A study of the color diversity around maximum light in Type Ia supernovae
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From a sample of 12 well-observed Type Ia supernovae, we ﬁnd clear evidence of correlations between early phase (U − B), (V − R), and (V − I) colors and the velocity shifts of the [Fe II] λ7155 and [Ni II] λ7378 nebular lines measured from late-phase spectra. As these lines are thought to trace the ashes of the initial deﬂagration process, our ﬁndings provide additional support to the new paradigm of oﬀ-center explosions in Type Ia supernovae, and we interpret these correlations as viewing angle eﬀects in the observed colors. We also show that the nebular velocity shifts are related to the strength and width of the Ca II H&K and IR-triplet lines near-maximum light. The evidence presented here implies that the viewing angle must be taken into account when deriving extinction values and distances in future cosmological studies based on Type Ia supernovae.