results to HK Tau B and IRAS 04302+ 2247. The effects found might also be of importance to atmospheric aerosols. Since large aggregates will scatter more radiation backward than smaller aggregates, they may influence the energy budget of Earth or other planets, and therefore this work has some relevance for climate modeling.