ANALYSIS OF THE POLARIZATION AND FLUX SPECTRA OF SN 1993J

Synthetic polarization and flux spectra are presented for aspherical, electron scattering-dominated photospheres of Type II supernovae (SN I I's) in general and the specific case of SN 1993J. Monte Carlo calcula tions are based on the following assumptions: (1) ellipsoidal envelope s with power-law density profiles; (2) occupation numbers given by loc al thermodynamical equilibrium (LTE); (3) pure electron scattering for continuum opacities; (4) lines treated in a Sobolev approximation wit h an assumed constant thermalization fraction; (5) line transitions re sult in depolarization; and (6) the temperature structure is given by a gray extended atmosphere. The observed luminosity of a Type II super nova depends on the unknown inclination angle i. Spectral analysis alo ne will fail to detect even strong deviations from spherical symmetry. Line scattering depolarizes incident polarized light, but the residua l intrinsic polarization does not completely vanish because of electro n scattering effects that depend on the electron density distribution. By combining results on the polarization and velocity structure of th e emission lines and the degree of polarization in the continuum, we p lace strong constraints on the degree of asphericity, the inclination of the system, and the electron density distribution. In addition, sin ce the problem is overconstrained, we can independently test for the c ontribution to the polarization caused by aligned interstellar grains between the supernova and Earth. These modeling techniques have been a pplied to SN 1993J. Both the flux spectra and the percentage of polari zation as a function of wavelength can be reproduced by an aspherical model with an axis ratio of 0.6, radial electron densities proportiona l to r(-5), and an effective temperature of 4800 K. In an oblate model , SN 1993J is seen almost equator-on. The line-forming region is still within the hydrogen-rich part of the envelope 3 weeks after the explo sion, although the continuum may form in deeper layers. Although the r esidual polarization across H alpha is not zero, the interstellar comp onent derived herein is consistent with that deduced and reported by T rammell et al. in 1993. The power of this technique for investigating the structure of other SN II's, as a method for independently deriving the interstellar polarization, and the implications of these results on the use of SN II's to determine distances through the Baade-Wesseli nk method are discussed.