Bipolar supernova explosions

We discuss the optical spectropolarimetry of several core-collapse supernov ae, SN 1996cb (Type IIB), SN 1997X (Type Ic), and SN 1998S (Type IIn). The data show polarization evolution of several spectral features at levels fro m 0.5% to above 4%. The observed line polarization is intrinsic to the supe rnovae and not of interstellar origin. These data suggest that the distribu tion of ejected matter is highly aspherical. In the case of the Type IIn SN 1998S, the major-to-minor axis ratio must be larger than 2.5 if the polari zation is 3% from an oblate spheroidal ejecta seen edge-on. A well-defined symmetry axis can be deduced from spectropolarimetry for SN 1998S, but the Type IIB events SN 1993J and SN 1996cb seem to possess much more complicate d geometries with polarization position angles showing larger irregular var iations across spectral features; the latter may be associated with large-s cale clumpiness of the ejecta. The observed degree of polarization of the T ype Ic SN 1997X is above 4%. The data reveal a trend that the degree of pol arization increases with decreasing envelope mass and with the depth within the ejecta. The high axial ratio of the ejecta is difficult to explain in terms of the conventional neutrino-driven core-collapse models for Type II explosions. Highly asymmetric explosion mechanisms such as the formation of bipolar jets during core collapse may be a necessary ingredient for models of all core-collapse supernovae.