The15O(α,γ)19Ne Reaction Rate and the Stability of Thermonuclear Burning on Accreting Neutron Stars

Abstract

Neutron stars in close binary star systems often accrete matter from their companion stars. Thermonuclear ignition of theaccretedmaterialintheatmosphereof theneutronstarleadstoathermonuclearexplosion,whichisobservedas an X-ray burst occurring periodically between hours and days depending on the accretion rate. The ignition conditions are characterized bya sensitive interplay between the accretion rate of the fuel supply and its depletion rate by nuclear burning in the hot CNO cycle and the rp-process. For accretion rates close to stable burning the burst ignition therefore depends critically on the hot CNO breakout reaction 15 O(�; � ) 19 Ne that regulates the flow between the hot CNO cycle and the rapid proton capture process. Until recently, the 15 O(�; � ) 19 Ne reaction rate was not known experimentally and the theoretical estimates carried significant uncertainties. In this paper we perform a parameter study of the uncertainty of this reaction rate and determine the astrophysical consequences of the first measurement of this reaction rate. Our results corroborate earlier predictions and show that theoretically burning remains unstable up to accretion rates near the Eddington limit, in contrast to astronomical observations. Subject headingg nuclear reactions, nucleosynthesis, abundances — stars: neutron — X-rays: bursts