Bipolar planetary nebulae
Astronomers have used the NTT (New Technology Telescope) of the ESO and the Hubble space telescope of the NASA/ESA to explore more than 100 planetary nebulae in the Galactic bulge. They found that the members of this family of cosmic butterfly-shaped tend to be mysteriously aligned – a surprising result given their different stories and different properties. In the final stages of the life of a star like the Sun’s outer layers are expelled into the surrounding space, forming what are known as planetary nebulae, with a large series of suggestive shapes. A particular type of these nebulae is known as bipolar planetary nebulae, creating ghostly structures like hourglass or butterfly shaped around the parent star.
The nebulae are formed in different places and have different characteristics. And neither the individual nor the nebulae stars that have formed usually interact with other planetary nebulae. A new study of astronomers of the University of Manchester, United Kingdom, shows striking similarities to some of them: many are aligned in the same way in heaven. The “major axis” of a bipolar planetary nebula is what passes between the wings of the butterfly, while “minor” axis passes through the body. “This is a truly amazing and, if confirmed, a very important discovery,” said Bryan Rees an astronomer at the University of Manchester, one of the two authors of the article. “Many of these butterflies seem to have the spectral axis aligned along the plane of the Galaxy. Using Hubble’s images and NTT can resume these items very well and study them in great detail “. Astronomers have observed 130 planetary nebulae in the central bulge of the Milky Way, have identified three different kinds and watched with attention to their characteristics and their appearance. The shapes of planetary nebulae are divided into three categories, by Convention: elliptic, with or without an internal structure aligned and 2-pole.
“While two of these populations the orientation in the sky was completely random, as expected, we found for the third – the bipolar nebulae – a surprising tendency to a particular alignment,” says the second author of the article Albert Zijlstra, also a professor in Astrophysics of the University of Manchester. “If the alignment itself is a surprise, looking in the crowded central region of the Andromeda Galaxy is even more unexpected”. It is thought that planetary nebulae are carved by the rotation of the star system from which is formed. This in turn depends on the properties of the system. The forms of bipolar nebulae are among the most extreme and probably caused by jets that are blowing away from the binary field perpendicular to the orbit. “The alignment that we see in these bipolar nebulae suggests something bizarre about star systems within the central bulge,” said Rees. “To align the way you see it, the system of stars that form these nebulae should rotate so that is perpendicular to the interstellar cloud from which it is formed, which is very strange”.
While the properties of ancestors affect the shape of nebulae, these new findings suggest a factor even more mysterious. Together these stellar complex features are those of the Milky Way: the entire central bulge revolves around the Galactic Center. This bulge might have a greater influence than previously thought across the Galaxy – through its magnetic fields. Astronomers suggest that the ordered performance of planetary nebulae may be caused by the presence of strong magnetic fields during the formation of the bulge.
As such nebulae closest to us doesn’t line up in the same way as ordered, these fields should be much stronger than what you see today in the local universe.
“We can learn a lot by studying these objects”, said Zijlstra. “If they really behave in unexpected ways, this has consequences not only for the past of individual stars, but for that of the Galaxy”.