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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, Lonely Planets More Common Than Sun-Like Stars

19 May

By Christopher Dombrowski, Ars Technica

Seems like every week astronomers find a new exoplanet, one that’s the biggest or the smallest or the hottest or most habitable. However, this week astronomers are announcing a truly unique and new class of exoplanets: Jupiter sized planets that are in extremely large orbits or completely unbound from a host star altogether. And there appear to be a lot of them, as these planets seem to be more common than main sequence stars.

Finding a planet that is not associated with a star is no easy task. In the new search, a team of researchers used a technique called gravitational microlensing. As you look at a background field of stars, if an object passes between you and one of the stars, there will be a temporary brightening of that star. This occurs as the gravity of the object bends light around itself, which acts as a lens for light from the background star, hence “gravitational lensing.” Microlensing occurs when the foreground object is too small to create measurable distortion of the background star and only a brightening is observed. This makes it an ideal detector for small, dim objects.

The mass of the lensing object determines the duration of the brightening event — the longer the duration, the more massive. A Jupiter-sized object would produce lensing event with a duration of around one day.

The odds of a microlensing event occurring are exceedingly small, as the lensing object has to line up exactly between you and the background star. To compensate, astronomers looked at 50 millions of stars over several years, which yielded 474 microlensing events. Out of those 474, 10 had durations of less than two days, consistent with a Jupiter mass object.

No host stars were observed within 10 astronomical units of the lensing object. Previous work from The Gemini Planet Imager had set limits of the population of Jupiter-sized planets in extended orbits. From that data, the astronomers were able to estimate that 75 percent of their observed planets were most likely not bound to a host star at all, and are instead loose within the galaxy.

By creating a galactic-mass density model that takes into account this new class of object, astronomers were able to predict how many of these unbound planets there might be. They found that there are ~1.8 times as many unbound Jupiter-sized object as there are main sequence stars in our galaxy.

This raises a number of questions. Did these planets from near a star only to be ejected from the system? And if they truly have never been bound to any stars, do these planets represent a new planetary formation process? In any case, these observations have discovered a whole new population of Jupiter-sized planets in the Milky Way, and there are a lot of them.

I wonder if these new planets are like our Jupiter and, like our Jupiter, have moons which are geologically active and warm. If so, these new planets may have significantly increased the number of places that life may exist.

Image: NASA/JPL-Caltech [full-resolution image]

Citation: “Unbound or distant planetary mass population detected by gravitational microlensing.” The Microlensing Observations in Astrophysics (MOA) Collaboration and The Optical Gravitational Lensing Experiment (OGLE) Collaboration. Nature, Vol. 473, Pg. 349–352, 19 May 2011. DOI: 10.1038/nature10092

Source: Ars Technica

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