The temperature and nuclear composition of the crust and ocean of an accreting neutron star depend on the mix of material ( the ashes ) that is produced at lower densities by fusion of the accreting hydrogen and helium .
The hydrogen/helium burning is thermally stable at high accretion rates , a situation encountered in weakly magnetic ( B \ll 10 ^ { 11 } { G } ) neutron stars accreting at rates \dot { M } > 10 ^ { -8 } { M _ { \odot } { yr ^ { -1 } } } and in most accreting X-ray pulsars , where the focusing of matter onto the magnetic poles results in local accretion rates high enough for stable burning .
For a neutron star accreting at these high rates , we calculate the steady state burning of hydrogen and helium in the upper atmosphere ( \rho < 2 \times 10 ^ { 6 } { g cm ^ { -3 } } ) , where T \approx ( 5 – 15 ) \times 10 ^ { 8 } { K } .
Since the breakout from the “ hot ” CNO cycle occurs at a temperature comparable to that of stable helium burning ( T \gtrsim 5 \times 10 ^ { 8 } { K } ) , the hydrogen is always burned via the rapid proton capture ( rp ) process of Wallace and Woosley .

The rp process makes nuclei far beyond the iron group , always leading to a mixture of elements with masses A \sim 60 – 100 .
The average nuclear mass of the ashes is set by the extent of helium burning via ( \alpha , p ) reactions , and , because these reactions are temperature sensitive , depends on the local accretion rate .
Nuclear statistical equilibrium , leading to a composition of mostly iron , occurs only for very high local accretion rates in excess of 50 times the Eddington rate .

We briefly discuss the consequences of our results for the properties of the neutron star .
The wide range of nuclei made at a fixed accretion rate and the sensitivity of the ash composition to the local accretion rate makes it inevitable that accreting neutron stars have an ocean and crust made up of a large variety of nuclei .
This has repercussions for the thermal , electrical and structural properties of the neutron star crust .
A crustal lattice as impure as implied by our results will have the conductivity throughout most of its mass set by impurity scattering , allowing for more rapid Ohmic diffusion of magnetic fields than previously estimated for mono-nuclear mixes .