The Polstar high resolution spectropolarimetry MIDEX mission

The ISM dust in turn reveals the composition and magnetic environment leading to new star formation, and the accretion disks of Herbig Ae/Be stars reveal how the ISM gas returns to make new massive stars. Polstar will combine high-resolution spectroscopy in the time domain with high-precision UV polarimetry. Doppler-shifted UV resonance line opacity will provide information about circumstellar kinematics, while polarization gives complementary geometric information about unseen structures.

The composition and magnetic alignment of the smallest interstellar dust grains provides a probe of the ISM utilizing radiative alignment theory (RAT). Polstar will operate in the far-UV (FUV) at 122-200 nm at high spectral resolution of around R similar to 30k, and at FUV and near-UV (NUV) wavelengths of 122-320 nm at lower spectral resolutions of 0.1 - 1k. Detection of polarization levels as weak as 0.1% are expected, with a temporal cadence ranging from 5-10 minutes for most wind variability studies, to hours or days for sampling rotation, to days or weeks for sampling binary orbits, to months to a year for sampling substructure in the inner regions of protoplanetary disks.

Sub-meter-class aperture is well suited to access this wide array of time domain science, made possible by restricting to a few hundred bright, massive stars, necessarily extincted by a small to moderate column of interstellar dust, informing both the attributes of the stars and the ISM through which they are seen. As such, the focus is on our own galaxy and its evolutionary drivers, but a few targets in the Magellanic clouds offer the potential to extend this understanding to low-metallicity environments.