Evidence for asphericity in a subluminous Type Ia supernova: Spectropolarimetry of SN 1999by

We present polarization spectra near maximum light for the strongly sublumi nous Type Ia supernova SN 1999by that show that the supernova is intrinsica lly polarized. SN 1999by has an observed, overall level of polarization of approximate to0.3%-0.8%, a rise of the polarization P redward of 6500 Angst rom, and a change in polarization across the Si II lambda 6150 feature of a bout 0.4%. The presentation of the polarization at different wavelengths in the Q-U plane is shown to be a powerful tool to determine the overall geom etry and the interstellar component. The distribution of points with wavele ngth using this empirical Q-U plane method reveals that SN 1999by has a wel l-defined axis of symmetry and suggests an interstellar polarization (ISP) vector with P-ISP = 0.3% and position angle Theta = 150 degrees with an err or circle in the Q-U plane of radius about 0.1%. Synthetic non-LTE spectra for axisymmetric configurations based on delayed-detonation models have bee n computed assuming ellipsoidal geometry. The input ejecta structure and co mposition are based on a Chandrasekhar mass delayed-detonation model. The p arameters of the explosion are chosen to reproduce the time evolution of IR spectra of SN 1999by without further adjustments. Spherical models are the n mapped onto ellipsoidal geometries and the axis ratio, viewing angle, and ISP adjusted to provide the best agreement with the polarization spectra. Both flux and polarization spectra can be reasonably well reproduced by mod els with an asphericity of approximate to 20% observed equator-on. The gene ral properties of the polarization can be understood as a consequence of th e structure of subluminous models. Best fits are obtained for the theoretic al models with P-ISP = 0.25% and Theta = 140 degrees, consistent with the e mpirical method. We discuss our results for this subluminous Type Ia in the context of "normally bright" Type Ia supernovae. For normally bright Type Ia, the photosphere is near the inner iron-rich layers at maximum light and the ubiquitous iron lines give a rapid variation to the model polarization spectra. In subluminous models, the photosphere near maximum is in the sil icon layers with fewer lines and a smoother overall polarization spectrum, as observed for SN 1999by. Though data are sparse, the low upper limits for polarization determined for many normal events in contrast to the high pol arization in SN 1999by may suggest a relation between the asymmetry we obse rved and the mechanism that produces a subluminous Type Ia. Among various m echanisms, rapid rotation of the progenitor white dwarf and/or an explosion during a binary white dwarf merger process are likely candidates to explai n the asphericity in SN 1999by.