SIMBAD references

An explanation is given of the low value of R_λ≡A_λ/E(B-V), the ratio of absolute to selective extinction deduced from Type Ia supernova observations. The idea involves scattering by dust clouds located in the circumstellar environment or at the highest velocity shells of the supernova ejecta. The scattered light tends to reduce the effective R_λin the optical but has an opposite effect in the ultraviolet. The presence of circumstellar dust can be tested by ultraviolet to near-infrared observations and by multiepoch spectropolarimetry of Type Ia supernovae.