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Dense, fast-moving ejecta knots in supernova remnants are prime sites formolecule and dust formation. We present SOFIA far-IR spectrometer FIFI-LSobservations of CO-rich knots in Cas A which cover a ~1 square arc minute areaof the northern shell, in the [O III] 52 and 88 micron and [O I] 63 micronlines. The FIFI-LS spectra reveal that the line profiles of [O III] and [O I]are similar to those of the Herschel PACS [O III] and CO lines. We find thatthe [O III] maps show very different morphology than the [O I] map. The [O III]maps reveal diffuse, large-scale structures and the ratio of the two [O III]lines imply the presence of gas with a range of density 500 - 10,000 per cm^3within the mapped region. In contrast, the [O I] map shows bright emissionassociated with the dense CO-rich knots. The 63 micron [O I] line tracescooled, dense post-shocked gas of ejecta. We find that IR-dominated [O III]emission is from post-shocked gas based on its morphology, high column density,and velocity profile. We describe multi-phase ejecta knots, a lifetime ofclumps, and survival of dust in the young supernova remnants.

SOFIA Far-infrared [O III] and [O I] Observations of Dense CO-knots in the Supernova Remnant Cassiopeia A: Multi-phase Ejecta