Can Light Echoes Account for the Slow Decay of Type IIn Supernovae?

The spectra of type IIn supernovae indicate the presence of apre-existingslow, dense circumstellar wind (CSW). If the CSW extends sufficiently far fromthe progenitor star, then dust formation should occur in the wind. The lightfrom the supernova explosion will scatter off this dust and produce a lightecho. Continuum emission seen after the peak will have contributions from boththis echo as well as from the shock of the ejecta colliding with the CSW, witha fundamental question of which source dominates the continuum. We calculatethe brightness of the light echo as a function of time for a range of dustshell geometries, and use our calculations to fit to the light curves of SN1988Z and SN 1997ab, the two slowest declining IIn supernovae on record. Wefind that the light curves of both objects can be reproduced by the echo model.However, their rate of decay from peak, color at peak and their observed peakabsolute magnitudes when considered together are inconsistent with the echomodel. Furthermore, when the observed values of M$_{B}$ are corrected for theeffects of dust scattering, the values obtained imply that these supernovaehave unrealistically high luminosities. We conclude that light echoes cannotproperly account for the slow decline seen in some IIn's, and that the shockinteraction is likely to dominate the continuum emission.Comment: 15 pages, 9 figure