Equation of State for warm Neutron Star outer crusts

TL;DR

This paper presents a refined equation of state for warm neutron star outer crusts using microscopic simulations, including electron screening and finite-size ion effects, with data and neural network models available publicly.

Contribution

It introduces a detailed EoS for warm neutron star crusts incorporating advanced physical effects and provides accessible tabulated data and neural network parametrizations.

Findings

Thermal effects of ions are significant at higher densities near the inner crust.

The EoS covers densities from 7.48e-10 to 2.09e-4 fm^{-3} and temperatures from 1 to 5 MeV.

Comparison with other EoS highlights the importance of including thermal ion effects.

Abstract

We describe the equation of state (EoS) of a warm ion plasma as obtained by performing microscopic many-body simulations using Molecular Dynamics computational techniques. Using the cold one-component plasma (OCP) composition in the Neutron Star (NS) outer crust assumed in Murarka et al. (2022) with a representative heavy nucleus for each density, we refine previous calculations. We include electron screening and modeling of ions as finite-size Gaussian distributions in the interaction potential, together with an efficient Ewald energy summation procedure. From this, the EoS relation $P(n_B,T)$ is obtained as a function of baryonic density and temperature in the NS outer crust under conditions $n_B\in[7.48\times 10^{-10},2.09\times10^{-4}]$ $ \rm fm^{-3}$ , $k_{B}T\in[1,5]$ MeV. In order to improve the usability of our results we provide tabulated data values along with a neural network parametrization available in the Zenodo repository, see https://zenodo.org/records/15348712. We find that even at moderate temperatures, thermal effects of ions are key in the higher density region closer to the inner crust, when described using a thermal effective parametrization based on the thermal adiabatic index $\Gamma_{th}$. We compare our results with other EoS in the literature performing a critical discussion.