A new insight into the classification of Type Ia supernovae

Type Ia supernovae (SNe Ia) spectra are compared using the coefficient of the largest wavelet scale in their decomposition. Two distinct subgroups have been identified, and their occurrence is discussed with regards to the use of SNe Ia as cosmological probes. Apart from the group of normal SNe, another trend characterized by intrinsically redder colours consists of many different SN events, which exhibit diverse properties. These include the interaction with the circumstellar material and the existence of a specific shell structure in or surrounding the SN ejecta or super-Chandrasekhar mass progenitors. Compared with normal objects, these SNe may violate the standard width-luminosity correction. This could influence the cosmological results if these are all calibrated equally, as their fraction among SNe Ia is not negligible when performing precision cosmology. Using the largest wavelet scale coefficient in combination with long-baseline B - I colours, we show how to disentangle the SN intrinsic colour from the part that corresponds to the reddening as a result of dust extinction in the host galaxy in the SALT2 colour parameter c. We discuss how the intrinsic colour differences may explain the different reddening laws for two subsamples. There are wavelength intervals for which the measured largest scale coefficient is invariant to the additional extinction applied to a spectrum. A combination of wavelet coefficients measured in different wavelength intervals can be used to develop a technique that allows for the estimation of extinction.