The nascent bipolar nebula surrounding the carbon-rich variable CIT 6: Transition to axisymmetry

CIT 6 is a luminous carbon star currently evolving off the asymptotic giant branch into a tiny proto planetary nebula. The star is surrounded by a thick dust shell that reprocesses much of the photospheric radiation, making it one of the brightest infrared sources in the sky. Spacecraft imaging at optical and near-IR wavelengths demonstrates that the nebulosity is already attaining a bipolar morphology, with strong polarization by scattering tracing out a centrosymmetric pattern around the central star. Long-term monitoring with optical polarimetry, spectroscopy, and spectropolarimetry demonstrates that the optical spectrum is comprised of two major components. The optical-near-IR spectrum longward of similar to6000 Angstrom depicts the photospheric features of the carbon star. This emission varies strongly on the similar to640 day stellar pulsation period and shows broad changes in the degree and position angle of polarization on a timescale of about a decade. At shorter wavelengths, a nearly featureless blue continuum is seen, strongly polarized at roughly right angles to the red component. This component does not vary significantly in either brightness or polarization on the pulsation period. Associating the spectral components with morphological features in the images, it is straightforward to ascribe the red light to emission from the carbon star that is scattered by dust within the nebular lobes. The blue continuum originates in the southern lobe, and the unique polarization of this component arises through scattering in a narrow fan that is observed in b-band images to protrude at right angles from the bipolar axis. While seemingly coincidental, a consistent explanation for this component is a main-sequence companion star of spectral type A-F, buried within the same nebular material. The separation of this probable companion is greater than 40 AU, implying an orbital period greater than 150 yr and making it an unlikely explanation for axially symmetric mass loss from the carbon star. The deepest NICMOS images reveal several faint, diffuse arcs of light 1-4 from the central star that are highly polarized (P similar to 30%-40%) and therefore shine primarily by reflection. These nearly concentric features have characteristic scattering optical depths 0.01 less than or similar to tau(s) less than or similar to 0.1 and individual masses similar to10(-8) to 10(-7) M-circle dot. The arcs attest to a recent period of spherically symmetric mass loss that was modulated by an as yet unknown mechanism. The subsequent transition to a superwind and axial symmetry occurred within the span of only a few hundred years.