Irreducible Representation of Shell Flashes on Accreting Neutron Stars

Abstract

Parameter dependences are studied for shell burnings induced by the accretion onto the surface of neutron stars. It is shown that their progress can be formulated with two parameters of the surface gravity and the column accretion rate, although the model parameters involve the mass and the radius of neutron stars and the mass accretion rate. By applying this formulation, the property of hydrogen shell burning and the condition to ignite helium shell flash are investigated. The structure of the envelope depends on surface gravity only weakly and therefore, the recurrence period of helium shell flashes is determined practically by the column accretion rate. The effect of varying CNO abundance in the accreted gas is also discussed to reveal the general tendency. Implications to X-ray burst phenomena, and especially the relevant range of the column accretion rate are discussed from the comparison of our results with their observations.