A jet-like flow of material, detected in the vicinity of a dying star, supports a model in which such jets shape the gas cloud around the star into a bipolar nebula. The jet probably comes from an unseen companion star.
As Sun-like stars exhaust the nuclear fuel in their cores, they become 'red giants', swelling to a size several hundred times that of the Sun. The stars begin to lose mass, emitting a dense wind of gas that, in most red giants, would be expected to be spherically symmetric. Using the Hubble Space Telescope, Sahai et al.1 have discovered a fast wind emanating from the vicinity of a red giant that is collimated, rather than spherical (page 261 of this issue). This, coupled with similar observations of another dying star2, suggests why the gas clouds that form around many red giants — known as planetary nebulae — are elongated, or bipolar. But this latest detection1, around the star V Hydrae as it begins its transition to a planetary nebula, also points more clearly to the origin of the cloud-shaping jets.
The evolution of a Sun-like star follows a particular cycle. For most of the nuclear-active life of such a star, hydrogen fuses to form helium in the star's core; later, helium fuses to form carbon and oxygen. Once the helium in the core is exhausted, nuclear burning continues in two thin shells surrounding the core: an inner shell of helium fusion to carbon and oxygen, and an outer shell of hydrogen fusion to helium. Many other elements are also formed during this last nuclear burning phase.
The size of the core with its two burning shells is typically only a few per cent of the size of the Sun today. But the star swells. As its production of energy reaches a rate 3,000 times higher than that of the Sun, its size might be up to three times the distance of the Earth from the Sun (hence the Earth may be swallowed by the Sun when our star reaches this final stage of its life, in around 7 billion years). To conserve angular momentum, the rotation of these stars must slow down substantially as their radius increases. So, as the red giants are at this stage losing mass at a very high rate, their slow rotation means that the winds they emit should be spherical. Later in the cycle of evolution, all that remains of the star is a hot bare core, which ionizes and heats its surrounding cloud of gas. This is a planetary nebula.
But most planetary nebulae have bipolar structures, rather than the spherical shape that would be the natural result of their formation from an even stream of gas emission by a red giant. They may be elliptical, or have two opposite narrow lobes or bubbles. This nonspherical structure is a puzzle that astronomers have been struggling to solve for more than 20 years. If the winds from most red giants are spherical, what then is the mechanism that shapes the nonspherical planetary nebulae3?
It has been suggested that jets of matter, thrown out in opposite directions by the dying star, are behind it. Not all astronomers agree — after all, many planetary nebulae are not shaped by jets, so these collimated flows cannot be a universal feature in planetary nebulae. There is also dispute over whether the jets, if they exist, are blown by the giant star itself or by a stellar companion. Increasingly, observations and calculations are pointing to stellar companions as the source of such jets.
Many so-called symbiotic binary systems, which contain a red-giant star and a smaller stellar companion, have bipolar nebulae around them that are remarkably similar to some planetary nebulae. This suggests that binary companions could be at the root of the shaping process in both symbiotic nebulae and planetary nebulae4. Some symbiotic systems are known to have jets5,6, and the newly found jets near red-giant stars1,2 strongly support jet shaping of some planetary nebulae as well. Sahai and colleagues' discovery1 is particularly suggestive on this point: the wind speed of the jet from around V Hydrae is much higher than the value expected for material ejected from giant stars; so it seems more likely that the jet is emanating from a much smaller stellar companion as it accretes matter from the dying star. Furthermore, V Hydrae is known to rotate more rapidly than expected, suggesting that its rate of spin is being affected by a companion star.
The existence of jets in planetary nebulae may also help to explain the similarity between bipolar structures in some planetary nebulae and those in some clusters of galaxies — even though the latter are a million times larger. Both can appear to have oppositely balanced pairs of bubbles in their structure (Fig. 1). In galaxy clusters, these are known to be formed by two oppositely ejected jets. The new finding1, then, brings us closer to describing a unified formation mechanism for pairs of bubbles in astrophysical systems, over many orders of magnitude in system size.
Stimulating and significant as this jet discovery is, some links are still missing. The collimated flow detected by Sahai et al.1 is less than three years old — is there also a long-lived jet in that direction, or does this indicate that mass loss is sporadic rather than continuous? A second jet to balance this one would also be expected. As Sahai et al. state, the counter-jet may be obscured by dense gas, but it or its signature should now be sought. And, of course, efforts must be made to find the stellar companion, the suspected source of the jets that will shape the planetary nebula of V Hydrae.
Sahai, R., Morris, M., Knapp, G. R., Young, K. & Barnbaum, C. Nature 426, 261–264 (2003).
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Brocksopp, C. et al. Mon. Not. R. Astron. Soc. (in the press).
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Monthly Notices of the Royal Astronomical Society (2006)
Advances in Space Research (2005)
The Astrophysical Journal (2004)