Neutron stars—the tiny collapsed remains of the cores of stars much more massive than the Sun—are remarkably complex systems. The inner layers are composed of a form of matter that exists nowhere else in the Universe, while the outermost layer is a "crust" of heavy atomic nuclei. When any matter falls onto the neutron star's surface, it can result in thermonuclear explosions and other, stranger energetic events.

A new study of neutron star nuclear physics showed that these objects might be even weirder than previously thought. H. Schatz and colleagues modeled reactions in the crust and found that much of the energy there is carried away by neutrinos. This involves processes known from exploding white dwarfs (the type Ia supernovas) but which had never been thought to take place in neutron stars. The surprising result is a decoupling between thermonuclear explosions outside the neutron star and reactions deep inside the crust.

Neutron stars and pulsars

Neutron stars are best known when they act as pulsars, where the intense magnetic fields channel matter into beams, which sweep across our field of view, causing regular pulses of light. While it's highly probable that all neutron stars are pulsars (many just don't point their pulses at Earth), the "neutron star" label is the more general term, since "pulsar" describes an observational behavior rather than the physical character of the object.



When a star roughly 8 times more massive than the Sun runs out of nuclear fuel, its core collapses while the outer layers are blown out in a supernova explosion. If the star is not too massive, the core remnant will stabilize before becoming a black hole. The object's self-gravity is strong enough to crush atoms and nuclei into a dense material that we don't think is found anywhere else in the cosmos. Because much of this material is composed of neutrons, the core remnant is known as a neutron star.

Neutron stars pack a mass equivalent to a moderately sized star into a sphere roughly 20 kilometers in diameter. The outermost kilometer is the crust, a solid shell of neutron-rich nuclei prevented from melting by high pressure.

Matter that falls onto a neutron star reaches very high density and pressure, sufficient to create nuclear reactions. Since the reactions happen at the surface, however, they are not contained as they are in the cores of stars: they are uncontrolled thermonuclear explosions, which astronomers observe as X-ray bursts. The "ashes" of these reactions—nuclei resulting from the explosions—fall onto the neutron star surface, adding to the crust.

The crust is very dynamic because it's affected by these ashes, the heat from the neutron star interior, and nuclear processes inside the crust itself. Because of its relatively low temperature (at least compared to the interior and the infalling matter), the most widely used model assumes a temperature of absolute zero for simplicity. That's not as crazy as it sounds: matter at these densities is dominated by quantum effects, and the combination of high density and pressure makes materials behave much like they do at very low temperature. This model also (reasonably) assumes that the types of nuclei are stratified based on atomic mass so that a given layer in the crust is composed of a single type of nucleus.

However, to achieve a full understanding of neutron star behavior (including X-ray bursts and "glitches"), astronomers need to know which approximations are justified and which lead to errors. That was the motivation behind the present study, which examined nuclear processes inside the crust at higher temperatures and with a mixture of nuclei in each layer.

The researchers found that the crust was divided into three distinct regions. The deepest region involved electron capture reactions, where a nucleus absorbs a free electron, converts a proton into a neutron, and shifts its chemical identity down the periodic table. Above that was a very active shell, where both electron capture and beta decay (capture in reverse) could occur. That region produced a huge number of neutrinos: fast-moving, very low-mass particles with no electric charge.

Because neutrinos pass through pretty much everything unimpeded, they constitute a very efficient cooling mechanism for a neutron star, carrying energy away from the outermost layer of the crust. This type of energy transfer is known as the Urca process, named for a casino in Rio de Janeiro. (The joke is that the neutrinos carry energy away like money flows out of your pocket at the roulette table.) The Urca process is expected to take place in type Ia supernovas, which are the explosions of white dwarfs, but because it requires non-zero temperature, nobody had included it in models for neutron star crusts.

The Urca process has observable consequences, and the new model suggests that there will be differences in X-ray burst activity that we can look for.

However, the inclusion of more realistic physical conditions raises another problem: in this new model, the neutron star surface (perhaps ironically) was now much less hot than in the earlier zero-temperature approximations. Yet material accreted by the neutron star must still be hot enough for thermonuclear explosions. The researchers cautiously proposed that there might be a new, unknown process responsible for heating material near the surface.

Nature, 2013. DOI: 10.1038/nature12757 (About DOIs).