The Rise of a Zombie Nebula

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Feast your eyes on this gorgeous planetary nebula that has been “re-animated.”

Abell 30, as astronomers call it, glows in X-ray light (shown in purple) and optical (green, blue, and orange). How did such a complex nebula form?

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A planetary nebula signals the quiet death of a star that is not large enough to produce a supernova. Instead, as the star’s core runs out of hydrogen for nuclear fusion, the outer layers puff up and cool down, making the star a red giant. Strong stellar winds blow material from the outer layers into a shell or some other strange shape, such as an hourglass. The stellar core becomes a hot white dwarf star, giving off plenty of ultraviolet radiation that causes the surrounding gas to glow.

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But sometimes, the dead star doesn’t quite stay dead.

In a “born again” planetary nebula, as described in the literature as early as 1983, nuclear fusion reignites some of the helium in the white dwarf star, making carbon and oxygen and creating a new phase of stellar winds.

According to the observations using the Chandra X-Ray Observatory, the Hubble Space Telescope, XMM-Newton, and telescopes at Kitt Peak National Observatory, the initial planetary nebula shell was ejected from the star 12,500 years ago, and the re-animation event occurred only 850 years ago. The stellar winds from the central white dwarf are now powering the diffuse x-ray emission (purple above) as the faster moving wind shocks the slower moving, older material, and it is sculpting the knots seen in the inset (where orange indicates oxygen-rich gas).

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There is also a point-source of x-ray emission that coincides with the central star of the planetary nebula. Though several hypotheses have been considered to explain the presence of such a point source, it is not yet clear how the x-rays are caused. This is an usual system with stellar winds of different speeds from different epochs all interacting and driving the evolution of this peculiar object.

On a historical note, I was a bit confused at the name of this planetary nebula. Usually, when we talk about an object that has been named after the astronomer George Abell, it is for a galaxy cluster. However, he also produced a catalog of planetary nebula using the Palomar Sky Survey plates made with the 48″ Schmidt Telescope. These happened to be in the field of view and were discovered (some, re-discovered) upon closer inspection and were cataloged in 1966.

Abell 30 is one of this rare class of “born-again” planetary nebulae, but I do hope the name “zombie nebula” catches on with further investigations.

Image Credit: Inset X-ray (NASA/CXC/IAA-CSIC/M.Guerrero et al); Inset Optical (NASA/STScI); Widefield X-ray (ESA/XMM-Newton); Widefield Optical (NSF/NOAO/KPNO)

This research was published in Astrophysical Journal, and a preprint is available at arxiv.org.

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