  We report the detection of carbon monoxide (CO) emission from the young
supernova remnant Cassiopeia A (Cas A) at wavelengths corresponding to the
fundamental vibrational mode at 4.65 micron. We obtained AKARI Infrared Camera
spectra towards 4 positions which unambiguously reveal the broad characteristic
CO ro-vibrational band profile. The observed positions include unshocked ejecta
at the center, indicating that CO molecules form in the ejecta at an early
phase. We extracted a dozen spectra across Cas A along the long 1 arcmin slits,
and compared these to simple CO emission models in Local Thermodynamic
Equilibrium to obtain first-order estimates of the excitation temperatures and
CO masses involved. Our observations suggest that significant amounts of carbon
may have been locked up in CO since the explosion 330 years ago. Surprisingly,
CO has not been efficiently destroyed by reactions with ionized He or the
energetic electrons created by the decay of the radiative nuclei. Our CO
detection thus implies that less carbon is available to form carbonaceous dust
in supernovae than is currently thought and that molecular gas could lock up a
significant amount of heavy elements in supernova ejecta.
