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Astron. Astrophys. 328, 290-310


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The remarkable asymmetric outflow from the Cygnus Egg Nebula

C.J. Skinner1,2,3, M. Meixner4,5,12, M.J. Barlow6, A.J. Collison7, K. Justtanont8, P. Blanco9, R. Piña9, J.R. Ball3,10,12, E. Keto11,12, J.F. Arens10, and J.G. Jernigan10

1Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA;
on assignment from the Space Sciences Department of the European Space Agency
(email: skinner@stsci.edu)
2Laboratory for Experimental Astrophysics, Lawrence Livermore National Laboratory
3Institute of Geophysics & Planetary Physics, Lawrence Livermore National Laboratory
4Department of Astronomy, University of California, Berkeley
5Department of Astronomy, MC221, 1002 W. Green Street, University of Illinois, Urbana, Il 61801, USA
6Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
7Department of Physics, Loomis Laboratory, 1110 W. Green Street, University of Illinois, Urbana, Il 61801, USA
8NASA-Ames Research Center, MS245-3, Moffett Field, CA 94035, USA
9Center for Astrophysics & Space Science, Code 0111, University of California San Diego, La Jolla, CA 92093-0111, USA
10Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
11Lawrence Livermore National Laboratory, L-59, P.O.Box 808, Livermore, CA 94551-9900, USA
12Visiting astronomer, NASA Infrared Telescope Facility.

Received 11 August 1996 / Accepted 20 May 1997

Abstract

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 AFGL2688, 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$^\circ$ 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$^\circ$, rather than 15$^\circ$, 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$^\circ$, rather than 20$^\circ$ or so as previously suggested. The bipolar flow axis is inclined by about 20-30$^\circ$, 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 $\sim$750 years. This model implies a much lower velocity for the 'fast' bipolar outflow than does the standard model, which is consistent with very recent Nobeyama Millimetre Array images in 13CO emission. In support of our new model, we present a full radiative transfer model for the source, in axial symmetry, which reveals that the final phase of heavy mass loss included a superwind phase which lasted about two hundred years and removed about 0.7 M$_{\odot}$ 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.

Key words: circumstellar matter - stars: evolution - stars: individual (AFGL2688) - stars: AGB and post-AGB - stars: mass-loss - infrared: stars

Send offprint requests to: C.J. Skinner


© European Southern Observatory (ESO) 1997

Online publication: October 30, 1997
Last change: March 24, 1998
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