2012ApJ...745..188B

We present four-color images of CRL 2688 obtained in 2009 using the Wide-Field Camera 3 on Hubble Space Telescope. The F606W image is compared with archival images in very similar filters to monitor the proper motions of nebular structure. We find that the bright N-S lobes have expanded uniformly by 2.5% and that the ensemble of rings has translated radially by 0".07 in 6.65 yr. The rings were ejected every 100 yr for ∼4 millennia until the lobes formed 250 yr ago. Starlight scattered from the edges of the dark E-W dust lane is coincident with extant H₂ images and leading tips of eight pairs of CO outflows. We interpret this as evidence that fingers lie within geometrically opposite cones of opening angles ~30° like those in CRL618. By combining our results of the rings with ¹²CO absorption from the extended asymptotic giant branch (AGB) wind we ascertain that the rings were ejected at ∼18 km/s with very little variation and that the distance to CRL 2688, v_{exp/{dot}theta}exp_, is 300-350 pc. Our 2009 imaging program included filters that span 0.6-1.6 µm. We constructed a two-dimensional dust scattering model of stellar radiation through CRL 2688 that successfully reproduces the details of the nebular geometry, its integrated spectral energy distribution, and nearly all of its color variations. The model implies that the optical opacity of the lobes ≳ 1, the dust particle density in the rings decreases as radius^–3, and that the mass and momentum of the AGB winds and their rings have increased over time.