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Posts Tagged ‘neutron star’

Super-Dense Stars May Squash Neutrons Into Cubes

16 Aug

Deep inside the super-dense hearts of exploding stars, gravity may squash neutron particles from spheres into cubes.

The idea could mean that neutron stars, as researchers call the stellar corpses, are denser than anyone expected. It could also question what stops them from collapsing into black holes and out of existence.

“If you take this result purely at face value, it means neutron star theoreticians are in trouble. [Neutron stars] should collapse into black holes at lower masses,” said theoretical physicist Felipe Jose Llanes-Estrada of Complutense University of Madrid, co-author of a study published Aug. 9 on the prepublication server arXiv.

“But that’s not what we observe. It’s possible there’s an additional repulsive interaction [between neutrons] to counter a collapse that we just haven’t thought of yet,” he said.

A star between nine and 20 times the sun’s mass detonates as a supernova toward the end of its life. At that weight, a star isn’t heavy enough to create a critical, ultra-dense state and shrink into a black hole. Instead, its core collapses into a sphere no bigger than 15 miles wide and so dense that a single teaspoon of it weighs as much as everyone on Earth, multiplied by 18.

Late last year, astronomers discovered the biggest-ever neutron star, called J1614-2230, that weighed in at 1.97 times the sun’s mass.  Prior to its discovery, the most massive neutron star weighed 1.67 solar masses.

The find left more than a few astrophysicists scratching their heads. Its existence ruled out some models of neutron stars that relied on exotic forms of matter and can’t explain the halt in the collapse of such a heavy object. Instead, the discovery supported models of neutron stars as containing only neutrons and protons.

When Llanes-Estrada and his university colleague Gaspar Moreno Navarro heard of J1614-2230, they wanted to know what might be happening inside of it.

The duo knew of a model from the 1970s suggesting pure neutrons could form a crystal lattice under incredible pressure (similar to how carbon forms diamonds in the bowels of the Earth). When they tweaked a familiar computer model to incorporate the idea, they discovered that — at the pressures anticipated deep in neutron stars — neutrons deformed from spheres into cubes.

“There’s an optimum packing density with spheres, including neutrons. It’s about 74 percent. No matter how efficiently you arrange them, like oranges on display at a supermarket, there’s always space in between,” Llanes-Estrada said. “If you want to be most efficient, you distort the oranges. Pack them a mile high and squish the ones on the bottom.”

Gravity shapes aggregate particles of matter into the simplest, most efficiently-packed object possible, normally a sphere like the Earth. The particles themselves, though, remain individually unaffected; gravity is too weak to overcome the strong interactions that hold neutrons and other particles together. But if gravity becomes intense enough, it might overpower the interactions.

So deep within the newly discovered neutron star — which may have a core pressure two times higher than the rest — a neutron’s most efficient shape may be a cube. “They’ll be flattened on all sides, like dice” starting at pressures found about 2.5 miles below the surface, Llanes-Estrada said.

So far, responses to the study have proven lukewarm.

Particle physicist Richard Hill of the University of Chicago, for example, noted the study looks at a neutron in isolation, not in aggregate.

“It’s an interesting idea, but what happens among the neutrons isn’t clear,” said Hill, who wasn’t involved in the study. At the densities in neutron stars, he noted, the “identities of individual neutrons may be blurred out.”

Llanes-Estrada acknowledged the criticism, which a second physicist who wished to remain anonymous also shared. But Llanes-Estrada said that pushing boundaries was, in part, the point.

“I think there is a large uncertainty of what happens to neutrons at very high compressions,” he said. “We should keep studying all of the possibilities.”

Updated: Aug. 17, 2011; 8:45 a.m. EDT

Images: 1) Illustration of a neutron star. (NASA/JPL-Caltech) 2) As pressure and density in a neutron star go up, normally sphere-like neutrons might take on an increasingly cubic shape. (F.J. Llanes-Estrada and G.M. Navarro/arXiv.org)

Via: MIT Technology Review

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Cold, Dead Stars Could Help Limit Dark Matter

15 Oct

Hunting for cold stellar corpses near the center of the galaxy or in star clusters could put new limits on the properties of dark matter.

