A Comparative Study of the Supernova Remnant Cassiopeia A from 2013 to 2020 Deep Fe ii + Si i Images

Abstract We present a comparative analysis of supernova remnant Cassiopeia A based on two deep, narrow-band images covering the Fe ii 1.644 μ m + Si i 1.645 μ m lines obtained in 2013 and 2020 with the same instruments on the UKIRT 3.8 m telescope. The identical setup and observing procedure allow for direct, accurate measurements of morphological and kinematic changes over a 7 yr baseline. We identified 263 compact knots in the 2020 image and, through comparison with the 2013 catalog of B.-C. Koo et al., classified them into quasi-stationary circumstellar knots and fast-moving knots (FMKs) of supernova ejecta. The FMKs show significant flux fluctuations, and many of those detected in 2013 are absent in the 2020 image. Proper-motion measurements derived from cross-correlation analysis indicate that most FMKs follow nearly ballistic expansion, whereas some, particularly those just beyond the eastern Fe-rich, X-ray-emitting ejecta region, exhibit noticeable deceleration. The proper motions of the main ejecta shell were also measured and modeled as a uniformly expanding shell with a systemic motion, which reproduces the observed geometric and kinematic asymmetries of the remnant.