Two White Dwarfs Found in Orbital Dance in Planetary Nebula Fleming 1

An international team of astronomers using ESO’s Very Large Telescope has discovered a pair of stars orbiting each other at the center of the remarkable planetary nebula Fleming 1.

This image shows the planetary nebula Fleming 1 in the constellation of Centaurus (ESO / H. Boffin)

Fleming 1 (FG 1 for short), also known as PN G290.5+07.9, ESO 170-6 and Hen 2-66, was discovered in 1910 by Williamina Fleming, a Scottish astronomer from Harvard College Observatory.

It is located in the southern constellation of Centaurus about 10,200 light years away and has strikingly symmetric jets that weave into knotty, curved patterns.

Astronomers have long debated how these symmetric jets could be created.

Now, the team led by Dr Henri Boffin of European Southern Observatory in Santiago, Chile, has combined new observations of the FG 1’s central star with existing computer modeling to explain how these bizarre shapes came about. They found that the nebula is likely to have not one but two white dwarfs at its center, circling each other every 1.2 days. Although binary stars have been found at the hearts of planetary nebulae before, systems with two white dwarfs orbiting each other are very rare.

“The origin of the beautiful and intricate shapes of FG 1 and similar objects has been controversial for many decades,” said Dr Boffin, who co-authored the study published in the journal Science.

“Astronomers have suggested a binary star before, but it was always thought that in this case the pair would be well separated, with an orbital period of tens of years or longer. Thanks to our models and observations, which let us examine this unusual system in great detail and peer right into the heart of the nebula, we found the pair to be several thousand times closer.”

When a star with a mass up to eight times that of the Sun approaches the end of its life, it blows off its outer shells and begins to lose mass. This allows the hot, inner core of the star to radiate strongly, causing this outward-moving cocoon of gas to glow brightly as a planetary nebula.

While stars are spherical, many of these planetary nebulae are strikingly complex, with knots, filaments, and intense jets of material forming intricate patterns. Some of the most spectacular nebulae present point-symmetric structures. For FG 1 it means that the material appears to shoot from both poles of the central region in S-shaped flows.

The study shows that these patterns for the nebula are the result of the close interaction between a pair of stars – the surprising swansong of a stellar couple.

This artist’s impression shows how the two stars at the heart of a planetary nebula like Fleming 1 can control the creation of the spectacular jets of material ejected from the object (ESO / L. Calçada)

“This is the most comprehensive case yet of a binary central star for which simulations have correctly predicted how it shaped the surrounding nebula – and in a truly spectacular fashion,” said study co-author Dr Brent Miszalski of the South African Astronomical Observatory and the Southern African Large Telescope Foundation.

The new observations have also led to the discovery of a knotted ring of material within the inner nebula. Such a ring of material is also known to exist in other families of binary systems, and appears to be a telltale signature of the presence of a stellar couple.

“Our results bring further confirmation of the role played by interaction between pairs of stars to shape, and perhaps even form, planetary nebulae,” Dr Boffin concluded.

_______

Bibliographic information: Henri M. J. Boffin et al. 2012. An Interacting Binary System Powers Precessing Outflows of an Evolved Star. Science, vol. 338, no. 6108, pp. 773-775; doi: 10.1126/science.1225386