Dense, fast-moving ejecta knots in supernova remnants are prime sites for molecule and dust formation. We present SOFIA far-IR spectrometer FIFI-LS observations of CO-rich knots in Cas A in the [O III] 52 and 88-micron and [O I] 63-micron lines. The [O I] map shows bright emission associated with the dense CO-rich knots and traces cooled, dense post-shocked gas of ejecta. In contrast, the [O III] maps reveal diffuse, larger-scale structures and the ratio of the two [O III] lines provide a density map. We conclude that far-IR [O III] emission is also post-shocked gas based on its morphology, high column density, and high-velocity profile. We describe multi-phase ejecta knots by comparing optical and X-ray maps and estimate the lifetime of ejecta/CO clumps and the time-scale of dust survival in the young supernova remnants.
Far-infrared [O III] and [O I] Observations of CO-knots in Cas A