Evidence for Asphericity in a Subluminous Type Ia Supernova: Spectropolarimetry of SN 1999by

We present polarization spectra near maximum light for the stronglysubluminous Type Ia supernova 1999by that show that the supernova isintrinsically polarized. SN 1999by has an observed, overall level ofpolarization of ~0.3 to 0.8%, a rise of the polarization P redward of 6500 A,and a change in polarization across the Si II 6150 A feature of about 0.4%. Thedistribution of points with wavelength using an empirical Q-U plane methodreveals that SN 1999by has a well-defined axis of symmetry and suggests aninterstellar polarization (ISP) vector with P(ISP)=0.3% and position angleTheta = 150 deg with an error circle in the Q-U plane of radius about 0.1%.Synthetic NLTE-spectra for axisymmetric configurations based on delayeddetonation models have been computed assuming ellipsoidal geometry. Both fluxand polarization spectra can be reasonably well reproduced by models with anasphericity of ~20 % observed equator-on. The general properties of thepolarization can be understood as a consequence of the structure of subluminousmodels. The low upper limits for polarization determined for many normal eventsin contrast to the high polarization in SN 1999by may suggest a relationbetween the asymmetry we observed and the mechanism that produces a subluminousType Ia. Among various mechanisms, rapid rotation of the progenitor whitedwarf, or an explosion during a binary white dwarf merger process are likelycandidates to explain the asphericity in SN 1999by.Comment: Accepted for publication in ApJ, 50 pages, 12 figures (5 color