Program: GN-2019A-Q-205

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 occurrence rates are non-negligible. 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 g-band 4-consecutive-night imaging observations (0.5+1+1+0.5 nights) with HSC, and immediate and continuous follow-up imaging and spectroscopic observations with GMOS. This proposal (1) constructs statistical sample of SBOs to clarify the population of SBO subtypes and (2) obtains well-sampled light curves and immediate spectra for low-$z$ samples (z<0.2) to understand the SBO physics. These reveal 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.