  We present the first images of the transition disk around the close binary
system HD 34700A in polarized scattered light using the Gemini Planet Imager
instrument on Gemini South. The J and H band images reveal multiple spiral-arm
structures outside a large (R = 0.49" = 175 au) cavity along with a bluish
spiral structure inside the cavity. The cavity wall shows a strong
discontinuity and we clearly see significant non-azimuthal polarization Uphi
consistent with multiple scattering within a disk at an inferred inclination
~42deg. Radiative transfer modeling along with a new Gaia distance suggest HD
37400A is a young (~5 Myr) system consisting of two intermediate-mass (~2Msun)
stars surrounded by a transitional disk and not a solar-mass binary with a
debris disk as previously classified. Conventional assumptions of the
dust-to-gas ratio would rule out a gravitational instability origin to the
spirals while hydrodynamical models using the known external companion or a
hypothetical massive protoplanet in the cavity both have trouble reproducing
the relatively large spiral arm pitch angles (~30deg) without fine tuning of
gas temperature. We explore the possibility that material surrounding a massive
protoplanet could explain the rim discontinuity after also considering effects
of shadowing by an inner disk. Analysis of archival Hubble Space Telescope data
suggests the disk is rotating counterclockwise as expected from the spiral arm
structure and revealed a new low-mass companion at 6.45" separation. We include
an appendix which sets out clear definitions of Q, U, Qphi, Uphi, correcting
some confusion and errors in the literature.
