Speaker
Description
The extreme conditions encountered in neutron stars (NS) require the extension of the quantum chromodynamics phase diagram from the temperature-density plane to the isospin dimension. The crucial parameter for the description of nuclear matter in these very neutron-rich environments appears to be the nuclear symmetry energy. The smallest proton fractions (down to ≈ 3 %) can be found deep in the crust of NS. This region is predicted [1,2] to consist of very exotic nuclear configurations resembling spaghetti, lasagna or even bucatini and commonly referred to as “nuclear pasta”. Because of their peculiar structure, these pasta phases alter the transport and mechanical properties of the crust, leaving their imprints on the magneto-thermal evolution, oscillations and the emission of gravitational waves by NSs. The abundance of nuclear pasta in NS is governed to some extent by the symmetry energy at sub-saturation densities [3,4].
Recently [3,4], we have investigated the existence of nuclear pasta within the semiclassical extended Thomas-Fermi (ETF) approach. We have employed a series of Brussel-Montreal parametrizations of generalized Skyrme functionals that covers different behaviors for the symmetry energy and at the same time accurately reproduces (i) experimental masses of thousands of nuclei and (ii) state-of-the-art ab initio predictions for pure neutron and symmetric nuclear matter. For all adopted parametrizations, we observe that pasta shape transitions occur whenever the volume fraction $u$ occupied by nuclear clusters fills some threshold value $u_t$. In particular, spherical clusters turn into pasta at $u_t$ ≈ 0.14. This quasi-universality of pasta transitions coincides with the predictions of Ref. [2] (however, their values obtained within a liquid-drop picture are systematically higher; the difference can be explained by the curvature correction). The symmetry energy $S(n)$ at baryon number density $n$ is then found to control the density dependence of $u(n)$ and therefore determines the density ranges of the various pasta structures: the higher the symmetry energy at relevant densities, the softer u(n), hence the wider the density range for each pasta configuration.
However, accounting for quantum shell and pairing effects perturbatively within the Strutinsky integral method leads to a substantial shrinking of the pasta layer, questioning its very existence [3,5]. To better assess the presence of nuclear pasta in NS, we follow a fully three-dimensional quantum treatment based on the self-consistent nuclear energy-density functional theory. In the talk, I will show that the accurate determination of such exotic nuclear configurations requires large computational domains (encompassing several pasta replicas) and special convergence strategies of the mean-field problem. The stability of pasta shapes will be discussed through full 3D Hartree-Fock plus BCS pairing numerical calculations using our latest generalized Skyrme functional BSkG4 [6].
References:
[1] D.G. Ravenhall, C.J. Pethick, J.R. Wilson, Phys. Rev. Lett. 50, 2066 (1983)
[2] M. Hashimoto , H. Seki , M. Yamada, Progr. of Theor. Phys. 71 (2), p. 320 (1984)
[3] N.N. Shchechilin, N. Chamel, J.M. Pearson, Phys. Rev. C 108 (2), id.025805 (2023)
[4] N.N. Shchechilin, N. Chamel, A.I. Chugunov, Eur. Phys. Journ. A, accepted (2025)
[5] N.N. Shchechilin, N. Chamel, J.M. Pearson, A.I. Chugunov, A.Y. Potekhin, Phys. Rev. C 109 (5), id.055802 (2024)
[6] G. Grams, N.N. Shchechilin, A. Sánchez-Fernández, W. Ryssens, N. Chamel, S. Goriely Eur. Phys. Journ. A 61 (2), id.35 (2025)
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