Scattering properties of protoplanetary dust analogs with microwave analogy: Rough compact grains

Scattering simulations of perfect spheres are not sufficient to explain the observations of scattered light coming from protoplanetary and debris disks, specially when dust sizes are of the same order of magnitude as the wavelength used to perform the observations. Moreover, examples of grains collected from the Solar System have proved that the morphology of interstellar dust is irregular. These evidences lead to consider that the dust that participates in these circumstellar disks has morphologies more complex than spheres. We aim to measure and simulate the scattering properties of six rough compact grains to identify how their morphology affect their scattering properties . These grains are intended to be dust analogs of protoplanetary and debris disks with convexity ranging from $75$ to $99$. Grains were 3D printed using stereolithography, controlling their shape and refractive index. These analogs were measured with our microwave scattering experiment (microwave analogy) at wavelengths ranging from $ meter $ to $ meter $, leading to size parameters from $X=1.07$ to $X=7.73$. In parallel, their scattering properties were simulated with our finite element method bf(FEM) which contained the same geometric file as the 3D printed grains. We retrieved five scattering properties of such grains, that is, the phase function, the degree of linear polarization (DLP), and three other Mueller matrix elements $ ij Two types of studies were performed. First, a study on the scattering properties averaged over several orientations of grains at different wavelengths. Second, a study on the same scattering properties where a power law size distribution effect was applied. The very good correspondence between the measured and simulated Mueller matrix elements demonstrated the accuracy of our measurement setup as well as the efficiency of our FEM simulations. For the first study, DLP proved to be a good indicator of the grain morphology, in terms of convexity and shape anisotropy . For the second study, backscattering enhancements of the phase function were related to the grains convexity. The maximum DLP and its negative polarization branches as well as the $ levels were related to the shape anisotropy of our grains.