Monday, September 16, 2013

Seen For The First Time Magnetic Jets From A Dying Star: How Stars Begin Their Final Transformation

An international team of astronomers have for the first time found a jet of high-energy particles emanating from a dying star. The discovery, by a collaboration of scientists from Sweden, Germany and Australia, is a crucial step in explaining how some of the most beautiful objects in space are formed – and what happens when stars like the sun reach the end of their lives.  

A star is “blooming” in the southern sky — and astronomers using a CSIRO telescope are a step closer to knowing why.

An old star, IRAS 15445-5449, has begun to push out a jet of charged particles that glow with radio waves.

A few old stars are known to have jets, “but this is the first one where the radio waves tell us the jet is held together by a strong magnetic field”, said Dr Jessica Chapman of CSIRO Astronomy and Space Science, a member of the research team.

A jet of energetic particles (shown in magenta) is shaping the environment around the star IRAS 15445-5449. Infrared light from dusty material which the jet has already shaped into a symmetric form is shown in green. The star itself is hidden by dust in its environment. 
iras15445 en small
Credits: E. Lagadec/ESO/A. Pérez Sánchez

“That’s a clue to what makes these jets switch on.”

At the end of their lives, stars like the sun transform into some of the most beautiful objects in space: amazing symmetric clouds of gas called planetary nebulae. But how planetary nebulae get their strange shapes has long been a mystery to astronomers.

The finding has been published online today in the journal Monthly Notices of the Royal Astronomical Society.

IRAS 15445−5449 lies 230 000 light-years away in the southern constellation Triangulum Australe (the Southern Triangle).

The radio waves from the star’s jets were detected with CSIRO’s Australia Telescope Compact Array, an array of six 22-m diameter radio dishes near Narrabri in New South Wales (eastern Australia).

The flowering star is turning into one of the most beautiful objects in space — a “planetary nebula”.

Planetary nebulae are large glowing objects that early astronomers thought looked like planets. In fact they are stars late in their lives that have shed much of their gas into space. The shed gas glows, powered by energy from the old star’s tiny core.

Like Spring flowers, planetary nebulae blossom and go. The “jet” phase seems to show the first stages of the star becoming a planetary nebula. This phase is just a “blink of an eye in a star’s life,” says Dr Chapman, “probably lasting only about a hundred years”.

Roughly half the known planetary nebulae are round blobs. The other half are long and symmetrical, often like a Christmas cracker.

“The question is, what makes this symmetry?” Dr Chapman said.

It could be that the outflowing gas is shaped by the presence of a companion to the old star — another star, or a planet.

A second idea, which Dr Chapman favours, is that it’s the magnetic field of the old star.

“The magnetic field may get twisted up as the star shrinks, perhaps launching these jets,” she said. But more detailed observations are needed to clarify how jets form.

Scientists at Chalmers University of Technology in Sweden have together with colleagues from Germany and Australia discovered what could be the key to the answer: a high-speed, magnetic jet from a dying star.

Using the CSIRO Australia Telescope Compact Array, an array of six 22-metre radio telescopes in New South Wales, Australia, they studied a star at the end of its life. The star, known as IRAS 15445−5449, is in the process of becoming a planetary nebula, and lies 230 000 light years away in the southern constellation Triangulum Australe (the Southern Triangle).

"In our data we found the clear signature of a narrow and extremely energetic jet of a type which has never been seen before in an old, sun-like star", says Andrés Pérez Sánchez, graduate student in astronomy at Bonn University, who led the study.

The strength of the radio waves of different frequencies from the star match the expected signature for a jet of high-energy particles which are, thanks to strong magnetic fields, accelerated up to speeds close to the speed of light. Similar jets have been seen in many other types of astronomical object, from newborn stars to supermassive black holes.

"What we're seeing is a powerful jet of particles spiralling through a strong magnetic field", says Wouter Vlemmings, astronomer at Onsala Space Observatory, Chalmers. "Its brightness indicates that it's in the process of creating a symmetric nebula around the star."

Right now the star is going through a short but dramatic phase in its development, the scientists believe.

"The radio signal from the jet varies in a way that means that it may only last a few decades. Over the course of just a few hundred years the jet can determine how the nebula will look when it finally gets lit up by the star", says team member Jessica Chapman, astronomer at CSIRO in Sydney, Australia.

The scientists don't yet know enough, though, to say whether our sun will create a jet when it dies.

"The star may have an unseen companion – another star or large planet – that helps create the jet. With the help of other front-line radio telescopes, like ALMA, and future facilities like the Square Kilometre Array (SKA), we'll be able to find out just which stars create jets like this one, and how they do it", says Andrés Pérez Sánchez.


Contacts and sources:
Helen Sim
Jessica Chapman
CSIRO
Royal Astronomical Society

Citation: Andres Pérez-Sánchez (Argelander-Institut für Astronomie, University of Bonn, Germany), Wouter Vlemmings (Onsala Space Observatory, Chalmers University of Technology, Sweden), Daniel Tafoya (Onsala Space Observatory, Chalmers University of Technology, Sweden and UNAM, Morelia, Mexico) and Jessica Chapman (CSIRO, Australia). “A synchrotron jet from a post-asymptotic giant branch star”. Monthly Notices of the Royal Astronomical Society http://dx.doi.org/10.1093/mnrasl/slt117

Also online at http://arxiv.org/abs/1308.5970 .

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