For decades, astronomers have suspected that planetary nebulae—dazzlingly colorful shrouds of gas cast off by dying stars—owe their weird, often symmetric shapes to the sculpting magnetic forces of two stars orbiting each other at the nebula’s center. Now, a new study has helped confirm theorists’ picture that a binary pair, orbiting each other so closely that they share the same atmosphere, explains many nebulae.
There are more than a thousand known planetary nebulae, and few have the simple, spherical shape that would be expected from the dying gasps of a solitary star expelling its outer layers. Instead, they often look like hourglasses or butterflies. For a binary pair to produce that shape, the nebula’s long axis of symmetry should point in the same direction as the axis around which the stars orbit.
So Todd Hillwig, an astronomer at Valparaiso University in Indiana, and his colleagues set out to find planetary nebulae with binary stars at their center where both axes could be measured. Doing so requires plenty of telescope time and a decent view through the nebula to track the stars’ orbits, plus a good understanding of the nebula’s shape. As a result, the team could only identify eight systems. But in each, allowing for uncertainties, the axes point the same way.
“Eight doesn’t seem like a lot,” he says. “But once we get to the kind of statistics we have here it becomes very convincing that there’s a real connection.” There’s only about a one-in-a-million chance that all the systems just happen to be aligned, according to the paper, which has been accepted for publication at The Astrophysical Journal.
“The conclusions of this work are on quite solid grounds,” says Henri Boffin, an astronomer at the European Southern Observatory in Garching, Germany, who did not participate in the work. Another outsider, Noam Soker, a theorist at the Technion-Israel Institute of Technology in Haifa, says the result is a big development. “It tells us, look: What shapes the planetary nebula is a binary system.”
That conclusion has been 4 decades coming. In 1975, astronomer Howard Bond, a co-author on Hillwig’s paper, spent a cloudy night at Kitt Peak National Observatory near Tucson, Arizona, browsing through the library. In reference materials, he noticed two objects suspiciously close to each other on the sky: UU Sagittae, an eclipsing binary pair discovered in 1932; and Abell 63, a planetary nebula found in 1966.
Bond soon confirmed that the nebula and stars were in one and the same place, making them the first example of a binary pair surrounded by a symmetrical nebula. But the two stars of UU Sagittae—a small dwarf star and the hot, shrunken remnant of a red giant that had recently shed its outer layers—were locked in an improbably close orbit, Bond says. “You have the core of a red giant, orbited by another star so close together that the star would have been inside the red giant.”
In 1976, Bohdan Paczyński, an astrophysicist at the Polish Academy of Sciences in Warsaw who was unaware of Bond’s discovery, predicted how such a binary pair could end up inside a nebula. Later, theorists would sketch out how these pairs might sculpt the gas. His starting point was a red giant with a companion star. When the giant sheds its envelope, it engulfs its partner, which spirals inward until it closely orbits the red giant core, gathering mass to itself in a disk in the same plane of the orbit. Magnetic fields in the disk send jets of material spurting from its poles, which then blow the outer atmosphere of the red giant into an elongated nebula, perhaps cinched around the waist by rings. That nebula glows, blasted by ultraviolet radiation from the exposed red giant core.
The details of how such “common envelope” systems work are still fuzzy, according to Soker. “Actually, it’s embarrassing how little we know about the common envelope evolution,” he says. But Hillwig’s work makes it clear that these turbulent binaries are at work in many planetary nebulae.
For Bond, the finding is bittersweet. On the one hand, he’s pleased to have observations that help confirm his predictions from 40 years ago. On the other hand, the link suggests that a single-star solar system like our own may end with a disappointing fizzle, contrary to the spectacular nebula many textbooks forecast, he says.
“We have this disappointment that maybe our own sun won’t participate in making a spectacular butterfly, or a cigar-shaped nebula.”