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The stability of thermonuclear burning of hydrogen and helium accreted ontoneutron stars is strongly dependent on the mass accretion rate. The burningbehavior is observed to change from Type I X-ray bursts to stable burning, withoscillatory burning occurring at the transition. Simulations predict thetransition at a ten times higher mass accretion rate than observed. Usingnumerical models we investigate how the transition depends on the hydrogen,helium, and CNO mass fractions of the accreted material, as well as on thenuclear reaction rates of triple alpha and the hot-CNO breakout reactions15O(a,g)19Ne and 18Ne(a,p)21Na. For a lower hydrogen content the transition isat higher accretion rates. Furthermore, most experimentally allowed reactionrate variations change the transition accretion rate by at most 10%. A factorten decrease of the 15O(a,g)19Ne rate, however, produces an increase of thetransition accretion rate of 35%. None of our models reproduce the transitionat the observed rate, and depending on the true 15O(a,g)19Ne reaction rate, theactual discrepancy may be substantially larger. We find that the width of theinterval of accretion rates with marginally stable burning depends strongly onboth composition and reaction rates. Furthermore, close to the stabilitytransition, our models predict that X-ray bursts have extended tails wherefreshly accreted fuel prolongs nuclear burning.

Reaction Rate and Composition Dependence of the Stability of Thermonuclear Burning on Accreting Neutron Stars