How do we know the chemical composition of the crust of neutron stars?

Question

Although neutron stars are mostly made of neutronium, the pressure at the surface is not very high which allows regular atomic matters to exist. Emission spectrum can reveal the chemical composition of distant stars. However, neutron stars are surrounded with extremely strong magnetic field which is enough to distort the atomic structures. Atomic nuclei should be more resistant against the magnetic field because they are much more compact and tightly bound, but I am not sure if atomic nuclei emit characteristic spectrum like the electron clouds.

If we can determine the composition of the neutron star crust, will there be any variations? Neutron stars formed due to iron core collapse should have an iron crust. Some neutron stars are formed due to the electron capture of the O-Ne-Mg core. Would these neutron stars have a different chemical composition in the crust? Many neutron stars are accreting hydrogen and helium gas from companion stars. Will the accreted matters show up on the surface?

Answer 1

The accreted matter often burns, in a thermonuclear sense. We attribute Type I X-ray bursts to episodic helium burning.

Answer 2

Observations of the surface cannot tell us (directly) about the composition of the crust.

The surface of a neutron star will likely consist of "normal" nuclei and partially ionised atoms including a mixture of helium, carbon, oxygen and iron-peak elements. Spectroscopic observations at X-ray wavelengths (for young, hot neutron stars) can tell us something about the surface composition from the departures from a simple blackbody spectrum.

The interior crust could be 1-2 km deep and it is in there where increasingly exotic, neutron-rich heavy nuclei are expected to be found. This material is hidden behind the physically thin, but optically thick surface layer.

The material in the crust is capable of achieving nuclear statistical equilibrium. That means, whatever the initial composition, the nucleons will rearrange themselves to arrive at a configuration the minimises the total local energy density. In practice this means the composition will depend (dominantly) on the local density, though the magnetic field and temperature could play a role closer to the surface.

The surface layers (maybe a few cm thick) might reflect the initial composition, but more likely they are accreted later from a companion or the interstellar medium.