Nuclear composition and heating in accreting neutron-star crusts

Nuclear reactions in accreting neutron-star crusts and the heat release accompanying them are studied, under different assumptions concerning the composition of the outermost layer formed of the ashes of X-ray bursts. Particular examples of ashes containing nuclides with A ≃ 90−110 are considered and compared with a standard A = 56 case. In all cases, evolution of a crust shell is followed from 10 g cm to a few times 10 g cm. The total crustal heating produced in the non-equilibrium processes in the accreting crust is 1.1 − 1.5 MeV per one accreted nucleon. The composition of the accreted crust at densities exceeding the threshold for the pycnonuclear fusion (ρ > 10 g cm) is essentially independent of the assumed initial composition of the X-ray burst ashes.