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First Stars Were Spinning Tops

27 April 2011 1:00 pm
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Adapted from A. Stacy et al., MNRAS, 413 (2011)

Twirling wildly. A simulation of a fast-spinning early star.

The first stars born in the universe are believed to have been massive objects, up to hundreds of times bigger than the sun. They were also spinning tops or "spinstars," according to a new study, that spent their lives whirling at incredible speeds. If true, astronomers may one day be able to glimpse the final days of these early suns.

Astronomers have yet to catch a glimpse of the earliest stars, which formed some 300 million years after the Big Bang and burned out by the time the universe was 1 billion years old. Made entirely of hydrogen and helium, these stars produced heavier elements in the process of consuming their fuel and ultimately died in explosions that spewed out the newly forged elements into interstellar space. Those elements were incorporated into later generations of stars. By measuring the relative proportions of heavy elements in second- and third-generation stars, many of which have survived to this day, astronomers can make inferences about their now-extinct ancestors.

That's exactly what a team led by Cristina Chiappini of the Leibniz Institute for Astrophysics in Potsdam, Germany, set out to do by analyzing the ratio of different elements in eight stars from NGC 6522, one of the oldest globular clusters in the Milky Way. The cluster is more than 12 billion years old, which means the stars that the researchers looked at formed only a few hundred million years after the death of first-generation stars. Studying the spectra of the stars, the researchers found unexpectedly high abundances of the heavy elements strontium (Sr) and yttrium (Y). Based on the other characteristics of the stars, it was evident that these two elements had not been made within the stars themselves but were likely present in the interstellar clouds from which the stars originated.

The researchers knew from theory that rare elements such as Sr and Y are forged at higher rates in rotating stars as a result of mixing between outer and inner layers of gas within the star. The nuclear reactions that result from this gas mixing produce a large supply of neutrons that are captured by the nuclei of heavy elements such as iron to make Sr and Y. Chiappini and her colleagues found that the best way to explain the pattern of abundances they had observed was to apply a stellar model involving a spinning velocity of 500 kilometers per second at the surface. In other words, the ancestral stars that spawned the eight stars observed in the study were likely to have been spinning at that velocity, the authors report online today in Nature. That's 250 times as fast as the sun.

If the first stars were indeed rapid spinners, they are likely to have ended their lives with a huge Gamma Ray Burst (GRB), producing an enormous flash of high-energy radiation. That augurs well for astronomers hoping to watch the first stars in the act of dying. "I think we have little hope of detecting individual first stars directly in the distant universe, but GRBs can be seen much further away than individual stars," says Jason Tumlinson, an astronomer at Space Telescope Science Institute in Baltimore, Maryland. "A higher frequency of bursts increases the chances of seeing the first generations directly," he says.

Volker Bromm, a theoretical astrophysicist at the University of Texas, Austin, says rapid rotation "could also lead to a special class of energetic supernova explosions called hypernovae, with unusual chemical abundance patterns." And, he says, high rates of spinning would induce deep mixing of currents inside the star, causing the star to evolve into a chemically homogeneous object. Future missions such as the Joint Astrophysics Nascent Universe Satellite—a small explorer mission being considered by NASA—could give astronomers their first look at gamma-ray bursts produced by these first-generation stellar objects.

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