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Evidence for Asphericity in the Type IIN Supernova SN 1998S
We present optical spectropolarimetry obtained at the Keck II 10 mtelescope on 1998 March 7 UT along with total flux spectra spanning thefirst 494 days after discovery (1998 March 2 UT) of the peculiar TypeIIn supernova (SN) 1998S. The SN is found to exhibit a high degree oflinear polarization, implying significant asphericity for itscontinuum-scattering environment. Prior to the removal of interstellarpolarization, the polarization spectrum is characterized by a flatcontinuum (at p~2%) with distinct changes in polarization associatedwith both the broad (symmetric, half-width near zero intensity>~10,000 km s-1) and narrow (unresolved, full width athalf-maximum less than 300 km s-1) line emission seen in thetotal flux spectrum. When analyzed in terms of a polarized continuumwith unpolarized broad-line recombination emission, an intrinsiccontinuum polarization of p~3% results, suggesting a global asphericityof >~45% from the oblate, electron-scattering dominated models ofHöflich. The smooth, blue continuum evident at early times is shownto be inconsistent with a reddened, single-temperature blackbody,instead having a color temperature that increases with decreasingwavelength. Broad emission-line profiles with distinct blue and redpeaks are seen in the total flux spectra at later times, suggesting adisklike or ringlike morphology for the dense(ne~107 cm-3) circumstellar medium,generically similar to what is seen directly in SN 1987A, although muchdenser and closer to the progenitor in SN 1998S. Implications of thecircumstellar scattering environment on the spectropolarimetry arediscussed, as are the effects of uncertainty in the removal ofinterstellar polarization; the importance of obtaining multiplespectropolarimetric epochs in future studies to help better constrainthe interstellar polarization value is particularly stressed. Usinginformation derived from the spectropolarimetry and the total fluxspectra, an evolutionary scenario for SN 1998S and its progenitor arepresented.
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