Compact Stars as Dark Matter Probes

TL;DR

This work investigates how DM accretion by compact stars can serve as a probe of DM properties. It develops a capture–annihilation framework, introducing the capture rate $\Gamma_c$, annihilation rate $\Gamma_a$, and equilibration timescale $\tau_{eq}$, and applies it to white dwarfs in globular clusters (notably M4) and neutron stars in high-DM environments. By modeling the DM density profiles and stellar structure, it derives potential DM-heating signatures and translates them into constraints on the DM density and the WIMP–nucleon cross section $\sigma_{si}$, including the possibility of DM forming self-gravitating cores or degenerate dark stars within neutron stars. Overall, the paper demonstrates that, in regions of elevated DM density, compact stars offer a complementary, astrophysical avenue to constrain DM properties beyond terrestrial experiments and collider searches.

Abstract

We discuss the consequences of the accretion of dark matter (DM) particles on compact stars such as white dwarfs and neutron stars. We show that in large regions of the DM parameter space, these objects are sensitive probes of the presence of DM and can be used to set constraints both on the DM density and on the physical properties of DM particles.

Figures (4)

• Figure 1: Density of baryons and DM in the central region of the globular cluster M4. The solid line shows the density of baryons as a function of radius. The dotted and dashed lines represent the density of DM for the normal NFW profile and for an adiabatically contracted profile, respectively.
• Figure 2: Luminosity vs. temperature for the white dwarfs in the inner field of the observations made in paper richerm4 (data points). Also plotted (lines) is the minimum luminosity expected for white dwarfs radiating as black bodies, for WIMP-nucleon cross sections of $10^{-43}$ cm$^2$ and $10^{-44}$ cm$^2$. See text for details.
• Figure 3: Temperature of neutron stars due to the accretion of WIMPs as a function of distance from the centre of the galaxy. The two lines correspond to two different density profiles, $\rho\propto r^{-1}$ and $\rho \propto r^{-1.5}$.
• Figure 4: Different outcomes of the accumulation of DM inside a neutron star, in the DM mass vs. annihilation cross section plane, for a capture rate of $\Gamma_c=({\rm 100 GeV}/m_{dm})10^{29}\rm s^{-1}$. In the top left corner we show the results of a scan of the supersymmetric parameter space, where neutralino models are compatible with accelerator and cosmological constraints (as obtained with DarkSUSY Gondolo:2004sc). The solid line is relative to viable DM models in Universal Extra-dimensions (see text for further details). In the shaded region below the dashed line, DM particles become self-gravitating before equilibrium between capture and annihilation is reached. For models on the right of the dotted vertical line, particles reach the critical mass for gravitational collapse in less than 1 Gyr. These models can likely be ruled out, since they lead either to large injection of energy in the NS core, or to gravitational collapse to a Black Hole, rapidly destroying the star.