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Perturbations of SNe Ia lightcurves, colors and spectral features by circumstellar dust

It has been suggested that multiple scattering on circumstellar dust could explain the non-standard reddening observed in the line-of-sight to Type Ia supernovae. In this work we use Monte Carlo simulations to examine how the scattered light would affect the shape of optical lightcurves and spectral features. We find that the effects on the lightcurve widths, apparent time evolution of color excess and blending of spectral features originating at different photospheric velocities should allow for tests of the circumstellar dust hypothesis on a case by case basis. Our simulations also show that for circumstellar shells with radii r=1e16-1e19 cm, the lightcurve modifications are well described by the empirical Dm_15 parameter and intrinsic color variations of order sigma_{BV}=0.05-0.1 arise naturally. For large shell radii an excess lightcurve tail is expected in B-band, as observed in e.g. SN2006X.