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tokens: light curve, which may help discriminate between different explosion scenarios. 4.1. Circumstellar Setup For some guidance in what sort of mass distributions to consider, we turn to studies of post-merger density distributions as presented by Pakmor et al. ( 2012 ), Schwab et al. ( 2012 ), and Shen et al. ( 2012 ), and summarized in Figure 9 . These calculations roughly show how circumstellar material evolves from immediately following the merger (∼seconds), to many viscous times (∼hours), to many thermal times (∼1000 years), respectively. Even over this large range of timescales, this comparison demonstrates that a profile is a good description of all of these density profiles. This is because the large heating near the base of the material leads to a roughly constant flux and a radiative envelope with an n = 3 polytropic index. Motivated by this, here we consider similar density distributions for extended material of , so that the mass of the extended material is simplywhere is the radius of the underlying WD and is a normalization constant for the density profile. Strictly speaking, the models that evolve for large periods of time following the merger are not expected to necessarily make SNe Ia, and instead lead to accretion-induced collapse (Saio Nomoto 1998 ; Shen et al. 2012 ; Schwab et al. 2015 ). Nevertheless, in the spirit of this work, we will still explore a variety of density distributions with different values of M e and R e , even for those cases that might not reach an SN Ia, to see what the corresponding observational signatures should be. Figure 9. Example density profiles showing a diversity of possible outcomes for the circumstellar material following a double WD merger. These profiles are a WD (green dashed line), equatorial material during a violent merger (Pakmor et al. 2012 , purple dot–dashed line), the nearly spherical profile after the material has viscously relaxed (Schwab et al. 2012 , red solid line), and the material after it has expanded to a red supergiant-like size (Shen et al. 2012 ; blue dotted line). 4.2. Extended Material Light Curves Our first set of circumstellar calculations are shown in Figure 10 . Here, we set the external mass to and vary the outer radius from up to . The explosion is triggered in the same way as all previous models, and the 56 Ni mixing is kept low with a boxcar width of , that is, the same as the red curves in Figures 1 , 2 , 5 , 7 , and 8 . The impact of the extended material on the light curve is dramatic, resulting in a bright first peak from the shock cooling of this extended material. This is reminiscent of the double-peaked light curves of some SNe IIb, which have also been attributed to extended material around the progenitor star (Bersten et al. 2012 ; Nakar Piro 2014 ). Furthermore, the roughly parabolic shape is similar to the analytic light curve expected for this material (Piro 2015 ). The observation of such a feature would be extremely useful for constraining the