Stellar Egg Reveals Organic-Rich Shell

June 9, 2009 -- The egg-shaped remains of a dying star has a hollow shell that is "unusually" rich in the complex organic molecules, say Australian and Vietnamese researchers.

The study, from the current issue of The Astrophysical Journal, used the Very Large Array (VLA) in New Mexico to probe the interior of the Egg Nebula, also known as CRL2688.

The nebula formed from the death throes of a red giant star that was slightly hotter than the sun and is located about 1370 light-years away. Although hidden by dust, the star inside the center of the nebula is still active, shooting fast jets of material from its poles.

These jets have blown away the material inside the nebula, leaving cavities and producing a "clumpy hollow shell, [which is] fragmented into several large blobs or arcs with missing portions", said Dinh-V-Trung and Jeremy Lim, both currently working at the Institute of Astronomy and Astrophysics, Academia Sinica in Taiwan.

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The researchers used the VLA to trace the emission lines of the complex organic molecule cyanoacetylene. Cyanoacetylene forms as a result of photochemical reactions in the outer shell of the carbon-rich expanding envelope.

Curiously, the molecule is also found in the interior of the Egg Nebula, indicating that the physical conditions of the star are unusual or there is another, as yet undiscovered way for the cyanoacetylene to form.

"In principle, photochemistry can operate only in the outer shell," said Dinh-V-Trung

According to Dinh-V-Trung, the inner envelope is very dense with a lot of gas and dust, so ultraviolet light shouldn't be able to penetrate this inner region.

"With no UV photon in the inner region, there will be no photochemistry and we expect, according to current chemical models, that very little [cyanoacetylene] should exist," he said.

The researchers also found evidence for jets around the equator of the star, indicating the central star may have one or more companions.

"The Egg ... is unusual in the sense that there are several pairs of jets seen along the famous optical lobes and also in the dark lane oriented perpendicular to these optical lobes," said Dihn-V-Trung. "That is very odd unless there are several binary companions around the central star."

Astrophysicist Tim Bedding from the University of Sydney agrees that the shape of the nebula probably indicates a binary in its center. He says the research represents an important piece of the puzzle in understanding "how stars die and the mess they make when they do so."

"The Egg Nebula is an extreme example of how beautiful and exotic planetary nebula can be," said Bedding. "These objects are all unique and different; a lot are spherical, and some are weird like this one. There's some fascinating chemistry going on inside. It shows that things in space are often more complicated than you think."

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