Program: GN-2019B-FT-209

Cassiopeia A (Cas A) is a nearby young supernova remnant (SNR) providing a unique laboratory for the study of core-collapse supernova explosion. It is probably the best core-collapse SNR where we can see in full detail the footprints of both the SN explosion and the mass loss just before the SN explosion. The purpose of this proposal is to explore the nature of the Cas A progenitor by studying the chemical abundance of the so-called ``quasi-stationary floculli (QSFs)', which are dense, CNO-processed circumstellar clumps ejected by the progenitor <100,000 yrs before the SN explosion. Previous studies in 1970s showed that two bright QSFs in the northern area of the remnant originated from the N-rich layer near the He core. There are now more than 40 QSFs visible in optical emission, and a detailed study with more samples of QSFs can give new insights on this unique SNR. We propose observing ~20 QSFs using the multi-object mode of GMOS. We will derive accurate abundances of He and N together with shock parameters from the ratios of optical H, He, N, O, and S lines using our shock code. By analyzing the result, we can learn how the Cas A progenitor stripped off its envelope and exploded, which will be very helpful for understanding the nature of SN IIb progenitors.