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Astronomers discover magnetic jets of a dying star

Jenny Winder, News Writer
Sep 18, 2013, 7:00 UTC

Sen—An international group of scientists have observed, for the first time, a jet of high-energy particles emanating from a dying star.

IRAS 15445−5449 is an old star that lies 23,000 light-years away in the southern constellation Triangulum Australe (the Southern Triangle). It has begun to push out a high-speed magnetic jet of charged particles that glow with radio waves.

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

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. "That's a clue to what makes these jets switch on."

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.

CSIRO observations of a jet of energetic particles emitting radio waves (shown in pink), coming from IRAS 15445-5449. Dusty material around the star (shown in green) was imaged with ESO's Very Large Telescope Interferometer.  Image credits: A. Pérez-Sánchez /ATCA/CSIRO; E. Lagadec/ESO

"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 flowering star is turning into a planetary nebula, one of the most beautiful objects in space. 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.

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, "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."

Roughly half the known planetary nebulae are round blobs. The other half are long and symmetrical, often like a Christmas cracker. How planetary nebulae get their strange shapes has long been a mystery to astronomers.

"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.

The researchers published their results in the journal Monthly Notices of the Royal Astronomical Society.