Heavy Find: Weighty neutron stars may rule out exotic core

Neutron stars may be weird, but they’re not so strange, a new study reveals.

STELLAR ANATOMY. Existence of superheavy neutron stars sets limits on composition of their inner cores. J. Lattimer, M. Prakash

Crushed by gravity, matter at the cores of neutron stars—the collapsed remains of heavyweight stars—is subject to a combination of enormously high pressure and low temperature that can’t be attained in any laboratory. A thimbleful of the stuff weighs more than the entire human population, and physicists have few clues about how matter behaves under such conditions.

For years, researchers have debated whether a neutron star core is composed predominantly of neutrons and a smattering of protons (and possibly a few other kinds of particles) or if it transforms into something much more exotic. Under gravity’s pull, run-of-the-mill subatomic particles could conceivably turn into individual quarks, the fundamental building blocks of matter. The quark matter would consist not only of the up and down quarks that make up protons and neutrons, but also strange quarks, which aren’t found in ordinary matter. Moreover, because quark matter is squishier than a mix of neutrons and protons, the stuff would collapse to form a black hole at a lower mass than a core of ordinary matter.

Astronomers studying several rapidly rotating neutron stars have now weighed in on the debate. Paulo C. Freire of Arecibo Observatory in Puerto Rico and his colleagues have found what appear to be two of the most massive neutron stars ever recorded. The stars are almost certainly too heavy to have quark cores, Freire reported in Austin, Texas, this week at a meeting of the American Astronomical Society.

He and his collaborators used the Arecibo radio telescope, as well as the Greenbank radio telescope in West Virginia, to study millisecond pulsars, radio wave–emitting neutron stars that rotate hundreds of times a second and orbit a normal-density companion star. The team restricted its 18-year study to those pulsars that reside in Milky Way globular clusters—crowded concentrations of several hundred thousand stars. Because of their close interactions with neighboring stars, pulsars in globular clusters have elongated orbits, a shape that makes it much easier to infer the mass of these superdense stars.

According to Einstein’s theory of general relativity, the direction along which the pulsar’s orbit is elongated should slowly vary, or precess. (Mercury’s precession about the sun was one of the first successful tests of general relativity.) Precession provides a measure of the total mass in a system.

Observations of the precession of the millisecond pulsar PSR B1516+02B, located some 25,000 light-years away in the globular cluster M5, indicate that the pulsar most likely has a mass equivalent to about 1.94 suns. Another pulsar, PSR J1748-2021B, which lies in the slightly more remote globular cluster NGC 6440, could have up to 2.74 times the mass of the sun, Freire says. In comparison, most quark-core models predict that a neutron star should be no heavier than about 1.6 suns. Additional mass would cause the squishy quark-core to collapse into a black hole.

Freire cautions that each measurement has a significant uncertainty, because precession directly reveals the total mass of a pulsar plus its companion star, rather than the mass of the pulsar alone. But in the case of PSR B1516+02B, chance observations with the Hubble Space Telescope indicate that the companion star must be tiny because it can’t be seen, he notes. Previous measurements of two pulsars in the globular cluster Terzan 5, reported by Freire’s team in 2005, also suggest that some neutron stars are heavier than quark models might allow.

The number of observations makes a compelling case for the existence of heavy neutron stars, comments Cole Miller of the University of Maryland in College Park. “We could be fooled in an individual case, but it would take malice from the universe to be fooled in all.”

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