Odd pair of aging stars sculpt spectacular shape of planetary nebula
The study confirms that precessing accretion disks within binary systems cause the stunningly symmetric patterns around planetary nebulae like Fleming 1.
November 8, 2012
Astronomers using the European Southern Observatory’s (ESO) Very Large Telescope (VLT) have discovered a pair of stars orbiting each other at the center of one of the most remarkable examples of a planetary nebula. The new result confirms a long-debated theory about what controls the spectacular and symmetric appearance of the material flung out into space.
The planetary nebula Fleming 1 seen with ESO’s Very Large Telescope. // Credit: ESO/H. Boffin
Planetary nebulae are glowing shells of gas around white dwarfs — Sun-like stars in the final stages of their lives. Fleming 1 is a beautiful example that has strikingly symmetric jets that weave into knotty, curved patterns. It is located in the southern constellation Centaurus the Centaur and was discovered just over a century ago by Williamina Fleming, a former maid who was hired by Harvard College Observatory after showing an aptitude for astronomy.
Astronomers have long debated how these symmetric jets could be created, but no consensus has been reached. Now, a research team led by Henri Boffin from ESO has combined new VLT observations of Fleming 1 with existing computer modeling to explain in detail for the first time how these bizarre shapes came about.
The team used ESO's VLT to study the light coming from the central star. They found that Fleming 1 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 rare.
"The origin of the beautiful and intricate shapes of Fleming 1 and similar objects has been controversial for many decades," said Boffin. "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, including Fleming 1, present point-symmetric structures. For this planetary nebula, it means that the material appears to shoot from both poles of the central region in S-shaped flows. This new study shows that these patterns for Fleming 1 are the result of the close interaction between a pair of stars — the surprising swansong of a stellar couple.
"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 Brent Miszalski from the South African Astronomical Observatory and the South African Large Telescope near Sutherland.
The pair of stars in the middle of this nebula is vital to explain its observed structure. As the stars aged, they expanded, and for part of this time, one acted as a stellar vampire, sucking material from its companion. This material then flowed in toward the vampire, encircling it with a disk known as an accretion disk. As the two stars orbited each other, they both interacted with this disk and caused it to behave like a wobbling spinning top — a type of motion called precession. This movement affects the behavior of any material that has been pushed outward from the poles of the system, such as outflowing jets. This study now confirms that precessing accretion disks within binary systems cause the stunningly symmetric patterns around planetary nebulae like Fleming 1.
The deep images from the VLT 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 it 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," said Boffin.