Rotating jets solve ‘merry-go-round’ problem of star formation
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Stars are born from collapsing clouds of gas and dust. But the material in the cloud doesn’t directly fall into the newborn star. Instead, it first heaps up in a flattened, rotating disk. And that poses a problem to astronomers. The disk rotates pretty fast, so how does the gas and dust manage to fall into the star at all?
Rotating stuff has a hard time moving inward. You may have experienced that yourself on a merry-go-round. If it’s rotating fast, you will have trouble making your way toward the central axis. Likewise, you would expect gas in the disk to have trouble falling into the central star. It just has too much rotational energy. Or, as astronomers say: it has too much angular momentum.
Apart from a rotating disk of material (known as an ‘accretion disk’), most young stars also exhibit two narrow jets of fast-moving gas. The jets point in opposite directions, perpendicular to the disk. Apparently, some of the gas from the inner parts of the disk is blown away into space at high speeds.
Astronomers have speculated that those jets could solve the angular momentum issue. Who knows, the jets may somehow carry away enough rotational energy. The result would be that the inner parts of the disk would have no problem falling into the star.
New ALMA observations of the jets of a young star seem to confirm this idea. The material in the jets isn’t flowing ‘straight’, like water from a garden hose. Instead, while the material is rapidly blown into space, the jets are also rotating, more or less like a drill.
The discovery reveals that the jets of protostars do indeed carry away rotational energy (angular momentum) from the inner parts of the accretion disk. As a result, it will be much easier for the gas at the inner edge of the disk to fall towards the newborn star. Problem solved!
What?
ALMA observed a protostar known as HH212, at a distance of 1,300 light-years in the constellation Orion. HH212 is very young: it was born some 40,000 years ago. It’s also light-weight: about 20 percent of the mass of our own Sun. The accretion disk and the jets of the protostar had been seen before. From Earth, the accretion disk is seen more or less edge-on. It’s about twice as large as the orbit of the planet Neptune in our own solar system. Because of dark material at the central plane, the disk looks a bit like a hamburger. The disk and the two jets have now been studied in much more detail by ALMA.
Who?
HH212 was observed by a team of astronomers led by Chin-Fei Lee and Paul Ho of the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan. They worked together with colleagues from Taiwan and the United States. The discovery of rotating jets has been published in a professional astronomy magazine, called Nature Astronomy.
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