A preplanetary nebula from Hubble and Wide Field Camera 3

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Image credit: ESA/Hubble, NASA

The preplanetary nebula is a distinct evolutionary phase for main sequence stars with less than 8 or 9 solar masses, when the star is moving between the end of its evolution as a giant on the asymptotic giant branch and its existence as the central star of a true planetary nebula – a hot white dwarf. It is a very short-lived phase, between 104 and 105 years. Consequently, examples of objects observed in this phase are rare. Preplanetary nebulae do not shine, but reflect the light of the central star. As the effective temperature of the star increases after leaving the tip of the asymptotic giant branch (AGB), the material in the shells seen above will eventually become ionised, at which point the object completes the transition to a true planetary nebula. Preplanetary nebulae can be thought of as a subset of a more diverse class of post-AGB stars; often the term is reserved for those examples sufficiently close for structure to be resolved by imaging observations such as these. This is the Egg Nebula, AFGL 2688. The distance is uncertain, probably around 1 kpc.

At the center of this image, and hidden in a thick cloud of dust, is the nebula’s central star. While we can’t see the star directly, four searchlight beams of light coming from it shine out through the nebula. It is thought that ring-shaped holes in the thick cocoon of dust, carved by jets coming from the star, let the beams of light emerge through the otherwise opaque cloud. The precise mechanism by which stellar jets produce these holes is not known for certain, but one possible explanation is that a binary star system, rather than a single star, exists at the center of the nebula. The onion-like layered structure of the more diffuse cloud surrounding the central cocoon is caused by periodic bursts of material being ejected from the dying star. The bursts typically occur every few hundred years.

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Previous imaging using NICMOS had revealed the structure of the nebula seen in infrared light (right) where blue corresponds to starlight reflected from dust and red to radiation from hot molecular hydrogen. The concentric structures seen in the main image are material ejected by thermal pulse episodes during prior evolution on the asymptotic giant branch. Moreover, such shells have been observed by Herschel in the far-infrared (image at lower left) and at visible wavelengths in dust-scattered, ambient galactic light, around CW Leonis, an extreme carbon giant which is still on the AGB.

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AFGL 2688 is also carbon-rich, and the progenitor star weighed around three solar masses when on the main sequence. For more images from WFC 3 see WFC 3 at STScI. Note that the term protoplanetary nebula is no longer used in the context of evolved stars, since it more usually now denotes material around young stars which is actively undergoing planet formation.

Update: In new work appearing on arXiv and to be published in the Astrophysical Journal, the proper motions of the material in the AFGL 2688 shells have been studied using archival Hubble data. The study provides additional evidence that the central star is a binary (hidden by a torus of dust and seen edge on) which acts to modulate the outflow in accordance with theoretical predictions.

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