“You can exclude a big class of theories that the experiments cannot exclude just by observing the temperature of a neutron star,” said physicist Chris Kouvaris of the University of Southern Denmark, lead author of a paper in the Sept. 28 Physical Review D. “Maybe by observations, which come cheaper than expensive experiments, we might get some clues about dark matter.”

Dark matter is the irritatingly invisible stuff that makes up some 23 percent of the universe, but makes itself known only through its gravitational tug on ordinary matter.

There are several competing theories about what dark matter actually is, but one of the most widely pursued is a hypothetical weakly interacting massive particle (WIMP). Physicists in search of WIMPs have placed experimental detectors deep underground in mines and mountains, and are waiting for a dark matter particle to hit them.

Others have proposed looking for the buildup of dark matter in stars like the sun or white dwarfs. But both subterranean and stellar-detection strategies will light up only for WIMPs larger than a certain size. That size is miniscule — about a trillionth of a quadrillionth of a square centimeter — but dark matter particles could be smaller still.

One way to rule out such diminutive particles is to look to neutron stars, suggest Kouvaris and co-author Peter Tinyakov of the Université Libre de Bruxelles in Belgium.

Neutron stars are the cold, dense remnants of massive stars that died in fiery supernova explosions. They tend to have masses similar to the sun, but in diameter they would barely stretch from one end of Manhattan to the other. This extreme density makes neutron stars exceptionally good nets for dark matter.

“For their size and their temperature, they have the best efficiency in capturing WIMPs,” Kouvaris said. Particles up to 100 times smaller than the ones underground experiments are sensitive to could still make a noticeable difference to neutron stars.

After the fires of their birth, neutron stars slowly cool over millions of years as they radiate photons. But if WIMPs annihilate each other whenever they meet — like a particle of matter meeting a particle of antimatter — as some models suggest they should, dark matter could reheat these cold stars from the inside.

Kouvaris calculated the minimum temperature for a WIMP-burning neutron star, and found it to be about 100,000 kelvins [about 180,000 degrees Fahrenheit]. That’s more than 10 times hotter than the surface of the sun, but more than 100 times cooler than the sun’s fuel-burning interior. It’s also much cooler than any neutron star yet observed.

Dark matter and ordinary matter are thought to clump up in some of the same places, like the center of the galaxy or globular clusters of stars. So Kouvaris and Tinyakov suggest that astronomers try to find a neutron star colder than the minimum temperature in a region with a lot of dark matter floating around.

“If you observe a neutron star with a temp below the one we predict, that excludes a whole class of dark-matter candidates,” Kouvaris said. It could mean the WIMPs are extra-small, or that they don’t annihilate when they meet each other — a property of WIMPs that experiments can’t get at.

“It’s an intriguing idea,” said observational astronomer David Kaplan University of Wisconsin-Milwaukee. “But I’m a little skeptical that it can be done immediately, or even in the near future.”

The center of the galaxy is dusty and difficult to observe, and most globular clusters are so far away that a cold, tiny neutron star hiding inside them would be beyond today’s telescopes. The next generation of ultraviolet telescopes could be up to the task, Kaplan suggests. “But that’s not to say that it will be easy.”

Astronomer Bob Rutledge of McGill University suggests an alternative approach: Rather than squinting for neutron stars’ dim light, astronomers could find them through ripples in space-time called gravitational waves. When two neutron stars merge, they are expected to throw off massive amounts of these waves, and Earth-based detectors like LIGO are already in place to catch them — although no waves have actually shown up yet.

“It would be technically hard, but a sound approach,” Rutledge said. “This sort of thing could become possible in the more distant future.”

Image: Artist’s impression of a neutron star with a powerful magnetic field, called a magnetar. Credit: NASA

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