Helium detonations on neutron stars

We present the results of a numerical study of helium detonations on the su rfaces of neutron stars. We describe two-dimensional simulations of the evo lution of a detonation as it breaks through the accreted envelope of the ne utron star and propagates laterally through the accreted material. The deto nation front propagates laterally at nearly the Chapman-Jouguet velocity, v = 1.3 x 10(9) cm s(-1). A series of surface waves propagate across the poo l of hot ash behind the detonation front with the same speed, matching the speed expected from shallow water wave theory. The entire envelope oscillat es in the gravitational potential well of the neutron star with a period of similar to 50 ks. The photosphere reaches an estimated height of 10 km abo ve the surface of the neutron star. Our study confirms that such a detonati on can insure the spread of burning over the entire neutron star surface on a timescale consistent with burst rise times. We analyze the sensitivity o f the results to the spatial resolution and the assumed initial conditions. We conclude by presenting a comparison of this model to type I X-ray burst s.