Bridging the waiting points: The role of two-proton capture reactions in the rp process

The influence of two-proton capture reactions on long-lived waiting point isotopes near the proton drip line is discussed for the rp-process reaction flow. These reactions are characterized by two successive proton captures, where the first reaction product is unstable against proton emission, but the second reaction results in a particle stable nucleus. The reactions which have this character and involve long-lived waiting points in the rp process are ¹⁵O(2p,γ)¹⁷Ne, ¹⁸Ne(2p,γ)²⁰Mg, and ³⁸Ca(2p,γ)⁴⁰Ti. Their rates are presented and compared with the rates of alpha-induced reactions for stellar conditions at high temperatures and densities. We find that the two-proton captures on ¹⁵O and ¹⁸Ne can only compete for unrealistically high densities, but that ³⁸Ca(2p,γ)⁴⁰Ti can act as an efficient reaction link at conditions typical for x-ray bursts on neutron stars.