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The spectra of Type Un supernovae indicate the presence of a preexisting slow, dense circumstellar wind (CSW). If the CSW extends sufficiently far from the progenitor star, then dust formation should occur in the wind. The light from the supernova explosion will scatter off this dust and produce a light echo. Continuum emission seen after the peak will have contributions from both this echo as well as from the shock of the ejecta colliding with the CSW, with a fundamental question of which source dominates the continuum. We calculate the brightness of the light echo as a function of time for a range of dust shell geometries and use our calculations to fit to the light curves of SN 1988Z and SN 1997ab, the two slowest declining Type Un supernovae on record. We find 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 observed peak absolute magnitudes when considered together are inconsistent with the echo model. Furthermore, when the observed values of MB are corrected for the effects of dust scattering, the values obtained imply that these supernovae have unrealistically high luminosities. We conclude that light echoes cannot properly account for the slow decline seen in some Type IIn supernovae and that the shock interaction is likely to dominate the continuum emission.

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Astrophysical Journal

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