LARGE LATE-TIME ASPHERICITIES IN THREE TYPE IIP SUPERNOVAE

Type II-plateau supernovae (SNe IIP) are the results of the explosions of red\nsupergiants and are the most common subclass of core-collapse supernovae. Past\nobservations have shown that the outer layers of the ejecta of SNe IIP are\nlargely spherical, but the degree of asphericity increases toward the core. We\npresent evidence for high degrees of asphericity in the inner cores of three\nrecent SNe IIP (SNe 2006my, 2006ov, and 2007aa), as revealed by late-time\noptical spectropolarimetry. The three objects were all selected to have very\nlow interstellar polarization (ISP), which minimizes the uncertainties in ISP\nremoval and allows us to use the continuum polarization as a tracer of\nasphericity. The three objects have intrinsic continuum polarizations in the\nrange of 0.83-1.56% in observations taken after the end of the photometric\nplateau, with the polarization dropping to almost zero at the wavelengths of\nstrong emission lines. Our observations of SN 2007aa at earlier times, taken on\nthe photometric plateau, show contrastingly smaller continuum polarizations\n(~0.1%). The late-time H-alpha and [O I] line profiles of SN 2006ov provide\nfurther evidence for asphericities in the inner ejecta. Such high core\npolarizations in very ordinary core-collapse supernovae provide further\nevidence that essentially all core-collapse supernova explosions are highly\naspherical, even if the outer parts of the ejecta show only small deviations\nfrom spherical symmetry.\n