  The Polstar mission will provide for a space-borne 60cm telescope operating
at UV wavelengths with spectropolarimetric capability capturing all four Stokes
parameters (intensity, two linear polarization components, and circular
polarization). Polstar's capabilities are designed to meet its goal of
determining how circumstellar gas flows alter massive stars' evolution, and
finding the consequences for the stellar remnant population and the stirring
and enrichment of the interstellar medium, by addressing four key science
objectives. In addition, Polstar will determine drivers for the alignment of
the smallest interstellar grains, and probe the dust, magnetic fields, and
environments in the hot diffuse interstellar medium, including for the first
time a direct measurement of the polarized and energized properties of
intergalactic dust. Polstar will also characterize processes that lead to the
assembly of exoplanetary systems and that affect exoplanetary atmospheres and
habitability. Science driven design requirements include: access to ultraviolet
bands: where hot massive stars are brightest and circumstellar opacity is
highest; high spectral resolution: accessing diagnostics of circumstellar gas
flows and stellar composition in the far-UV at 122-200nm, including the NV,
SiIV, and CIV resonance doublets and other transitions such as NIV, AlIII,
HeII, and CIII; polarimetry: accessing diagnostics of circumstellar magnetic
field shape and strength when combined with high FUV spectral resolution and
diagnostics of stellar rotation and distribution of circumstellar gas when
combined with low near-UV spectral resolution; sufficient signal-to-noise
ratios: ~1000 for spectropolarimetric precisions of 0.1% per exposure; ~100 for
detailed spectroscopic studies; ~10 for exploring dimmer sources; and cadence:
ranging from 1-10 minutes for most wind variability studies.
