2012ApJ...744..133M

The evolved carbon-rich asymptotic giant branch 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-infrared, and mid-infrared interferometry has revealed a dynamic dust formation zone. We have obtained diffraction-limited imaging and grism spectroscopy of IRC +10216 at the 6.5 m MMT in the N band (∼8-13 µm). 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 µm and a reduction in the continuum at 13 µm. 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 full width at half-maximum of the object increases from 0".43 (∼ 56 AU) for λ < 10 µm to 0".58 (∼75 AU) at 11.8 µm, then decreases to 0".5 (∼65 AU) at 12.7 µm. 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/s.