1997A&A...328..290S

We present ground based continuum images in the infrared, from 1.2 to 19µm, and an H₂ 2.122µm line emission image of the post-AGB star AFGL 2688, the Cygnus Egg Nebula. We show that the standard model of this source, comprising a fast wind focussed by a dense, equatorial, dusty torus into a bipolar flow at position angle 15° and close to the plane of the sky, cannot explain the combination of kinematic information from previous studies and morphological information in our own observations. Nor are the images consistent with a classical bipolar flow, since the apex of the two lobes observed in scattered light in the visible and near-IR are offset in R.A. with respect to one another. We suggest a model which is physically similar, but substantially different geometrically, in which there is a bipolar flow at a position angle closer to 60°, rather than 15°, still collimated by a dense, equatorial, dusty torus, but the opening angle of the cones out of which the fast bipolar flow is directed is closer to 90°, rather than 20° or so as previously suggested. The bipolar flow axis is inclined by about 20-30°, rather than in the plane of the sky as in previous models. The dust distribution in the nebula has to be extremely clumpy, and there is evidence that large scale mass loss from the progenitor AGB star occurred in discrete phases, recurring on a timescale of ∼750 years. This model implies a much lower velocity for the `fast' bipolar outflow than does the standard model, which is consistent with very recent Nobeyama Millimetre Array images in ¹³CO emission. In support of our new model, we present a full radiative transfer model for the source, in axial symmetry, which reveals that the final phase of heavy mass loss included a superwind phase which lasted about two hundred years and removed about 0.7M_☉ from the envelope of the progenitor AGB star. Our results imply that the progenitor star must have been a relatively high mass AGB star. Our radiative transfer model also demonstrates convincingly that, in contrast with previous models, the core of the nebula has to be exceptionally optically thick, with an optical depth greater than unity even at 10µm.