Ultraviolet spectropolarimetry with polstar: interstellar medium science

Abstract Continuum polarization over the UV-to-microwave range is due to dichroic extinction (or emission) by asymmetric, aligned dust grains. Scattering can also be an important source of polarization, especially at short wavelengths. Because both grain alignment and scattering physics, wavelength dependence generally, traces size Similarly because differing dependencies two generally reliably separated. Ultraviolet (UV) polarimetry therefore provides a unique probe smallest grains (diameter $< 0.09~\upmu \text{m}$ < 0.09 ?m ), their mineralogy interaction with environment. However, current observational status interstellar UV very poor less than 30 lines sight probed. With modern, quantitative well-tested, theory now available, we have opportunity advance understanding medium (ISM) executing systematic study in ISM Milky Way near-by galaxies. The Polstar mission will provide sensitivity observing time needed carry out such program (probing hundreds stars dozens LMC/SMC), addressing questions composition as function location, radiation- magnetic-field characteristics well unveiling carrier 2175 Å feature. In addition, using high-resolution line spectroscopy search for of, from, atoms ions - so called “Ground State Alignment”, potentially powerful new magnetic fields diffuse ISM.