An explanation is given of the low value of Rλ≡Aλ/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 Rλ in 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 multi-epoch spectropolarimetry of SNe Ia.
@article{arxiv.astro-ph/0511003,
title = {Dust around Type Ia supernovae},
author = {Lifan Wang},
journal= {arXiv preprint arXiv:astro-ph/0511003},
year = {2009}
}