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D. C. Leonard, A. V. Filippenko, E. C. Moran (UCBerkeley), A. J. Barth (CfA)
We present high quality spectropolarimetry of seven young, nearby supernovae (SNe) observed with the Keck-II telescope. Our sample includes all major supernova types: Ia (1997dt), Ib (1998T), Ib/c (1997dq), Ic-pec (1997ef), II-P (1997ds), IIn (1997eg), and IIn-pec (1998S). All objects show evidence for intrinsic linear polarization, though the degree and detailed characteristics of the polarization vary considerably. Rather than relying on the overall continuum polarization level, which depends strongly on the value chosen for the (uncertain) interstellar component, we focus our analysis on specific line features, whose interpretation is much less dependent on interstellar polarization removal. The most prominent feature common to all objects is a change in both the magnitude and direction of the polarization across strong lines.
For SNe 1997dt, 1998T, 1997dq, 1997ef, and 1997ds we measure changes in polarization ranging from \Delta {\rm P}~.3% {\rm \ to\ } 2% associated with the absorption troughs of the strongest P-Cygni features. One interpretation of this signature is that it results from global asymmetry of the electron scattering atmosphere and/or the underlying continuum region (McCall, 1984: MNRAS, 210, 829). This sets a cautionary note on the use of these objects as extragalactic distance indicators through the Baade method.
The optical spectra of the type IIn SNe 1997eg and 1998S are dominated by strong, multi-component emission lines, thought to be produced by an intense interaction between the supernova and its circumstellar environment. In both objects, we see clear changes in the polarization degree (up to \Delta{\rm P}~%) across the strong lines. Further, abrupt changes in position angle are seen at distinct velocities in the line profiles. An interpretation of these changes in terms of the scattering origins of the broad, intermediate, and narrow line components is presented.
The author(s) of this abstract have provided an email address for comments about the abstract: dleonard@astron.berkeley.edu