Influence of the nucleon radius on the properties of slowly rotating neutron stars

We have studied the effect of free space nucleon radius on the nuclear matter and slowly rotating neutron-star properties within a relativistic mean-field model with the inclusion of $\omega -\rho$ mixing nonlinear term. We use the same density-dependent radius as in our previous work. However, in the present work, we utilize the parameter sets with more proper symmetric nuclear matter (SNM) predictions for a number of different nucleon radii. To this end, the isoscalar parameters in each parameter set and the cutoff parameter $\beta$ are adjusted by fitting the corresponding SNM properties at subsaturation density to the prediction of IUFSU parameter set. Our results show that the assumption of composite nucleons with free space radius $r\le 0.83$ fm is still compatible with the currently acceptable nuclear matter (NM) properties beyond the saturation density. We have also found that the effect of free space nucleon radius could be imprinted in the radius, moment of inertia, and crust properties of the slowly rotating neutron star.

Figure 1

Cutoff parameter $\beta$ as a function of the free space nucleon radius.

Figure 2

(a) Pressure as a function of the ratio between nucleon and nuclear saturation densities. (b) Energy per particle as a function of the density around the normal density for the symmetric nuclear matter with different nucleon radii. Shaded area in panel (a) corresponds to the heavy-ion experimental data taken from Ref. [33]. The shaded area in the inset of panel (b) displays the constraint of nuclear matter EOS near twice the saturation density from the FOPI data [32].

Figure 3

(a) Pressure as a function of the ratio between nucleon and nuclear saturation densities. (b) Energy per particle as a function of the density at low-density region for pure neutron matter with different nucleon radii. Shaded area in panel (a) corresponds to the heavy-ion experimental data based on soft and stiff symmetry energies taken from Ref. [33], whereas shaded area in panel (b) exhibits the pure neutron matter result of Ref. [34].

Figure 4

(a) Neutron star matter EOS, (b) mass as a function of the central pressure, and (c) mass-radius relation of the NS. Horizontal shaded bands in panels (b) and (c) are the pulsar mass constraint from Ref. [3]. The circle, triangle, square, and diamond in panels (b) and (c) denote the points with the maximum mass.

Figure 5

(a) Neutron star matter EOS and (b) SNM EOS of the standard IUFSU [26], NL3 [37], and FSU [38] parameter sets for comparison.

Figure 6

The influence of $\gamma$ on (a) NS matter EOS, (b) NS mass as a function of the central pressure, and (c) NS mass-radius relation for $r=0.67$ fm.

Figure 7

The influence of $\gamma$ on (a) NS matter EOS, (b) NS mass as a function of the central pressure, and (c) NS mass-radius relation for $r=0.83$ fm.

Figure 8

(a) Moment of inertia as a function of NS mass, (b) the predicted radius of canonical NS and the maximum NS mass as a function of the nucleon radius, and (c) the NS moment of inertia as a function the NS radius for $M=1.338M_{\odot }$. Shaded region in panel (c) exhibits the approximated range of the moment of inertia obtained by Lattimer and Schutz [40] for $M=1.338M_{\odot }$. Note that in panel (b) the results obtained for $M=1.338M_{\odot }$ and $M=1.4M_{\odot }$ are coincident.

Figure 9

(a) The ratio of the crust moment of inertia to the NS moment of inertia, (b) the ratio of the crust mass to the corresponding NS mass, and (c) the thickness of the crust as a function of the NS mass. Shaded area below the thick line in panel (a) shows the excluded ratio from the pulsar timing data if the giant glitches in the Vela pulsar (PSR B0833-45) originate only from the NS crusts [14].