Tyger! Tyger! burning bright
In the forests of the night,
What immortal hand or eye
Could frame thy fearful symmetry?
— William Blake, Songs of Experience (1794)
A team of astronomers led by Brent Miszalski of the Anglo-Australian Observatory has settled a 30-year argument, ruling out a one-size-fits-all mechanism for shaping some of the most beautiful objects in space—“planetary nebulae”.
Miszalski will present this work, published in Astronomy & Astrophysics (1), on 17 June in an invited talk at a meeting on planetary nebulae held in Rochester, New York(2).
Planetary nebulae have nothing to do with planets. Rather, they are enlarged glowing objects that early astronomers thought looked like planets. In fact they are stars late in their lives that have shed much of their gas into space.
Some planetary nebulae appear as round blobs. But usually we see the gas sculpted into a symmetrical form aligned along an axis, making shapes that can look like an eye, an ant or an hourglass.
The question is, what forms these shapes?
There have been two ideas. One is that a planetary nebula arises from a single star in the late stage of its life (a so-called asymptotic giant branch or AGB star), and that the magnetic field of this star controls the shape of the outflowing gas.
But astronomers now think the star’s magnetic field wouldn’t last long enough to do the trick.
The second idea is that two stars are involved. One aged star provides the gas. Its orbiting companion might “spin up” the primary star, thus boosting the strength of its magnetic field. Or the companion might create a belt of material, an “accretion disk”, around the first star, giving it an axis of symmetry.
Previous researchers have argued that most planetary nebulae are shaped in this way, by the influence of a companion star.
The “binary” (two-star) model has been discussed for thirty years. Miszalski and his colleagues have now ruled it out as a general mechanism for shaping the majority of planetary nebulae.
If most planetary nebulae were shaped by binaries, you’d expect to see a lot of them with a binary star system at their centre. So Miszalski’s team went looking.
They drew on a number of catalogues of planetary nebulae, particularly those derived from the Macquarie/AAO/Strasbourg H-alpha surveys, to amass a collection of hundreds of possible central stars in planetary nebulae. Using the OGLE-III photometric survey, they then looked for periodic variability in the light of the central star, which could be a sign that the central star was, in fact, a binary—two stars orbiting each other.
After refining their sample by knocking out dubious objects, the team was left with a set of about 300 planetary nebulae, of which 21 showed evidence for a central binary. This more than doubled the number of planetary nebulae known to have central binaries.
The team concluded that close binaries occur in 10-20% of planetary nebulae. Their figure agrees with, and provides the first independent support for, an estimate of 10-15% published by Howard Bond and his colleagues in 2000.
The team also found that the periods of the detected binaries were much shorter than had been predicted. “Why, we don’t know yet,” Miszalski said. “All we can say is that the existing models don’t well match what we’ve found.”
“Binarity is not a precondition for the formation of planetary nebulae and … close binaries do not play a dominant role in the shaping of nebular morphologies,” Miszalski’s team concludes in its research paper, which has been published in the journal Astronomy & Astrophysics.
Anglo-Australian Observatory, Macquarie University and Université de Strasbourg
Contactable only by email while travelling:
Dr Quentin Parker
+61 2 9850 8910
+61 (0)408 640 092
Miszalski, B.; Acker, A.; Moffat, A. F. J.; Parker, Q. A.; Udalski, A. “Binary planetary nebulae nuclei towards the Galactic bulge. I. Sample discovery, period distribution, and binary fraction.” Astronomy and Astrophysics, Volume 496, Issue 3, 2009, pp. 813-825
Towards Understanding Asymmetric Planetary Nebulae: Strategic Research Collaborations. Rochester NY, 17-19 June 2009. Contact: Adam Frank, firstname.lastname@example.org