Unusual pair of ageing stars creates spectacular shape of planetary nebula
Astronomers have discovered a pair of stars orbiting each other at the centre of one of the most remarkable examples of a planetary nebula, using ESO's Very Large Telescope.
The new result confirms a long-debated theory about what controls the spectacular and symmetric appearance of the material flung out into space.
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 of 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 (ESO, Chile) has combined new Very Large Telescope (VLT) observations of Fleming 1 with existing computer modelling 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 centre, 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.
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," explained co-author Brent Miszalski, from SAAO and SALT (South Africa).
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 towards the vampire, encircling it with a disc known as an accretion disc.
As the two stars orbited one another, they both interacted with this disc and caused it to behave like a wobbling spinning top -- a type of motion called precession. This movement affects the behaviour of any material that has been pushed outwards from the poles of the system, such as outflowing jets. This study now confirms that precessing accretion discs within binary systems cause the stunningly symmetric patterns around planetary nebulae like Fleming 1.
The results are published in the 9 November 2012 issue of the journal Science.