2025PASJ...77S.144B

The expansion structure of supernova remnants is important for understanding not only how heavy elements are distributed into space, but also how supernovae explode. The ejecta expansion structure of the young core-collapse supernova remnant Cas A is investigated, with Doppler parameter mapping of the Fe-K complex by the Resolve microcalorimeter onboard the X-ray Imaging and Spectroscopy Mission (XRISM). It is found that the Fe ejecta are blueshifted in the south-east and redshifted in the north-west, indicating an incomplete shell structure, similar to intermediate-mass elements (IMEs) such as Si and S. The Fe has a velocity shift of $\sim$1400 and $\sim$2160 km s$^{-1}$ in the north-west and south-east regions, respectively, with the error range of a few hundred km s$^{-1}$. These values are consistent with those for the IMEs in the north-west region, but larger than those for the IMEs in the south-east region, although the large error region prevented us from concluding which component has significantly higher velocity. The line broadening is larger in the center with values of $\sim$2000-3000 km s$^{-1}$, and smaller near the edges of the remnant. The radial profiles of the Doppler shift and broadening of the IMEs and Fe indicate that the Fe ejecta may expand asymmetrically as IME ejecta, although the large error regions do not allow us to confirm this. Moreover, we see little bulk Doppler broadening of the Fe lines in the north-eastern jet region whereas the IME lines exhibit significant broadening. No such narrow lines are detected in the north-west region. These findings suggest an asymmetric expansion of the ejecta potentially driven by large-scale asymmetries originating from the supernova explosion. This interpretation aligns with the large-scale asymmetries predicted by models of neutrino-driven supernova explosions.