Mathematics
Scientific paper
Sep 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt.........2t&link_type=abstract
Ph.D. Thesis Delaware Univ., Newark.
Mathematics
Neutron Stars, Stellar Atmospheres, Stellar Mass Accretion, Stellar Models, X Ray Sources, Carbon, Exosat Satellite, Mathematical Models, Nitrogen, Oxygen, Steady State, Stellar Structure
Scientific paper
We use the observed variation of burst properties of x-ray burst sources, with the persistent flux to determine if nuclear reactions are proceeding in a carbon poor or carbon rich environment. We find that by reducing the CNO cycle burning rate, which corresponds to a carbon poor environment, we cannot reproduce observed variations in the burst properties with persistent flux. Recent work on the surface boundary conditions of accreting neutron stars indicates that appreciable amounts of CNO elements can be destroyed by spallation. Previous authors assume an accretion of near solar abundance material which is not altered by boundary effects. Here we construct steady-state models of accreting neutron stars assuming all but traces of metals are destroyed due to spallation in the surface layers. We also construct models with solar accretion for comparison. The models are compared with EXOSAT observations of 4U/MXB 1636-53. We find that the simplification introduced by assuming CNO destruction does not bring theory closer to describing what is observed. Elemental diffusion has never been incorporated into the equations of stellar structure when solving for the steady-state structure of an accreting neutron star. A simple order of magnitude estimate of the timescale for diffusion indicates that this process may be important, particularly for bursts with long recurrence intervals. We solve the equations of stellar structure with diffusion and show that the results indicate that the star may not be in a steady-state.
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