High-cadence transient surveys discover transients with short timescales of <1 days. One of such short transients is a shock breakout (SBO) taking place at the beginning of supernovae (SNe). Two subtypes of SBOs with different timescale have been detected; an SBO at a stellar surface and in a dense circumstellar medium (CSM). The SBO offers a new insight on the final stage of the evolution of massive stars. The SBO at the stellar surface and in the dense CSM are observed as rapidly declining and rising transients, respectively. This proposal aims at revealing the last moments of massive stars with enhancing sample of rapidly varying transients with the early spectral evolution and the multicolor light curves from the SBO to the tail. We propose a g-band 6-consecutive-half-night imaging observation with HSC, and immediate and continuous follow-up imaging/spectroscopic observations with GMOS. Immediate objectives are (1) determining the fraction of massive stars with dense CSM, (2) measuring the density and extention of CSM, and (3) clarifying the relations among the progenitor masses, envelope masses, and mass loss rates. Furthermore, comparisons between these observations and theoretical models promote an understanding of the last moments of massive stars.
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