Astrophysical nuclear reactions and the break-out from the hot CNO cycles

Thermonuclear stellar explosions depend critically on the available fuel and the ignition through nuclear reactions. In this paper we intend to demonstrate the sensitive interplay between nuclear fuel supply and depletion by nuclear reaction processes at high temperature and densities. We demonstrate this on the example of X-ray bursts which are explained as thermonuclear explosions in the atmosphere of an accreting neutron star. These bursts occur periodically between hours and days depending on the accretion rate. The burst itself is triggered by the 15O(α,γ)19Ne reaction regulating the flow between the β-limited hot CNO cycle and the rapid proton capture process. The 18Ne(α,p)21Na reaction on the other hand provides a continuous processing flow of light 4He fuel towards heavier elements. The reaction rates of both α capture processes carry significant uncertainties. In this paper we report on the measurements of the critical nuclear parameters for the determination of the rate of the 15O(α,γ)19Ne and the 18Ne(α,p)21Na reactions. We demonstrate and analyze the impact of these experimental results on the burst pattern and burst periodicity for a range of accretion rates.