Models of X-ray bursters with radius expansion

A series of models of helium shell flashes on a neutron star is presented. To make models simple, no hydrogen was present and a small nuclear reaction network was used. General relativity was fully taken into account. Variation of electron scattering opacity with temperature and density was allowed for, and it was most essential for producing hydrostatic expansion of the outer layers of models. The models which expanded had relatively large helium envelope masses, developed strong shell flashes, and displayed a double peak variation of surface temperature with time. They may help to understand some X-ray bursters.