Ultraviolet Spectropolarimetry with Polstar: Interstellar Medium Science

TL;DR

Polstar's UV spectropolarimetry will significantly enhance understanding of interstellar dust grain properties, magnetic fields, and the composition of the interstellar medium in the Milky Way and nearby galaxies.

Contribution

This work proposes a systematic UV polarization study using Polstar, leveraging modern grain alignment theory to explore dust, magnetic fields, and atomic alignment in the ISM.

Findings

Potential to identify dust mineralogy and size distribution.

Probing magnetic field structures via ground state alignment.

Advancing knowledge of the 2175Å extinction feature carrier.

Abstract

Continuum polarization over the UV-to-microwave range is due to dichroic extinction (or emission) by asymmetric, aligned dust grains. Because of both grain alignment and scattering physics, the wavelength dependence of the polarization, generally, traces the size of the aligned grains. Ultraviolet (UV) polarimetry therefore provides a unique probe of the smallest dust grains (diameter$<0.09\mu$m), their mineralogy and interaction with the environment. However, the current observational status of interstellar UV polarization is very poor with less than 30 lines of sight probed. With the modern, quantitative and well-tested, theory of interstellar grain alignment now available, we have the opportunity to advance the understanding of the interstellar medium by executing a systematic study of the UV polarization in the ISM of the Milky Way and near-by galaxies. The Polstar mission will provide the sensitivity and observing time needed to carry out such a program, addressing questions of dust composition as a function of size and location, radiation- and magnetic-field characteristics as well as unveiling the carrier of the 2175\AA\ extinction feature. In addition, using high-resolution UV line spectroscopy Polstar will search for and probe the alignment of, and polarization from, aligned atoms and ions - so called "Ground State Alignment", a potentially powerful new probe of magnetic fields in the diffuse ISM.