  Cassiopeia A (Cas A) is one of the best studied young Galactic supernova
remnants. While providing a rare opportunity to study in detail the remnant of
a Type IIb supernova, questions remain regarding the nature of its progenitor,
its mass-loss history, and its pre-SN evolution. Here we present an optical
investigation of the circumstellar environment around Cas A and find clumpy and
filamentary Halpha emission nebulosities concentrated 10-15 pc (10-15
arcminutes) to the north and east. First reported by Minkowski as a faint H II
region, these nebulosities exhibit distinct morphological and spectroscopic
properties relative to the surrounding diffuse emissions. Compared to
neighboring H II regions, these nebulae show stronger [N II] 6548, 6583 A and
[S II] 6716, 6731 A emissions relative to Halpha. We show that Cas A's
highest-velocity ejecta knots are interacting with some of the closest
projected emission nebulae, thus providing strong evidence that these nebulae
lie at the same distance as the remnant. We interpret these surrounding
nebulosities to be the remains of the progenitor's red supergiant wind which
accumulated against the southern edge of a large extended H II region located
north of Cas A. Our findings are consistent with the view that Cas A's
progenitor underwent considerable mass-loss, first from a fast main-sequence
wind, then from a slower, clumpy red supergiant wind, and finally from a brief
high-velocity wind, like that from a yellow supergiant.
