Mueller Polarimetry of Experimental Interstellar Dust Analogs

Interstellar asymmetric dust grains aligned with the magnetic field polarize starlight in the optical-to-infrared spectral range. Anisotropic dichroic extinction behaviour of the dust grains results in partial polarization of light along the direction of minimum extinction. (Whittet 2022) Polarization measurements of interstellar dust are commonly used to study the magnetic field in the interstellar medium. (Andersson, Lazarian, and Vaillancourt 2015) Studies of the alignment process and dust polarization have been largely theoretical and simulation-based. This work aims to provide a laboratory foundation for interpreting astrophysical polarization observations. Experimental analogs of interstellar dust are created using the Caltech Ice Dust Plasma experiment. Ice grains are formed in a low-temperature (50–170 K), low-pressure (100–2000 mTorr) weakly ionized plasma, with an ionization factor of ∼ 10^-6. Ice grains grow to sizes up to 700 μm, exhibit fractal morphology, and are continuously levitated due to plasma confinement forces. (Nicolov, Gudipati, and Bellan 2024) A dual-rotating polarizer Mueller polarimeter is being constructed to measure the Mueller matrix of the ice cloud. We expect the ice grains to maintain partially uniform alignment in different regions of the ice cloud. By measuring the Mueller matrix of the dust cloud, we plan to investigate how grain alignment correlates with spatial distribution, temperature, pressure, and evolution over time.