Cooling of dark neutron stars

TL;DR

This study evaluates the cooling of dark-matter-admixed neutron stars by combining a realistic Brueckner-Hartree-Fock nuclear EOS with a fermionic self-interacting DM component described by mass $μ$ and fraction $f$. Using a two-fluid TOV framework and NSCool cooling simulations, the authors show that DM modifies the internal density profiles and global structure, thereby shifting direct Urca (DU) and proton 1S0 pairing thresholds and altering cooling trajectories. With pairing quenched and non-quenched DU channels, the cooling behavior spans slow to fast regimes across the $(f, μ)$ parameter space, enabling scenarios where very massive stars cool slowly or very light stars cool rapidly. The results highlight potential observational signatures of DM in neutron stars and emphasize the dependence on the nuclear EOS and DM model, while acknowledging speculative formation mechanisms and the need for additional data to constrain DM-NS scenarios.

Abstract

We study the cooling of isolated dark-matter-admixed neutron stars, employing a realistic nuclear equation of state and realistic nuclear pairing gaps, together with fermionic dark matter of variable particle mass and dark-matter fraction. The related parameter space is scanned for the stellar structural and cooling properties. We find that a consistent description of all current cooling data requires fast direct Urca cooling and reasonable proton 1S0 gaps. Dark matter affects the cooling properties by a modification of the nuclear density profiles, but also changes stellar radius and maximum mass. Possible signals of a large dark matter content could be a very massive but slow-cooling star or a very light but fast-cooling star.

Figures (10)

• Figure 1: The threshold condition for the DU process (a), the proton fraction (b), the scaled p1S0 gap (c), and the NS mass (d) vs. the nucleon (central) density for the V18 EOS. Vertical lines indicate the onset of DU cooling, the disappearance of the p1S0 gap, and the $M_\text{max}$ configuration.
• Figure 2: Mass-radius relations of pure dark stars for different values of the DM particle mass $\mu=0.5,1,2\;\text{GeV}$ [and associated interaction parameter $y$, Eq. (\ref{['e:ymu']})] (colored curves), in comparison with the NM V18 EOS (black curve). The horizontal lines indicate the maximum mass of a pure NS, $M_\text{max}=2.34\,M_\odot$, and $M=1.4,2.0\,M_\odot$. Observational constraints on masses Cromartie20Romani22 and radii $R_{1.4}$ and $R_{2.0}$ from NICER Miller21Rutherford24 are included.
• Figure 3: The DNS mass-radius relations for fixed DM fractions $f=0,0.1,...,1$ (colored curves) and for different DM particle masses $\mu=0.5,1,2\;\text{GeV}$ (panels). The solid black curves indicate the sequence of maximum masses. The broken black curves connect the $R_N=R_D$ positions $M_\text{cr}$ on the fixed-$f$ curves. The insets visualize both curves in the $(M,f)$ plane. DM-core and DM-halo domains are indicated. Note the different $R$ scales. The $M/\,M_\odot=1.4,2.0$ horizontal lines are to guide the eye.
• Figure 4: The radial oscillation frequencies of the DNS configurations in Fig. \ref{['f:mrf']}.
• Figure 5: Profiles of energy densities $\epsilon_N$ and $\epsilon_D$ (upper row) and nucleon density $\rho$ (lower row) for fixed $f=0.5$, $\mu=0.5,1,2\;\text{GeV}$, $M=1.0\,M_\odot,1.1\,M_\odot,...,M_\text{max}$ (indicated in the legend). The vertical lines indicate the radii $R_N$ and $R_D$, and the horizontal lines the densities of DU onset and vanishing p1S0 gap. In the top right panel, $\epsilon_D$ is scaled down by a factor 1/4. Note the different $r$ scales.