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

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 ${}_{}{}^{15}{}_{}{}^{}\mathrm{O}$(2p,γ${)}^{17}$Ne, ${}_{}{}^{18}{}_{}{}^{}\mathrm{Ne}$(2p,γ${)}^{20}$Mg, and ${}_{}{}^{38}{}_{}{}^{}\mathrm{Ca}$(2p,γ${)}^{40}$Ti. 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 ${}_{}{}^{15}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{18}{}_{}{}^{}\mathrm{Ne}$ can only compete for unrealistically high densities, but that ${}_{}{}^{38}{}_{}{}^{}\mathrm{Ca}$(2p,γ${)}^{40}$Ti can act as an efficient reaction link at conditions typical for x-ray bursts on neutron stars.