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% 12 C and 80% 56 Fe, we test different 12 C abundances mixed with scaled rp-process ashes. Results: Superbursts can ignite with only 10% 12 C when the remaining 90% consists of rp-process ashes. Their luminosity is comparable to that of the canonical fuel, whereas 10% 12 C mixed with 90% 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.