Analysis of the Flux and Polarization Spectra of the Type Ia Supernova SN 2001el: Exploring the Geometry of the High‐Velocity Ejecta

SN 2001el is the first normal Type Ia supernova to show a strong, intrinsicpolarization signal. In addition, during the epochs prior to maximum light, theCaII IR triplet absorption is seen distinctly and separately at both normalphotospheric velocities and at very high velocities. The high-velocity tripletabsorption is highly polarized, with a different polarization angle than therest of the spectrum. The unique observation allows us to construct arelatively detailed picture of the layered geometrical structure of thesupernova ejecta: in our interpretation, the ejecta layers near the photosphere(v \approx 10,000 km/s) obey a near axial symmetry, while a detached,high-velocity structure (v \approx 18,000-25,000 km/s) with high CaII lineopacity deviates from the photospheric axisymmetry. By partially obscuring theunderlying photosphere, the high-velocity structure causes a more incompletecancellation of the polarization of the photospheric light, and so gives riseto the polarization peak and rotated polarization angle of the high-velocity IRtriplet feature. In an effort to constrain the ejecta geometry, we develop atechnique for calculating 3-D synthetic polarization spectra and use it togenerate polarization profiles for several parameterized configurations. Inparticular, we examine the case where the inner ejecta layers are ellipsoidaland the outer, high-velocity structure is one of four possibilities: aspherical shell, an ellipsoidal shell, a clumped shell, or a toroid. Thesynthetic spectra rule out the spherical shell model, disfavor a toroid, andfind a best fit with the clumped shell. We show further that differentgeometries can be more clearly discriminated if observations are obtained fromseveral different lines of sight.Comment: 14 pages (emulateapj5) plus 18 figures, accepted by The Astrophysical Journa