Program: GN-2018A-Q-204

A shock breakout (SBO) is the brightest phenomenon in a supernova. We have started SBO surveys with HSC since the S14A semester. In our previous three observing runs, we detected ~20 SBO candidates and had confirmed the high-z nature of 6 objects (z>0.4). As a result, we found two subtypes of SBOs taking place at the stellar surface and in the dense wind, and their non-negligible occurrence rates as high as ~9% of CCSN rate. However, no objects with the rising and decline of SBOs, the flash spectrum, and the evolution from SBO to the plateau have been obtained yet. Therefore, we aim at obtaining the first sample of each subtype of SBOs with these complete observations for further understanding of their population. We propose (1) g-band 4-continuous-night imaging observations (0.5+1+1+0.5 nights) with HSC, and (2) immediate and continuous follow-up imaging and spectroscopic observations with GMOS. One of the co-Is also proposes a coordinated ToO observation using Keck/LRIS via Caltech. This proposal obtains well-sampled light curves with high signal-to-noise ratios and immediate spectra for low-z samples (z<0.2), and reveals variety of the last moments of massive stars, e.g., the density in the circumstellar medium and the mass loss shortly prior to the explosion.