Diffraction-limited near-IR imaging at Keck reveals asymmetric, time-variable nebula around carbon star CIT 6

We present multiepoch, diffraction-limited images of the nebula around the carbon star CIT 6 at 2.2 and 3.1 mum from aperture masking on the Keck I Telescope. The near-IR nebula is resolved into two main components, an elongated, bright feature showing time-variable asymmetry and a fainter component about 60 mas away with a cooler color temperature. These images were precisely registered (similar to 35 mas) with respect to recent visible images from the Hubble Space Telescope (Trammell et al.), which showed a bipolar structure in scattered light. The dominant near-IR feature is associated with the northern lobe of this scattering nebula, and the multiwavelength data set can be understood in terms of a bipolar dust shell around CIT 6. Variability of the near-IR morphology is qualitatively consistent with previously observed changes in red polarization, caused by varying illumination geometry due to nonuniform dust production. The blue emission morphology and polarization properties cannot be explained by the above model alone, but require the presence of a wide binary companion in the vicinity of the southern polar lobe. The physical mechanisms responsible for the breaking of spherical symmetry around extreme carbon stars, such as CIT 6 and IRC +10216, remain uncertain.