From Medium: “Powerful jets erupted from the Universe’s earliest stars — dispersing the seeds of future stars”

From Medium

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The first generation of stars exploded into supernovas dispersing elements from which later stars formed, new research suggests.

May 8, 2019
Robert Lea

Hundreds of millions of years after the Big Bang, the very first stars flared into the universe as massively bright accumulations of hydrogen and helium gas. Within their cores, extremely powerful thermonuclear reactions forged the first heavier elements — such as carbon, iron, and zinc.

These first stars were likely immense, short-lived fireballs, and until now scientists have assumed that they exploded as similarly spherical supernovae.

Anna Frebel, an associate professor of physics at MIT and a member of MIT’s Kavli Institute for Astrophysics and Space Research and MIT postdoc Rana Ezzeddine suspect these assumptions were incorrect.

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Rana Ezzeddine and Anna Frebel of MIT have observed evidence that the first stars in the universe exploded as an asymmetric supernova, strong enough to scatter heavy elements such as zinc across the early universe. (Melanie Gonick)

In fact, they have found that these first stars may have blown apart in a more powerful, asymmetric fashion — spewing forth jets violent enough to eject heavy elements into neighbouring galaxies. Elements that ultimately served as seeds for the second generation of stars — some of which can still be observed today.

In a paper published today in The Astrophysical Journal, the researchers report a strong abundance of zinc in HE 1327–2326 in an ancient, surviving star that is among the universe’s second generation of stars. They suspect that the star could only have acquired such a large amount of zinc after an asymmetric explosion of one of the very first stars had enriched its birth gas cloud.

Anna Frebel, an associate professor of physics at MIT and a member of MIT’s Kavli Institute for Astrophysics and Space Research, says: “When a star explodes, some proportion of that star gets sucked into a black hole like a vacuum cleaner.

“Only when you have some kind of mechanism, like a jet that can yank out material, can you observe that material later in a next-generation star. And we believe that’s exactly what could have happened here.”

MIT postdoc Rana Ezzeddine, the study’s lead author, explains: “This is the first observational evidence that such an asymmetric supernova took place in the early universe.

“This changes our understanding of how the first stars exploded.”

A sprinkle of elements delivered by jet

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A simulation shows what the first supernovae could have looked like: Instead of spherical as many scientists have assumed, these brilliant explosions may have been asymmetric jets that shot heavy elements such as zinc (green dots) out into the early universe. This simulation shows the shape of the supernova, 50 seconds after the initial explosion. (Melanie Gonick)

HE 1327–2326 was discovered by Frebel in 2005. At the time, the star was the most metal-poor star ever observed — astronomers define ‘metal’ as any element heavier than helium. So HE 1327–2326 had extremely low concentrations of elements heavier than hydrogen and helium — a strong indication that it formed as part of the second generation of stars, at a time when most of the Universe’s heavy element content had yet to be forged.

Frebel says: “The first stars were so massive that they had to explode almost immediately.

“The smaller stars that formed as the second generation are still available today, and they preserve the early material left behind by these first stars. Our star has just a sprinkle of elements heavier than hydrogen and helium, so we know it must have formed as part of the second generation of stars.”

Observing orbits with Hubble

In May of 2016, the team was able to observe the star which orbits within just 5,000 light years of Earth — relatively close on a cosmic scale. Using NASA’s Hubble Space Telescope over two weeks, they recorded the starlight over multiple orbits, using an instrument aboard the telescope — the Cosmic Origins Spectrograph — to measure the minute abundances of various elements within the star.

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Hubble’s Cosmic Origins Spectrograph (COS) instrument (NASA)

The high precision spectrograph is designed to pick up faint ultraviolet light. Some of those wavelengths are absorbed by certain elements, such as zinc. The researchers made a list of heavy elements that they suspected might be within such an ancient star — elements that they planned to look for in the UV data — including silicon, iron, phosphorous, and zinc.

Ezzeddine recalls: “I remember getting the data and seeing this zinc line pop out, and we couldn’t believe it, so we redid the analysis again and again.

“We found that, no matter how we measured it, we got this really strong abundance of zinc.”

Simulating the Zinc signal

The next step for Frebel and Ezzeddine was to contact collaborators in Japan specialising in developing simulations of supernovae and the secondary stars that form in their aftermath.

The researchers ran over 10,000 simulations of supernovae — each with different explosion energies, configurations, and other parameters — finding, that while most of the spherical supernova simulations were able to produce a secondary star with the elemental compositions the researchers observed in HE 1327–2326, none of them could reproduce the zinc signal.

The only simulation that could explain the star’s makeup — particularly its high abundance of zinc — was one of an aspherical, jet-ejecting supernova of a first-generation star. Such a supernova would have been extremely explosive, with a power equivalent to about 10³⁰ times that of a hydrogen bomb.

As Ezzeddine recalls: “We found this first supernova was much more energetic than people have thought before, about five to 10 times more.

“In fact, the previous idea of the existence of a dimmer supernova to explain the second-generation stars may soon need to be retired.”

A new perspective on a crucial point in the Universe’s history

The team’s results may shift scientists’ understanding of reionization, a pivotal period during which the gas in the universe morphed from being completely neutral to ionized — a state that made it possible for galaxies to take shape.

Frebel points out: “People thought from early observations that the first stars were not so bright or energetic, and so when they exploded, they wouldn’t participate much in reionizing the universe.

“We’re in some sense rectifying this picture and showing, maybe the first stars had enough oomph when they exploded, and maybe now they are strong contenders for contributing to reionization, and for wreaking havoc in their own little dwarf galaxies.”

These first supernovae could have also been powerful enough to shoot heavy elements into neighbouring ‘virgin galaxies’ — galaxies that had yet to form any stars of their own.

Frebel adds: “Once you have some heavy elements in a hydrogen and helium gas, you have a much easier time forming stars, especially little ones.

“The working hypothesis is, maybe second generation stars of this kind formed in these polluted virgin systems, and not in the same system as the supernova explosion itself, which is always what we had assumed, without thinking in any other way. So this is opening up a new channel for early star formation.”

See the full article here .

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