Diffraction‐limited Near‐IR Imaging at Keck Reveals Asymmetric, Time‐variable Nebula around Carbon Star CIT 6

Abstract

We present multiepoch, diffraction-limited images of the nebula aroundthe carbon star CIT 6 at 2.2 and 3.1 {μ}m from aperture masking on theKeck I Telescope. The near-IR nebula is resolved into two maincomponents, an elongated, bright feature showing time-variable asymmetryand a fainter component about 60 mas away with a cooler colortemperature. These images were precisely registered (~35 mas) withrespect 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 ofthis scattering nebula, and the multiwavelength data set can beunderstood in terms of a bipolar dust shell around CIT 6. Variability ofthe near-IR morphology is qualitatively consistent with previouslyobserved changes in red polarization, caused by varying illuminationgeometry due to nonuniform dust production. The blue emission morphologyand polarization properties cannot be explained by the above modelalone, but require the presence of a wide binary companion in thevicinity of the southern polar lobe. The physical mechanisms responsiblefor the breaking of spherical symmetry around extreme carbon stars, suchas CIT 6 and IRC +10216, remain uncertain.