Dust around Type Ia Supernovae

An explanation is given of the low value of $R_\lambda \equivA_\lambda/E(B-V) $, the ratio of absolute to selective extinction deduced fromType Ia supernova observations. The idea involves scattering by dust cloudslocated in the circumstellar environment, or at the highest velocity shells ofthe supernova ejecta. The scattered light tends to reduce the effective$R_\lambda$ in the optical, but has an opposite effect in the ultraviolet. Thepresence of circumstellar dust can be tested by ultraviolet to near infraredobservations and by multi-epoch spectropolarimetry of SNe Ia.Comment: ApJL, in pres