A second case of variable Na_I D lines in a highly reddened type Ia supernova

Recent high-resolution spectra of the Type Ia supernova SN 2006X have revealed the presence of time-variable and blueshifted Na I D features, interpreted by Patat et al. as originating in circumstellar material within the progenitor system. The variation seen in SN 2006X induces relatively large changes in the total Na I D equivalent width (ΔEW 0.5 Å in just over two weeks) that would be detectable at lower resolutions. We have used a large data set comprising 2400 low-resolution spectra of 450 Type Ia supernovae (SNe Ia) obtained by the CfA Supernova Program to search for variable Na I D features. Out of the 31 SNe Ia (including SN 2006X) in which we could have detected similar EW variations, only one other (SN 1999cl) shows variable Na I D features, with an even larger change over a similar 10 day timescale (ΔEW = 1.66 ± 0.21 Å). Interestingly, both SN 1999cl and SN 2006X are the two most highly reddened objects in our sample, raising the possibility that the variability is connected to dusty environments.