On the Inner Crusts of Neo-Neutron Stars: exotic light nuclei, diffusional and thermodynamical stability

TL;DR

This study uses an extended Nuclear Statistical Equilibrium (eNSE) model to characterize the composition and stability of warm, beta-equilibrated inner crusts in young neo-neutron stars, focusing on densities below saturation. A key finding is the emergence of an almost pure layer of neutron-rich light nuclei, notably $^{14}\mathrm{He}$ (and sometimes $^{7}\mathrm{H}$), due to translational degrees of freedom at finite temperature, with implications for transport and crystallization. The work also shows that the inner crust is stable against diffusion (buoyancy) under the eNSE composition, and that clusterization removes thermodynamic instabilities that affect sub-saturated nuclear matter. These results, sensitive to the chosen nuclear pools and impermeability assumptions, suggest that warm NS crusts possess transport properties and phase behavior distinct from cold, catalyzed crusts and motivate future exploration of in-medium cluster effects and pasta phases.

Abstract

Based on an extended nuclear statistical equilibrium model, we investigate the properties of non-accreted crusts of young and warm neo-neutron stars, i.e., of finite-temperature inhomogeneous dense matter in beta equilibrium. An interesting feature is the appearance, in the deep inner crust, of an extensive and almost pure layer of neutron-rich light nuclei that extends up to the density of the transition to homogeneous matter. Most probably, this layer emerges due to translational degrees of freedom of the nuclei. If confirmed, it will significantly impact the transport and elastic properties of the crust and its crystallization process. Then, we demonstrate that our inner crust is stable with respect to the diffusion of ions, which is in contrast with some of the predictions made in the literature for cold crusts. Finally, we show that clusterization completely exhausts the density instabilities that affect sub-saturated nuclear matter.

Figures (4)

• Figure 1: Composition of the neo-NS crust and visualization of the employed mass tables. White areas represent the nuclides with positive neutron and proton separation energies, while the nuclides with negative neutron and proton separation energies are marked in violet and magenta, respectively. Light (dark) hues are used for the T1 (reference) set. The gray hatched domain corresponds to nuclei for which nuclear mass data are not available in T1. Cyan and red show the "stable" and beyond neutron-drip nuclei present in the crust. The arrows show the direction of increase of $n_\mathrm{B}$. The results correspond to T1 run, $T=0.4$ MeV and BSk25 BSk22-BSk26 effective interaction.
• Figure 2: Mass fractions of "heavy" ($A \geq 20$) nuclei, light neutron-rich nuclei $^{14}$He (for the reference and T1 runs) and $^{7}$H (for T2 run), and unbound nucleons as functions of $n_\mathrm{B}$ for $\beta$-equilibrated crust at $T=0.1~\mathrm{MeV}$ (bottom) and $T=0.4~\mathrm{MeV}$ (top). eNSE results are compared with the zero temperature results of Ref. Pearson_MNRAS_2018. In all cases, the BSk25 BSk22-BSk26 effective interaction was employed. Vertical dotted lines mark the transition to homogeneous matter, $n_{\mathrm{tr}} = 0.0742~\mathrm{fm}^{-3}$ and $0.0839~\mathrm{fm}^{-3}$ for eNSE reference run and the results of Ref. Pearson_MNRAS_2018, respectively.
• Figure 3: Average mass to average charge numbers ratio as a function of $n_\mathrm{B}$. eNSE results for beta-equilibrated matter are compared with the zero temperature results of Ref. Pearson_MNRAS_2018. Results corresponding to BSk25 BSk22-BSk26. As in Fig. \ref{['Fig:MassFrac']}, vertical dotted lines mark the transition to homogeneous matter.
• Figure 4: Thermodynamic (in)stability of cold NS matter ($T=0.1$ MeV). Left and right panels show the baryon chemical potential ($\mu_\mathrm{B}=\mu_n$) as a function of particle number density ($n_\mathrm{B}$ or $n_n$) and pressure ($P$ or $P_\mathrm{B}$), respectively. NM without Coulomb interaction is computed at a fixed $\mu_p=-30$ MeV. NS matter with electrons is computed at a fixed $\mu_\mathrm{lep} = 0$ (beta equilibrium). BBSk1 Raduta_AA_2025 effective interaction. See text for details.