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D. C. Leonard (UCBerkeley/UMass Amherst), A. V. Filippenko (UCBerkeley)
Since supernovae (SNe) are unresolvable during the early phases of their evolution, the geometry of the explosion and expanding ejecta has been a difficult subject to approach observationally. Fortunately, geometric information is encoded in the polarization properties of supernova (SN) light, with higher polarization generally indicating a greater departure from spherical symmetry. We investigate the geometry of young core-collapse SNe through a spectropolarimetric study of Type II events, with particular emphasis placed on how the results impact their cosmological utility through the Expanding Photosphere Method (EPM) of extragalactic distance determination.
We find asphericity to be ubiquitous among core-collapse SNe, although the nature and degree of the asphericity varies greatly among objects. For example, SN 1998S, a peculiar Type II event in which the progenitor star had lost much of its envelope prior to exploding, is highly polarized (p \approx 3%) at early times, implying an asphericity of at least 45%. By contrast, SN 1999em, which had a massive hydrogen envelope intact when the star exploded, is only weakly polarized at early times (p \approx 0.2%), implying a substantially spherical geometry; it became more polarized (up to p \approx 0.5%) as its photosphere receded, however, perhaps indicating greater asphericity deeper into the ejecta.
Although still limited by the small sample size, it seems that the closer we probe to the heart of the explosion the greater the evidence for asphericity in core-collapse events becomes. This supports the conclusion that the explosion itself is fundamentally asymmetric, with the resulting asphericity damped by the addition of envelope material. This result, coupled with the lack of significant systematic scatter in the Hubble diagram for previous EPM distances, suggests that asphericity does not significantly hamper the cosmological use of SNe II whose progenitors have massive hydrogen envelopes intact at the time of explosion.
The author(s) of this abstract have provided an email address for comments about the abstract: leonard@nova.astro.umass.edu