2008ApJ...686L.103G

The effective extinction law for supernovae surrounded by circumstellar dust is examined with Monte Carlo simulations. Grains with light scattering properties as for interstellar dust in the Milky Way (MW) or the Large Magellanic Clouds (LMC), but surrounding the explosion site, would cause a semidiffusive propagation of light up to the edge of the dust shell. Multiple scattering of photons predominantly attenuates photons with shorter wavelengths, thus steepening the effective extinction law as compared to the case of single scattering in the interstellar medium. Our simulations yield typical values for the total-to-selective extinction ratio R_V∼1.5-2.5, as seen in recent studies of Type Ia supernova colors, with a steepening differential extinction toward shorter wavelengths.