Equation of State for Warm Neutron Star Outer Crusts

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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 U. Murarka et al. 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 number density n B ∈ [7.48 × 10 −10 , 2.09 × 10 −4 ] fm −3 and temperature k B T ∈ [1, 5] MeV in the NS outer crust conditions. In order to improve the usability of our results, we provide tabulated data values along with a neural network parameterization available in the Zenodo repository; see DOI: 10.5281/zenodo.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 parameterization based on the thermal adiabatic index Γ th . We compare our results with other EoSs in the literature and provide a critical discussion.
Funding Information
  • Ministerio de Ciencia, Innovación y Universidades (PID2022137887NB-100)
  • Junta de Castilla y León (SA101P24)

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