Nucleon Direct Urca Research Articles

Inferring the nuclear symmetry energy at suprasaturation density from neutrino cooling

An ambitious goal of the astrophysical community is not only to constrain the equation of state (EOS) of neutron star (NS) matter by confronting it with astrophysics observations, but ultimately also to infer the NS composition. Nevertheless, the composition of the NS core is likely to remain uncertain unless we have an accurate determination of the nuclear symmetry energy at supra saturation density ($\rho>\rho_0$). We investigate how the nucleonic direct Urca (dUrca) processes can be used as an effective probe to constraint the high density nuclear symmetry energy. A large number of minimally constrained EOSs has been constructed by applying a Bayesian approach to study the correlations of the symmetry energy at different densities with a few selected properties of a NS. The nuclear symmetry energy above the baryon density 0.5 fm$^{-3}$ ($\sim 3 \rho_0$) is found to be strongly correlated with NS mass at which the onset of nucleonic dUrca neutrino cooling takes place in the core. This allows us to constrain the high density behavior of nuclear symmetry energy within narrow bounds. {The pure neutron matter pressure constraint from chiral effective field theory rules out the onset of nucleonic dUrca in stars with a mass $\lesssim$ 1.4 $M_\odot$.} The onset of dUrca inside 1.6 M$_\odot$ to 1.8 M$_\odot$ NS implies a slope of the symmetry energy $L$ at $\sim 2.5~\rho_0$, respectively, between 54 and 48 MeV.

Cooling of isolated neutron stars with pion condensation: Possible fast cooling in a low-symmetry energy model

In this paper, we studied thermal evolution of isolated neutron stars (NSs) including the pion condensation core, with an emphasis on the stiffness of equation of state (EOS). Many temperature observations can be explained by the minimal cooling scenario which excludes the fast neutrino cooling process. However, several NSs are cold enough to require it. The most crucial problem for NS cooling theory is whether the nucleon direct Urca (DU) process is open. The DU process is forbidden if the nucleon symmetry energy is significantly low. Hence, another fast cooling process is required in such an EOS. As the candidate to solve this problem, we consider the pion condensation. We show that the low-symmetry energy model can account for most cooling observations including cold NSs, with strong neutron superfluidity. Simultaneously, it holds the [Formula: see text] observations even if the pion condensation core exists. Thus, we propose the possibility of pion condensation, as an exotic state to solve the problem in low-symmetry energy EOSs. We examined the consistency of our EOSs with other various observations as well.

Neutrino emissivity of the nucleon direct URCA process for rotational traditional and hyperonic neutron stars**Supported by National Natural Science Foundation of China (11545011, 11405096), the Shandong Natural Science Foundation (ZR2014AQ012), the Young Scholars Program of Shandong University, Weihai (2015WHWLJH01) and the Fundamental Research Funds for the Central Universities (lzujbky-2016-30)

Based on covariant density functional theory, we study the effects of rotation on the nucleon direct URCA (N-DURCA) process for traditional and hyperonic neutron stars. The calculated results indicate that, for a fixed mass sequence of rotational traditional neutron stars, the neutrino emissivity of the star is nearly invariant with increasing frequency, while it always increases for rotational hyperonic neutron stars. Thus, rotation has different effects on the N-DURCA process for these two kinds of neutron stars.