Parity-doubled nucleons can rapidly cool neutron stars

Abstract

In confined hadronic matter, the spontaneous breaking and restoration of chiral symmetry can be described by considering nucleons, $N_{+}(939)$, and excited states of opposite parity, $N_{-}(1535)$. In a cold, dense hadronic phase where chiral symmetry remains spontaneously broken, direct Urca decay processes involving the $N_{-}$ are possible, e.g. $N_- \rightarrow N_+ + e^- + \barν_e$. We show that at low temperature and moderate densities, because the $N_-$ is much heavier than the $N_+$, such cooling dominates over standard $N_+$ direct Urca processes. This provides a strong astrophysical signature of the pattern of chiral symmetry restoration in neutron stars.

Figures (3)

• Figure 1: The deficit of momentum forbidding the direct Urca process as a function of baryon number density. A Fermi sea of $n_{-}$'s appears at $n_B=2.94\ \text{n}_0$; for $p_{-}$'s, at $n_B=5.31\ \text{n}_0$. The dot-dashed line indicates where the $n_{-} \rightarrow p_{+} + e^- + \bar{\nu}_e$ process terminates at $n_B = 3.25\ \text{n}_0$. The grey region indicates where direct Urca is kinematically forbidden.
• Figure 2: The rate of neutrino emission at $T=100$ keV as a function of baryon number density. Direct Urca emissivities are in solid colors and modified Urca emissivities are dashed. We vary $g_A^*$ from $-1.267$ to $+1.267$, and indicate the effect by the blue shaded region. In our model, $n_{-} \rightarrow p_{+} + e^- + \bar{\nu}_e$ is allowed between the onset of $n_{-}$'s at $n_B = 2.94\ \text{n}_0$ and their termination at $n_B = 3.25\ \text{n}_0$. Since $n_{+} \rightarrow p_{+} + e^- + \bar{\nu}_e$ is not allowed in our model, for comparison we include the results from a different model, the IUF model of Ref. Fattoyev:2010mx.
• Figure 3: The core temperature of various mass neutron stars over time since birth. The $n_{-}$ onset density is only reached in the $1.95\ \text{M}_{\odot}$ and $2.19\ \text{M}_{\odot}$ stars (dashed and solid orange curves) and therefore those stars cool faster due to the $n_{-} \rightarrow p_{+} + e^- + \bar{\nu}_e$ process. For the $2.19\ \text{M}_{\odot}$ star we vary $g_A^*$; the effect is shown with the orange shaded region. The parity singlet model (PSM) and IUF model only include $N_{+}$ and cool primarily by modified and direct Urca processes, respectively.