What sodium absorption lines tell us about type Ia supernovae

We propose that the sodium responsible for the variable Na ID absorption lines in some Type Ia supernovae (SN Ia) originate mainly from dust residing at ∼1 pc from the supernovae. In this Na-from-dust absorption (NaDA) model, the process by which the SN Ia peak luminosity releases sodium from dust at ∼1 pc from the SN is similar to the processes by which solar radiation releases sodium from cometary dust when comets approach a distance of ≲1 au from the Sun. The dust grains are not sublimated but rather stay intact, and release sodium by photon-stimulated desorption (or photosputtering). Some of the Na might start in the gas phase before the explosion. Weakening in absorption strength is caused by Na-ionizing radiation of the SN. We apply the NaDA model to SN 2006X and SN 2007le, and find it to comply better with the observed time variability of the Na ID absorption lines than the Na recombination model. The mass in the dusty shell of the NaDA model is much too high to be accounted for in the single-degenerate scenario for SN Ia. Therefore, the presence of variable Na ID lines in some SN Ia further weakens the already very problematic single-degenerate scenario for SN Ia.