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The evolved carbon-rich AGB star IRC +10216 (CW Leo) is the brightestmid-infrared source outside the solar system, as well as one of the closestexamples of an evolved star losing mass. It has a complex and variablecircumstellar structure on small scales in the near-IR, and mid-IRinterferometry has revealed a dynamic dust formation zone. We have obtaineddiffraction limited imaging and grism spectroscopy of IRC +10216 at the 6.5mMMT in the N-band (~8-13 microns). These new observations show that a changehas occurred in the dust shell surrounding IRC +10216 over the last twodecades, which is illustrated by a change in the apparent shape of the wellknown SiC spectral feature at ~11 microns and a reduction in the continuum at13 microns. As expected, our diffraction limited spatial information shows anextended circumstellar envelope. We also demonstrate that the dusty envelopeappears to be ~30% larger at the wavelengths of the SiC feature, likely due tothe increased opacity of SiC. The deconvolved FWHM of the object increases from0.43" (~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'ssize allow us to plausibly tie the change in the spectrum over the last 12.5years to the evolution of the dusty circumstellar envelope at speeds of 12-17km/sec.

Four Decades of IRC +10216: Evolution of a Carbon Rich Dust Shell Resolved at 10 microns with MMT Adaptive Optics and MIRAC4