  Cassiopeia A is a nearby young supernova remnant that provides a unique
laboratory for the study of core-collapse supernova explosions. Cassiopeia A is
known to be a Type IIb supernova from the optical spectrum of its light echo,
but the immediate progenitor of the supernova remains uncertain. Here we report
results of near-infrared, high-resolution spectroscopic observations of
Cassiopeia A where we detected the pristine circumstellar material of the
supernova progenitor. Our observations revealed a strong emission line of iron
(Fe) from a circumstellar clump that has not yet been processed by the
supernova shock wave. A comprehensive analysis of the observed spectra,
together with an HST image, indicates that the majority of Fe in this
unprocessed circumstellar material is in the gas phase, not depleted onto dust
grains as in the general interstellar medium. This result is consistent with a
theoretical model of dust condensation in material that is heavily enriched
with CNO-cycle products, supporting the idea that the clump originated near the
He core of the progenitor. It has been recently found that Type IIb supernovae
can result from the explosion of a blue supergiant with a thin hydrogen
envelope, and our results support such a scenario for Cassiopeia A.
