The cosmos is green: Researchers catch nature in the act of 'recycling' a star (w/Animations)

May 21, 2009
Neutron star with accretion disk (left) drawing material from companion star (right). CREDIT: Bill Saxton, NRAO/AUI/NSF

( -- For the first time, researchers have observed a singular cosmic act of rebirth: the transformation of an ordinary, slow-rotating pulsar into a superfast millisecond pulsar with an almost infinitely extended lifespan.

The discovery was made during a large radio sky survey by an international team of astrophysicists at McGill University, the University of British Columbia (UBC), West Virginia University, the U.S. (NRAO) and several other institutions in the United States, the Netherlands and Australia.

The sky survey used the Robert C. Byrd radio telescope at Green Bank, West Virginia to observe nearly a third of the celestial sphere. The team's results will be published online by the journal Science on May 21.

Animation of orbits of neutron star with accretion disk and "normal" companion star during mass-transfer phase. ANIMATION CREDIT: Bill Saxton, NRAO/AUI/NSF

The discovery was made by astrophysics PhD candidate Anne Archibald and her supervisor, Prof. Victoria Kaspi of the McGill Pulsar Group. "This survey has found many new pulsars, but this one is truly special -- it is a very freshly 'recycled' pulsar that is emerging straight from the recycling plant." said Archibald. The McGill researchers worked with Asst. Prof. Ingrid Stairs of UBC and Scott Ransom of NRAO as well as others from the collaboration to carry out more observations of this unusual pulsar.

Pulsars are rapidly rotating, highly magnetized , the remnants left after massive stars have exploded as supernovae. Pulsars emit lighthouse-like beams of that sweep around as the star rotates. Most rotate relatively slowly, ten times a second or less, and their magnetic fields ordinarily slow them down even further over the course of millennia. Millisecond pulsars, however, rotate hundreds of times a second.

Animation of J1023 binary system showing period of mass transfer from companion star to the neutron star, then cessation of mass transfer and turn-on of radio pulsar. ANIMATION CREDIT: Bill Saxton, NRAO/AUI/NSF

"We know normal pulsars typically pulsate in the for one million to ten million years, but eventually they slow down enough to die out," explained Kaspi. "But a few of these old pulsars get 'recycled' into millisecond pulsars. They end up spinning extremely fast, and then they can pulsate forever. How does nature manage to be so green?"

It has long been theorized that millisecond pulsars are created in double-star systems when matter from the companion star falls into the pulsar's gravity well and increases the rotation speed, but until now the process has never been observed directly.

"Imagine a ping-pong ball in the bathtub, and then you take the plug out of the drain," explained Archibald. "All the water swirling around the ping-pong ball suddenly makes it spin a lot faster than when it was just bobbing on the surface.

"We've seen systems that are undergoing spin-up, because when the matter is falling in, the stars get really bright in X-rays and they're easy to see," she added. "But we've never seen radio pulsations from these stars during the process of spin-up. At last we've found a true radio pulsar that shows direct evidence for having just been recycled."

The pulsar found by the survey team was fortuitously observed by an independent, optical research group to have had swirling matter surrounding it roughly a decade ago -- the blink of an eye in astronomical time. That group recorded the observation as puzzling, never dreaming that a full-fledged radio would emerge.

"In other words, for the first time, we have caught a glimpse at an actual cosmic recycling factory in action," said Ingrid Stairs of UBC, who has been visiting the Australia Telescope National Facility and Swinburne University of Technology this year. "This system gives us an unparalleled cosmic laboratory for studying how millisecond pulsars evolve and get reborn."

Source: McGill University (news : web)

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User comments : 12

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3.7 / 5 (3) May 21, 2009
The amount of momentum transferred in such a short time (10 years?) is staggering.
1 / 5 (5) May 21, 2009

Nice animation, but less exciting than reality.

The birth of the solar system was similar but more exciting. See "Strange xenon, extinct superheavy elements, and the solar neutrino puzzle," Science, vol. 195, Jan. 14, 1977, pp. 208-210 and "Isotopes of tellurium, xenon and krypton in the Allende meteorite retain record of nucleosynthesis", Nature 277, 615-620 (1979); doi:10.1038/277615a0
"Isotopic ratios in Jupiter confirm intra-solar diffusion",
Meteoritics and Planetary Sci. 33, A97, abstract 5011 (1998).

With kind regards,
Oliver K. Manuel
not rated yet May 21, 2009
2.6 / 5 (5) May 22, 2009
"with an almost infinitely extended lifespan...." and "...they can pulsate forever" ??? Very unscientific and quite rediculous statements.
not rated yet May 22, 2009
My understanding is that when a massive star goes supernovae, then what remains, in this case a pulsar, is left sitting alone at the center of the nebula that the explosion creates. Something like the Cat's Eye Nebula, though that just has a white dwarf at the center, so it wasn't a massive star to begin with. My point is that it is sitting there alone. Does this mean that the companion star was captured as it was passing, by the pulsar's gravity, and that it's gas started being sucked up by the pulsar creating the ecretion disk around the pulsar? If something like that is happening, then I agree with Honor, and don't see the recycling. It seems like the story of recycling would be somehow related to the nebular gas from the original massive star, which the research team does not even mention. Perhaps someone can help me with my confusion? Thank you.
5 / 5 (1) May 22, 2009
I have to agree with Vsha, because at some point the companion star will explode, and wipe out the pulsar. Unless it is now able to push away the comapnion star, and send it back on its way. Even so, what is "almost infintely"?
3 / 5 (2) May 22, 2009
What a stupid, obvious headline! We are all 'recycled' stars.
"Ashes to ashes, dust to dust."
4.5 / 5 (2) May 22, 2009
Anybody know what braking force is operating on the matter from the companion star, allowing the pulsar to accrete it?
5 / 5 (1) May 22, 2009
I would think frictional dynamics within the accretion disk itself would be the predominant braking force for matter trying to rain down onto the WD's surface. At the same time, I sure the sheer EM output from the surface of the super hot white dwarf would would interact with the inner accretion disk. That this accretion disk was observed for a short period and then vanished around the spun-up WD is both fortuitous and a pretty strong case for spun-up compact objects (white dwarfs, millisecond pulsars, etc.) forming through some type of accretion from an orbiting donor objects. BTW-white dwarfs are not formed by supernova explosions. They are the core remnants of low-mass cooling stars.
not rated yet May 22, 2009
Firstly I understood this was a neutron star, not a white dwarf.

I would have thought that frictional forces would have taken longer than 10 years to dissipate so much energy. Presumably the pulsar was already spinning very fast before this particular accretion disk was 'digested'. Either that, or we've been very lucky in catching the very last 10 year's 'digestion' at the tail end of a much longer process.

Also, after having expended the energy to get down to the pulsar's surface, why is there so much angular momentum left over to speed the pulsar's rate of rotation to such a pitch? I'd have thought that the orbital speed of the infalling matter would be similar to the original rate of rotation of the star, when it first became a neutron star? In which case the neutron star started life as a millisecond pulsar and the accretion disk just served to keep it going like a 'whip and top'.
not rated yet May 22, 2009
I thought the pulsar used the accretion disk as a power source to increase its rotation speed. As though the accretion disk was a power cord that had been used to transfer energy from the companion star.
not rated yet May 23, 2009
The reason for faster spinning is that angular momentum is preserved. If something is rotating with long radius and then the radius is made smaller, the speed of that rotating increases in terms of arc seconds per second. This is similar to the usual phenomena observed with rotating figure skaters. It does not matter what the mass eater is, it must preserve the momentum it gets while it's collecting matter.

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