  One objective of the Polstar spectropolarimetry mission is to characterize
the degree of nonconservative mass transfer that occurs at various stages of
binary evolution, from the initial mass reversal to the late Algol phase. The
proposed instrument combines spectroscopic and polarimetric capabilities, where
the spectroscopy can resolve Doppler shifts in UV resonance lines with 10 km/s
precision, and polarimetry can resolve linear polarization with 1e-3 precision
or better. The spectroscopy will identify absorption by mass streams seen in
projection against the stellar disk as a function of orbital phase, hot
accretion spots, as well as scattering from extended splash structures,
circumbinary disks, and other flows in and above/below the orbital plane (e.g.
jets) that fail to be transferred conservatively. The polarimetry affects more
the light coming from material not seen against the stellar disk, allowing the
geometry of the scattering to be tracked, resolving ambiguities left by the
spectroscopy and light-curve information. For example, nonconservative mass
streams ejected in the polar direction will produce polarization of the
opposite sign from conservative transfer accreting in the orbital plane. Also,
time domain coverage over a range of phases of the binary orbit are well
supported by the Polstar observing strategy. Combining these elements will
significantly improve our understanding of the mass transfer process and the
amount of mass that can escape from the system, an important channel for
changing the final mass, and ultimate supernova, of the large number of massive
stars found in binaries at close enough separation to undergo interaction.
