FOUR decades of IRC+10216: Evolution of a carbon-rich dust shell resolved at 10 μm with MMT adaptive optics and MIRAC4

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 6.5m 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 043 (∼56AU) for λ < 10 μm to 058 (∼75AU) at 11.8 μm, then decreases to 05 (∼65AU) 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 12-17kms^-1.