The importance of 15O(α,γ)19Ne to X-ray bursts and superbursts

One of the two breakout reactions from the hot CNO ( HCNO) cycle is O-15(alpha,gamma)Ne-19, which at low temperatures depends strongly on the resonance strength of the 4.033MeV state in Ne-19. An experimental upper limit has been placed on its strength, but the lower limit on the resonance strength and therefore the astrophysical reaction rate is unconstrained experimentally. However, this breakout reaction is crucial to the thermonuclear runaway that causes type I X- ray bursts on accreting neutron stars. In this paper we exploit astronomical observations in an attempt to constrain the relevant nuclear physics and deduce a lower limit on the reaction rate. Our sensitivity study implies that if the rate were sufficiently small, accreting material would burn stably without bursts. The existence of type I X-ray bursts and superbursts consequently suggests a lower limit on the O-15(alpha,gamma) Ne-19 reaction rate at low temperatures.