  The evolved carbon-rich AGB star IRC +10216 (CW Leo) is the brightest
mid-infrared source outside the solar system, as well as one of the closest
examples of an evolved star losing mass. It has a complex and variable
circumstellar structure on small scales in the near-IR, and mid-IR
interferometry has revealed a dynamic dust formation zone. We have obtained
diffraction limited imaging and grism spectroscopy of IRC +10216 at the 6.5m
MMT in the N-band (~8-13 microns). These new observations show that a change
has occurred in the dust shell surrounding IRC +10216 over the last two
decades, which is illustrated by a change in the apparent shape of the well
known SiC spectral feature at ~11 microns and a reduction in the continuum at
13 microns. As expected, our diffraction limited spatial information shows an
extended circumstellar envelope. We also demonstrate that the dusty envelope
appears to be ~30% larger at the wavelengths of the SiC feature, likely due to
the increased opacity of SiC. The deconvolved FWHM of the object increases from
0.43" (~56 AU) for wavelength < 10 microns to 0.58" (~75 AU) at 11.8 microns,
then decreases to 0.5" (~65 AU) at 12.7 microns. Our estimates of IRC +10216's
size allow us to plausibly tie the change in the spectrum over the last 12.5
years to the evolution of the dusty circumstellar envelope at speeds of 12-17
km/sec.
