Physics – Plasma Physics
Scientific paper
Nov 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004aps..dppji2003d&link_type=abstract
American Physical Society, 46th Annual Meeting of the Division of Plasma Physics, 15-19 November, 2004, Savannah, GA. MEETING I
Physics
Plasma Physics
Scientific paper
The endpoint of stellar evolution of 98% of stars is the white dwarf phase in which nuclear fuel no longer burns, residual energy radiates away, and the star slowly cools. The cooling process in white dwarfs has important astrophysical consequences and is interesting in its own right because it calls for detailed knowledge of thermodynamic and transport properties of dense, strongly-coupled plasmas. Under these extreme conditions, ions are strongly coupled and ionic quantum effects can be important. Indeed, white dwarfs are made of a dense core of fully ionized mixtures, mainly carbon and oxygen, sustained against gravitational collapse by the pressure of degenerate, relativistic electrons. Energy outflow from the core is regulated by thin surface layers of helium and hydrogen. The most important sources of energy within the core are associated with crystallization; however, it has been suggested(*) that the cooling could be strongly delayed by the gravitational settling of ^22Ne impurities in liquid white dwarfs interiors, a mechanism previously overlooked. Simple models for this process treat the sedimentation of ^22Ne as diffusion in a classical, strongly coupled ionic plasma immersed in a uniform neutralizing electron background. Such a model suffers from uncertainties in the diffusion coefficient, which cannot be obtained via standard plasma kinetic theory. We first assessed this uncertainty by computing the diffusion coefficient using particle-particle particle-mesh molecular dynamics over the entire strongly coupled fluid regime. An analytical model was developed to extend the results into the quantum regime. These results were extended to the more realistic situation of ionic mixtures. These findings and their impact on the cooling process will be discussed. (*) L. Bildsten and D.M. Hall, ApJ 549, L219 (2001)
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