Researcher Studies the Neutron Star Crust

Dr. Huaiyu Duan, professor at the Department of Physics and Astronomy at the University of New Mexico, along with his colleagues Dr. Reddy at the University of Washington, Seattle, Dr. Carlson at LANL, and student Sajad Abbar are interested in the outer crust of the neutron star. Why study the nueutron star crust? Although it is believed that the outer crust is well described by the existing physics theories, the outer crust connects the observations of the phenomena (such as X-ray bursts) that occur on the surface with the physics at the inner region of the neutron star. Because of the high densities the neutron star crust is actually “cold” in the physics standard even at tens of millions of degrees and quantum effects can be significant.

What exactly is a neutron star and how is it formed? First, one must understand that a star is powered by the nuclear reactor at its center. When the nuclear fuel in the core is exhausted, the star will die and will leave a compact remnant such as a neutron star. A neutron star is an amazing object with matter of more than 1.4 solar masses compressed into a sphere of radius less than 10 miles. In contrast, the Earth has matter of 3 millionth of solar mass and a radius of 4000 miles. Neutron stars are the densest and tiniest stars known to exist in the universe. (Image credit: NASA/Dana Barry)

A neutron star is extremely hot at birth, and its surface temperature can be as high as tens of billions of degrees. It cools down slowly over time, first by radiating neutrinos and then by electromagnetic radiations. If a neutron star has not been disturbed for a long time, the shallow layer of the neutron star is made of the crystal of Iron-56 ions, which is the mostly tightly bound nucleus under normal conditions, immersed into a sea of electrons. This is very similar to the steel we use in ever day life except for one thing -- even on the surface of the neutron star the density is so high that the iron atom is fully pressure ionized and loses all its electrons.

As one goes deeper in the neutron star crust, the pressure increases dramatically such that the crystal is made of nuclei of larger neutron fraction and of more nucleons. This is because, as you can imagine, electrons are “compressed” into the nuclei and convert protons to neutron. Around just one-fifth mile below the neutron star surface the matter is so dense, hundreds of billions of times denser than the water on Earth, and the nuclei are so rich with neutrons that neutrons begin to “drip out” of the nuclei. According to some studies, nuclear matter can take peculiar shapes (such as rods and slabs) in the inner crust, which is between the neutron-drip sphere and the core. The physical conditions inside the core of a neutron star are so extreme that physicists are not sure of its state or constituents.

To summarize the results, Dr. Huaiyu Duan's research uses several methods to compute the thermal conductivity of the neutron star crust. The results of this research sketches the regimes where the quantum effects must be taken into account and provide guidance for future calculations. Duan's research is funded by Los Alamos National Laboratory with support by the New Mexico Consortium.


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