Hyperons and massive neutron stars: Vector repulsion and SU(3) symmetry

With the discovery of massive neutron stars such as PSR J1614-2230, the question has arisen of whether exotic matter such as hyperons can exist in the neutron star core. We examine the conditions under which hyperons can exist in massive neutron stars. We consistently investigate the vector baryon-meson coupling, going from the SU(6) quark model to a broader SU(3) symmetry. We propose that the maximum neutron star mass decreases linearly with the strangeness content $f_s$ of the neutron star core as $M_{\text{max}}\left(f_s\right)=M_{\text{max}}\left(0\right)-0.6M_{\odot }(f_s/0.1)$, which seems to be independent of the underlying nuclear equation of state and the vector baryon-meson coupling scheme. Thus, pulsar mass measurements can be used to constrain the hyperon fraction in neutron stars.

Figure 1

Relative vector meson coupling constants as functions of the $g_8/g_1$ ratio $z$ for fixed ${\alpha }_V=1$. The value $z=1/\sqrt{6}$ corresponds to the SU(6) case.

Figure 2

EoS for different $g_8/g_1$ ratios $z$ within a nonlinear $\sigma -\omega$ model with additional $\phi$ meson and the full baryon octet for GM1 parameter set. The EoS get stiffer with decreasing $z$.

Figure 3

Mass-radius relations for the EoS displayed in Fig. 2. The maximum mass is obtained for the case $z=0$, where all baryons couple to the vector mesons with equal strengths.

Figure 4

Maximum masses as functions of the $g_8/g_1$ ratio $z$ for NL3, GM1, and TM1 parameter sets. For each parametrization, the case of pure nucleonic matter is also displayed.

Figure 5

Particle fractions for the GM1 parameter set for three different values of $z$: $z=0.8$ (a), $z=0.408$ (b) corresponding to the SU(6) case, and $z=0$ (c). The threshold for the appearance of the hyperons $\Lambda$, ${\Xi }^{-}$, and ${\Xi }^0$ is pushed to higher densities with decreasing $z$.

Figure 6

Maximum masses of hyperonic neutron stars as functions of effective nucleon mass $m_N^{*}$ for different values of the $g_8/g_1$ ratio $z$. For comparison, a line for nucleonic stars and points to mark RMF sets (e.g., TM1, NL3) corresponding to the SU(6) case are also given.

Figure 7

Vector meson coupling constants as functions of the $F/(F+D)$ ratio ${\alpha }_V$. The $g_8/g_1$ ratio is fixed to its SU(6) value $z=1/\sqrt{6}$ and ideal mixing is assumed. The SU(6) case is given by ${\alpha }_V=1$.

Figure 8

EoS for “model $\sigma \omega \rho \phi$” in GM1 parametrization for different values of ${\alpha }_V$. $z$ is fixed to its SU(6) value $z=1/\sqrt{6}$ and ideal mixing is assumed. The EoS become stiffer with decreasing ${\alpha }_V$.

Figure 9

Mass-radius relations obtained from the EoS in Fig. 8.

Figure 10

Maximum masses of neutron stars as functions of strangeness fraction $f_s$ in the neutron star core for four different EoS. The maximum mass decreases linearly with the strangeness fraction approximately as $M_{\text{max}}\left(f_s\right)=M_{\text{max}}\left(0\right)-0.6M_{\odot }(f_s/0.1)$.