Supernova Polarization Spectra Calculated Using the Sobolev-H Method

Abstract

<p>In order to analyze supernova spectropolarimetry a radiative transfer technique is required that can treat polarization and introduces polarizing effects. To do this a modified Sobolev method, here called the Sobolev-H method, was developed for axisymmetric atmospheres with large velocity gradients. The method uses the Stokes parameters to treat the radiation field. It incorporates Hamilton's phase-matrix for resonance scattering by atomic transitions (1947), and thus allows for the polarizing effect of resonance scattering.</p> <p>The interest in supernova spectropolarimetry is to determine whether supernovae are spherically symmetric or not; the net radiation flux from a spherically symmetric supernovae would not be polarized. A computer program written using the Sobolev-H method calculates the P-Cygni line profiles emerging from homologously expanding atmospheres. A parameter survey of axisymmetric prolate and oblate models has been performed using this program. The survey demonstrates that there is considerable polarization structure associated with the P-Cygni lines. The emission and absorption polarization features have their position angle of polarization shifted from each other by 90° for both prolate and oblate models.</p> <p>An analysis of the March 6-7 polarization data for Supernova 1987a has been performed. Provided the polarization of 1987a's flux arises from oblate shape asymmetry, the analysis indicates a 50% asymmetry (ξobl = .5). A similar asymmetry would be required if 1987a were prolate. Since the polarization data indicates that an intrinsic continuum polarization exists, a method here called the discretized continuous opacity or DCO method has been devised in order to calculate continuum polarization. Calculations with the DCO method show that good qualitative agreement with the observed continuum polarization may be achievable.</p>