Program: GN-2020B-FT-213

Despite their importance as the cosmic distance indicator, the origin of type Ia supernovae (SNe Ia) is still obscured. Recent studies indicated that SNe Ia discovered within a few days of their explosions, especially for those with brightness excess at such early time (the so-called early-excess SNe Ia) play an irreplaceable role in addressing the long-standing progenitor and explosion mechanism issues. Using the most powerful wide-field survey facility mounted on the 8-m class telescopes–––the Subaru Hyper Suprime-Cam (Subaru/HSC), we are able to find numerous early-phase SNe Ia even in a short-term observing campaign. At the end of 2020, we will carry out the 2020 observing run of the MUlti-band Subaru Survey for Early-phase SNe Ia (MUSSES), a multi-band deep imaging survey with Subaru/HSC (down to 26.5 mag in g and r bands) for about 40 sq. deg sky in ten consecutive half-nights (the survey is the largest open-use supernova survey with Subaru/HSC so far) aiming to discover a half-dozen SNe Ia within one day of their explosions. These SNe are of legacy value as the high-cadence photometry soon after their explosions will be a milestone achievement of the Subaru time-domain science that will not be superseded by any other telescopes for the foreseeable 10 years. Given the importance of spectral follow-up observations in comprehensively understanding the nature of SNe Ia, by combining with our Gemini DDT project focusing on the early-phase spectroscopic follow-up in December, here we propose the rest of Gemini/GMOS follow-ups through the Gemini FT to further investigate the spectral evolution of ultra-early SNe Ia discovered in the December MUSSES observing run. The combination of the unprecedented 10-half-nights Subaru/HSC survey and the proposed Gemini follow-up observation will provide conclusive evidence on the progenitor and explosion mechanism of SNe Ia for the first time.