Stars freeze. Neutron stars, stellar remnants leftover from supernova explosions, have a solid outer later made of remarkable materials. These 'astromaterials' have structures similar to terrestrial crystalline solids and liquid crystals, though they are over a trillion times denser.
The material properties of these astromaterials affect the observable properties of neutron stars, so astromaterials must be understood to interpret astronomical observations. Astromaterial science is an interdisciplinary field, using techniques from material science to study nuclear physics systems with astrophysical relevance.
My research primarily uses large scale computer simulations to calculate the properties of these astromaterials to determine how their presence might affect neutron stars.
For a review of my research, see my article published in Reviews of Modern Physics.
|The Elasticity of Nuclear Pasta||Read|
|Simulations of nuclear pasta find that the deep layers of neutron star crusts may be the strongest known material in the universe.|
|M. E. Caplan, A. S. Schneider, C. J. Horowitz, (2018) arxiv:1807.02557|
|Domains and defects in nuclear "pasta"||Read|
|A companion paper to the one above, this work studies the formation of defects in nuclear pasta and develops an algorithm to identify them, showing how nuclear pasta can buckle over long distances and form many subdomains.|
|A. S. Schneider, M. E. Caplan, D. K. Berry, C. J. Horowitz, (2018) arxiv:1807.00102|
|Polycrystalline Crusts in Accreting Neutron Stars||Read|
|Molecular dynamics simulations and multicomponent phase diagrams show that the ashes produced in X-ray bursts will phase separate to produce a polycrystalline crust, formed of domains with distinct compositions.|
|M. E. Caplan et al (2018), ApJ., 860, 148|
Deep in the crusts of neutron stars, where matter is a trillion times denser than anything on earth, nuclear matter undergoes a phase transition. At depths of approximately one kilometer, directly above the neutron star core, nuclei start to touch. They rearrange and form exotic shapes, such as planar "lasagna" and cylindrical "spaghetti," which have been whimsically named "nuclear pasta."
I study the structure and properties of nuclear pasta to determine the effects it might have on observable neutron star properties. To do this, I use large scale classical molecular dynamics simulations to simulate thousands to millions of nucleons. A sample of these simulations are shown above.
In the outer layers of a neutron star we find a solid crust. Here, the immense pressure and density due to the compact star's gravity is great enough to compress nuclei into a crystalline lattice, even despite the enormous temperature. The structure and composition of these Coulomb crystals will be determined by both astrophysics and nuclear physics while their exact properties are determined by material science, making their study uniquely interdisciplinary.
The figure above shows a molecular dynamics simulation of the phase separation that occurs at the crust-ocean interface in accreting neutron stars. This work finds that neutron star crusts are likely polycrystalline, formed of "compositional domains." These are subcrystals with distinct compositions.
Department of Physics
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