Thursday, March 09, 2006

My niece the astrophysicist

...has published her dissertation. I'm not linking, but here is part of it.... [I've had to rewrite some of the numbers because I don't know how to code it properly, so please forgive the non-science stuff in brackets]:

1.5.1 Nuclear Pasta

At the beginning of collapse, when the central density is comparitively low, the nuclei behave as a gas and are spherical. As the density increases and the transition to nuclear matter takes place, between [~ 0.1 Ps], the nuclei minimize their energy by changing shape. This is known as the "nuclear pasta" phase (Ravenhall, Pethick & Wilson 1983, Bethe 1990). As the density increases, the spheres deform into prolate spheroids which align parallel to each other. These spheroids elongate into long cylinders, which then join together to give alternating flat plates of nuclear material and nucleon gas (which consists mainly of neutrons that have "dripped" out of nuclei). After this phase the stages invert, so that spaghetti-like gas spaces form between between nuclear matter, which shorten into spheroids and finally produce spherical bubbles of gas in nuclear matter - "Swiss cheese". The transition from Swiss cheese to uniform nuclear matter occurs when the density is around [0.8 Ps].

The nuclear pasta phases were originally ascertained from phase diagrams of the equilibrium state of nuclear material. The core collapse environment, with rapidly increasing density, is far from equilibrium. Recently, computational investigations into whether these phases can arise during core collapse have been carried out, which show that they do indeed occur (Watanabe et al. 2005). This is an important result, as pasta phases increase the neutrino opacity of matter (Horowitz, Perez-Garcia & Piekarewicz 2004), which in turn affects the mass of the inner core and the success of the shock.
Got that?

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