Program: GS-2023A-Q-601

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. In our previous observing runs, we detected 1 rapidly declining transients and 5 rapidly rising transients at high redshift (z>0.4), corresponding to the SBO at the stellar surface and in the dense CSM, respectively. This proposal aims at revealing the last momenets of massive stars with enhancing sample of rapidly varying transients and obtaining their early spectral evolution. 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 surrounded by dense CSM with the HSC imaging observation, and (2) measuring the density and extention of CSM with the GMOS follow-up. Furthermore, comparisons between these observations and theoretical models will promote an understanding of the last moments of massive stars.