Program: GN-2018B-Q-142

In this decade, the time scale shorter than 1 day is intensively investigated as a new frontier by transient surveys with widefield cameras. The short-timescale transients offer a new insight, especially, on the final stage of the evolution of massive stars, i.e., supernova (SN) progenitors and circumsteller medium (CSM). In contrast to the past HSC-SSP transient survey, the first opportunity for the g-band high-cadence survey with long-term multicolor observations with HSC is available in the S18B semester. We fully utilize this unique opportunity to explore the early SN emission. We propose ToO spectroscopic follow-up observations with Gemini/GMOS. We pick up 3 relatively low-z and bright (g < 24 mag) short-timescale transients and perform 3 spectroscopic observations for each candidate. The rapid evolution of early spectra is achieved in the first and second observations, while the SN type is identified in the third observation. The spectroscopic observations clarify the CSM structure, the mass-loss rate shortly prior to the explosion, the presupernova radius, and the temperature evolution during the SBO. The spectroscopic observations are essential to classify mechanisms of early SN emission and to reveal last moments of massive stars.