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D. A. Howell (LBL)
We use spectropolarimetry and environmental correlations to constrain the shape and (progenitor) ages of some Type Ia supernovae, setting new limits on possible progenitor scenarios. Polarization spectra near maximum light are presented for the strongly subluminous Type Ia supernova 1999by that show that the supernova is intrinsically polarized. SN 1999by has an observed, overall level of polarization of 0.4 to 0.8%, a rise of the polarization, P, redward of 6500 Å, and a change in polarization across the Si II 6150 Åfeature of about 0.4%. The measurement implies that SN 1999by is aspherical. The distribution of points with wavelength in the Q-U plane reveals that SN 1999by has a well-defined axis of symmetry and suggests an interstellar polarization vector with PISM=0.3 ±0.1% and position angle \theta = 150 ± 5 \deg. If the asphericity of the SN is modeled as an oblate spheroid, then the axis ratio is ~ 1.2. Several mechanisms that could produce the asymmetry are considered, including an off-center deflagration to detonation transition, and a hole in the SN ejecta created by the interaction with a secondary star. Rapid rotation of the progenitor WD or an exposion during the merger of two white dwarfs seem more likely candidates to explain the asphericity.
Additionally, we find that subluminous SNe come from an old population. Of the thirteen subluminous SNe known, nine are found in E/S0 galaxies, and the remainder are found in early-type spirals. Earlier findings that overluminous supernovae come from a young progenitor population are confirmed, as 16 of 20 are from Sbc or later host galaxy types. These findings indicate that the mass of the secondary star may play a role in determining the luminosity of SNe. If the traditional picture of accretion fron a nondegenerate companion onto a WD is correct, then there must be at least two different types of companion systems. Alternatively, SNe Ia may arise from the mergers of CO WDs. The fact that overluminous SNe occur in younger populations is a prediction of the merger scenario.
The merger of two CO WDs can explain both the polarization of SN 1999by and the ages of progenitor systems.
The author(s) of this abstract have provided an email address for comments about the abstract: howell@barneys.lbl.gov