Near-infrared Spectroscopy of Dense Ejecta Knots in the Outer Eastern Area of the Cassiopeia A Supernova Remnant

The Cassiopeia A supernova remnant has a complex structure, manifesting the multidimensional nature of core-collapse supernova explosions. To further understand this, we carried out near-infrared multiobject spectroscopy on the ejecta knots located in the northeastern (NE) jet and Fe K plume regions, which are two distinct features in the outer eastern area of the remnant. Our study reveals that the knots exhibit varying ratios of [S ii ] 1.03, [P ii ] 1.189, and [Fe ii ] 1.257 μ m lines depending on their locations within the remnant, suggesting regional differences in elemental composition. Notably, the knots in the NE jet are mostly S-rich with weak or no [P ii ] lines, implying that they originated below the explosive Ne-burning layer, consistent with the results of previous studies. We detected no ejecta knots exhibiting only [Fe ii ] lines in the NE jet area that are expected in the jet-driven supernova explosion model. Instead, we discovered a dozen Fe-rich knots in the Fe K plume area. We propose that they are dense knots produced by a complete Si burning with α -rich freeze-out in the innermost region of the progenitor and ejected with the diffuse X-ray-emitting Fe ejecta but decoupled after crossing the reverse shock. In addition to these metal-rich ejecta knots, several knots emitting only He i 1.083 μ m lines were detected, and their origin remains unclear. We also detected three extended H emission features of circumstellar or interstellar origin in this area and discuss their association with the supernova remnant.