Neutron star inner crust: Nuclear physics input

Andrew W. Steiner

Phys. Rev. C 77, 035805 – Published 18 March 2008

Abstract

A fully self-consistent model of the neutron star inner crust based upon models of the nucleonic equation of state at zero temperature is constructed. The results nearly match those of previous calculations of the inner crust given the same input equation of state. The extent to which the uncertainties in the symmetry energy, the compressibility, and the equation of state of low-density neutron matter affect the composition of the crust are examined. The composition and pressure of the crust is sensitive to the description of low-density neutron matter and the nuclear symmetry energy, and the latter dependence is nonmonotonic, giving larger nuclei for moderate symmetry energies and smaller nuclei for more extreme symmetry energies. Future nuclear experiments may help constrain the crust and future astrophysical observations may constrain the nuclear physics input.

Received 15 January 2008

DOI:https://doi.org/10.1103/PhysRevC.77.035805

Authors & Affiliations

Andrew W. Steiner

Joint Institute for Nuclear Astrophysics, National Superconducting Cyclotron Laboratory and the Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA

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Issue

Vol. 77, Iss. 3 — March 2008

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Images

Figure 1
A survey of the equations of state used in this work. Plotted in each panel are the energy per baryon of nuclear matter (lower set of curves) and neutron matter (upper set of curves) as a function of baryon density, $n_{B}$ . The upper left panel shows the models APR, SLy4, and SkM $^{*}$ , the upper right panel shows the model NL4Q (based on NL4) and shows how neutron matter was modified to match APR at low densities, and the lower panels show how the symmetry energy was modified in the schematic equation of state.Reuse & Permissions
Figure 2
A comparison of the composition of the neutron star crusts in models NL4Q, NL4QN, and NL4QN2, designed to demonstrate the sensitivity of the composition to the equation of state of low-density neutron matter. The proton nuber, $Z$ , the atomic number $A$ (left axis) and the number density of the dripped neutrons $n_{n, drip}$ are displayed (right axis). A comparison the composition given the two different mass models. The curve labeled LDM (solid lines) is obtained from Eq. (1), and the curve labeled FRDM (dashed lines) is obtained from Ref. [17]. The left axis is for $A$ and $Z$ and the right axis is for $n_{n, drip}$ .Reuse & Permissions
Figure 3
A comparison of the present work (solid lines) with the results of Ref. [25] (dashed lines) using the same input equation of state, SLy4. The left axis is for $A$ and $Z$ and the right axis is for $n_{n, drip}$ . A comparison of the present work (solid lines) with the results from Ref. [28] (labeled “JML” and plotted with dashed lines) using the same input equation of state, SkM $^{*}$ . The sharp dropoff in JML at high-densities is due to the transition to homogeneous nucleonic matter. The slightly jagged nature of the JML results is due to the naive interpolation employed in this work and is not necessarily present in the original table. The left axis is for $A$ and $Z$ and the right axis is for $n_{n, drip}$ .Reuse & Permissions
Figure 4
A comparison of the composition of the crust given different symmetry energies. The bold solid line is the baseline model, the dashed and dotted lines show the variation in γ, and the dashed-dotted and thin solid line give the variation in the magnitude of the symmetry energy at the saturation density.Reuse & Permissions
Figure 5
A comparison of the composition of the crust given different values of the symmetry energy at the saturation density at a fixed density of $n_{B} = 0.01$ fm $^{- 3}$ . The top panel gives the total binding energy per baryon, and the separate contributions from dripped neutrons (“n”), electrons (“e”), and nuclei (“Nuc”). The bottom panel shows the neutron and proton number of nuclei as well as the volume fraction, χ.Reuse & Permissions
Figure 6
A survey of the pressure of the crust as a function of density, scaled by the baryon density, $n_{B}^{4 / 3}$ . The curves are nearly flat where the adiabatic index is expected to be nearly 4/3 since the baryon density nearly scales with the energy density.Reuse & Permissions

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