Compositional Freeze-Out of Neutron Star Crusts

TL;DR

This study models the chemical evolution of neutron star crusts during cooling to determine how different cooling processes influence the composition of the star's atmosphere and envelope, affecting observable spectra.

Contribution

It introduces a detailed nuclear reaction network simulation of neutron star crusts without fallback accretion, linking cooling models to atmospheric composition predictions.

Findings

Atmospheric composition varies with cooling process: Si for modified Urca, Cr for thick crust, Ca for thin crust.

Light elements dominate the neutron star atmosphere, lighter than iron.

Different cooling models lead to distinct surface compositions, impacting observable spectra.

Abstract

We have investigated the crustal properties of neutron stars without fallback accretion. We have calculated the chemical evolution of the neutron star crust in three different cases (a modified Urca process without the thermal influence of a crust, a thick crust, and a direct Urca process with a thin crust) in order to determine the detailed composition of the envelope and atmosphere as the nuclear reactions freeze out. Using a nuclear reaction network up to technetium, we calculate the distribution of nuclei at various depths of the neutron star. The nuclear reactions quench when the cooling timescale is shorter than the inverse of the reaction rate. Trace light elements among the calculated isotopes may have enough time to float to the surface before the layer crystallizes and form the atmosphere or envelope of the neutron star. The composition of the neutron-star envelope determines the total photon flux from the surface, and the composition of the atmosphere determines the emergent spectrum. Our calculations using each of the three cooling models indicate that without accretion of fallback the neutron star atmospheres are dependent on the assumed cooling process of the neutron star. Each of the cooling methods have different elements composing the atmosphere: for the modified Urca process the atmosphere is $^{28}$Si, the thick crust has an atmosphere of $^{50}$Cr, and the thin crust has an atmosphere of $^{40}$Ca. In all three cases the atmospheres are composed of elements which are lighter then iron.