Carnegie Supernova Project-II: A New Method to Photometrically Identify Sub-types of Extreme Type Ia Supernovae

We present a new method to photometrically delineate between various sub-types of Type Ia supernovae (SNe Ia). Using the color-stretch parameters, s_BV or s_gr, and the time of I-band primary maximum relative to the B-band ( ${t}_{\max }^{I-B}$ ) or g-band ( ${t}_{\max }^{I-g}$ ) maximum it is demonstrated that 2003fg-like, 1991bg-like, and 2002cx-like SNe Ia can be identified readily. In the cases of these extreme SNe Ia, their primary I-band maximum occurs after the time of the B- or g-band maxima. We suggest that the timing of the I-band maximum can reveal the physical state of the SN Ia explosion as it traces: (I) the speed of the recombination front of iron group elements in the ejecta, (II) the temperature evolution and rate of adiabatic cooling in the ejecta and, (III) the presence of interaction with a stellar envelope. This photometric sub-typing can be used in conjunction with other SNe analysis, such as the Branch diagram, to examine the physics and diversity of SNe Ia. The results here can also be used to screen out non-Ia SNe from cosmological samples that do not have complete spectroscopic typing. Finally, as future surveys like that of the Vera C. Rubin Observatory (previously referred to as the Large Synoptic Survey Telescope) create large databases of light curves of many objects this photometric identification can be used to readily identify and study the rates and bulk properties of peculiar SNe Ia.