Supernovae (SNe) that change their spectral appearance from one type to another, termed the ‘morphing SNe’ here, are not as rare as previously thought, reflecting much larger variety of stellar evolutionary channels than we currently understand based on the standard stellar evolution theory. The discovery of these SNe has been accelerated by recent development in transient survey and follow-up facilities/programs. We propose to observe ∼ 10 SNe in the late-phase (≥ 100 days after the explosion) in search of the morphing SNe. The late-time spectra also provide various diagnostics on the nature of the progenitors and explosions, plying a key role in uncovering the origin of enigmatic transients and SNe so far. Combining the data obtained in the proposed observations with intensive follow-up observational data starting within a few days of the explosion, we aim at quantifying the connections between the types of SNe and various evolutionary channels, which forms a basis toward a new-generation stellar evolution theory.
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