On the superbursts as photodisintegrators of ashes from the rp-process -- Numerical simulations of superbursts with MESA II

Context: Superbursts are energetic explosions in the crusts of neutron stars in Low-Mass X-ray Binaries (LMXBs). At temperatures above 4 GK, photodisintegration of heavy rp-process ashes may play a major role alongside ${}^{12}$C fusion. Aims: We investigate how photodisintegration of rp-process ashes affects superburst properties and the initial ${}^{12}$C abundance required for ignition. Methods: We simulate superbursts with MESA v.24.08.1. Besides the canonical fuel of $20\mathrm{\%}$ ${}^{12}$C and $80\mathrm{\%}$ ${}^{56}$Fe, we test different ${}^{12}$C abundances mixed with scaled rp-process ashes. Results: Superbursts can ignite with only $10\mathrm{\%}$ ${}^{12}$C when the remaining $90\mathrm{\%}$ consists of rp-process ashes. Their luminosity is comparable to that of the canonical fuel, whereas $10\mathrm{\%}$ ${}^{12}$C mixed with $90\mathrm{\%}$ ${}^{56}$Fe yields lower luminosity. ${}^{56}$Fe remains the most abundant ash nuclide. Although photodisintegration drives nucleosynthesis, neutron, proton and alpha captures release energy at late times. We also find that $A=64$ nuclides or ${}^{20}$Ne help reduce the ${}^{12}$C abundance needed for ignition. Conclusions: The composition of accreted material - not just its ${}^{12}$C content - substantially affects superburst ignition, luminosity and nucleosynthesis.