Evidence for Asphericity in the Type IIn Supernova 1998S

We present optical spectropolarimetry obtained at the Keck-II 10-m telescope on 1998 March 7 UT along with total flux spectra spanning the first 494 days after discovery (1998 March 2 UT) of the peculiar type IIn supernova (SN) 1998S. The SN is found to exhibit a high degree of linear polarization, implying significant asphericity for its continuum-scattering environment. Prior to removal of the interstellar polarization, the polarization spectrum is characterized by a flat continuum (at p ≈ 2%) with distinct changes in polarization associated with both the broad (FWZI & 20, 000 km s) and narrow (unresolved, FWHM < 300 km s) line emission seen in the total flux spectrum. When analyzed in terms of a polarized continuum with unpolarized broadline recombination emission, an intrinsic continuum polarization of p ≈ 3% results (the highest yet found for a SN), suggesting a global asphericity of & 45% from the oblate, electron-scattering dominated models of Höflich (1991). The smooth, blue continuum evident at early times is shown to be inconsistent with a reddened, single-temperature blackbody, instead having a color temperature that increases with decreasing wavelength. Broad emission-line profiles with distinct blue and red peaks are seen in the total flux spectra at later times, perhaps suggesting a disk-like or ring-like morphology for the dense (ne ≈ 10 7 cm) circumstellar medium. Implications of the circumstellar scattering environment for the spectropolarimetry are discussed, as are the effects of uncertain removal of interstellar polarization. Subject headings: circumstellar matter — polarization — stars: mass-loss — supernovae: individual (SN 1998S) — techniques: polarimetric