Large Late-time Asphericities in Three Type IIP Supernovae
  Type II-plateau supernovae (SNe IIP) are the results of the explosions of red
supergiants and are the most common subclass of core-collapse supernovae. Past
observations have shown that the outer layers of the ejecta of SNe IIP are
largely spherical, but the degree of asphericity increases toward the core. We
present evidence for high degrees of asphericity in the inner cores of three
recent SNe IIP (SNe 2006my, 2006ov, and 2007aa), as revealed by late-time
optical spectropolarimetry. The three objects were all selected to have very
low interstellar polarization (ISP), which minimizes the uncertainties in ISP
removal and allows us to use the continuum polarization as a tracer of
asphericity. The three objects have intrinsic continuum polarizations in the
range of 0.83-1.56% in observations taken after the end of the photometric
plateau, with the polarization dropping to almost zero at the wavelengths of
strong emission lines. Our observations of SN 2007aa at earlier times, taken on
the photometric plateau, show contrastingly smaller continuum polarizations
(~0.1%). The late-time H-alpha and [O I] line profiles of SN 2006ov provide
further evidence for asphericities in the inner ejecta. Such high core
polarizations in very ordinary core-collapse supernovae provide further
evidence that essentially all core-collapse supernova explosions are highly
aspherical, even if the outer parts of the ejecta show only small deviations
from spherical symmetry.
