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

Abstract

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 0farcs43 (~ 56 AU) for λ < 10 μm to 0farcs58 (~75 AU) at 11.8 μm, then decreases to 0farcs5 (~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^-1.

Based in part on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.