Astro2020 Science White Paper Determining the Composition of Interstellar Dust with Far-Infrared Polarimetry

Does interstellar dust come in distinct silicate and carbonaceous varieties, or are interstellar grains a homogeneous mishmash of different materials? State-of-the-art spectropolarimetry of dust extinction features, measurements of the far-infrared dust SED, and determinations of the submillimeter dust polarization fraction as a function of frequency appear to be telling conflicting stories. In this white paper, we argue that sensitive far-infrared dust polarimetry in the 100–850μm wavelength range can provide a definitive test of the multi-component dust paradigm, with far-reaching implications from the evolution of metals in the interstellar medium to strategies for component separation in Cosmic Microwave Background experiments. We also advocate for the construction of polarized dust SEDs over small sky regions at ground-accessible frequencies below 353 GHz to test for grain components like magnetic nanoparticles and the presence of polarized anomalous microwave emission. Finally, we argue that a renewed interest in spectropolarimetry in the dust extinction features at 3.4, 9.7, and 18μm would be a timely complement to FIR polarimetry to elucidate the physical properties of interstellar dust.