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

Abstract

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.