An investigation of $^{56}$Ni shells as the source of early light curve bumps in type Ia supernovae

An excess of flux (i.e. a bump) in the early light curves of type Ia supernovae has been observed in a handful of cases. Multiple scenarios have been proposed to explain this. It has been shown that for at least one object (SN~2018oh) the excess emission observed could be the result of a large amount of $^{56}$Ni in the outer ejecta ($\sim$0.03~$M_{\rm{\odot}}$). We present a series of model light curves and spectra for ejecta profiles containing $^{56}$Ni shells of varying masses (0.01, 0.02, 0.03, and 0.04~$M_{\rm{\odot}}$) and widths. We find that even for our lowest mass $^{56}$Ni shell, an increase of \textgreater2 magnitudes is produced in the bolometric light curve at one day after explosion relative to models without a $^{56}$Ni shell. We show that the colour evolution of models with a $^{56}$Ni shell differs significantly from those without and shows a colour inversion similar to some double-detonation explosions. Spectra of our $^{56}$Ni shell models show that strong suppression of flux between $\sim$3\,700 -- 4\,000~$\AA$ close to maximum light appears to be a generic feature for this class of model. Comparing our models to observations of SNe~2017cbv and 2018oh, we show that a $^{56}$Ni shell of 0.02 -- 0.04~$M_{\rm{\odot}}$ can match shapes of the early optical light curve bumps, but the colour and spectral evolution are in disagreement. This would indicate that an alternative origin for the flux excess is necessary. Based on existing explosion scenarios, producing such a $^{56}$Ni shell in the outer ejecta as required to match the light curve shape, without the presence of additional short-lived radioactive material, may prove challenging. Given that only a small amount of $^{56}$Ni in the outer ejecta is required to produce a bump in the light curve, such non-monotonically decreasing $^{56}$Ni distributions in the outer ejecta must be rare, if they were to occur at all.