Rapid neutron star cooling triggered by dark matter

Abstract

We study the effect of asymmetric fermionic dark matter (DM) on the thermal evolution of neutron stars (NSs). No interaction between DM and baryonic matter is assumed, except the gravitational one. Using the two-fluid formalism, we show that DM accumulated in the core of a star pulls inwards the outer baryonic layers of the star, increasing the baryonic density in the NS core. As a result, it significantly affects the star’s thermal evolution by triggering an early onset of the direct Urca process and modifying the photon emission from the surface caused by the decrease of the radius. Thus, due to the gravitational pull of DM, the direct Urca process becomes kinematically allowed for stars with lower masses. Based on these results, we discuss the importance of NS observations at different distances from the Galactic center. Since the DM distribution peaks towards the Galactic center, NSs in this region are expected to contain higher DM fractions that could lead to a different cooling behavior.

The Impact of Asymmetric Dark Matter on the Thermal Evolution of Nucleonic and Hyperonic Compact Stars

“…The model reproduced the Cas A data with 1.66 M ⊙ and 1.91 M ⊙ stars with the inclusion of neutron and proton singlet pairings, as well as with the 1.96 M ⊙ star for unpaired matter [140]. As it was shown in [90] the FSU2R EoS describes the Cas A cooling with a combination of n 1 S 0 (SFB model), p 1 S 0 (CCDK model), n 3 P 2 pairing (T72 model) with the maximum critical temperature T c = 7.105 • 10 8 K. In Ref. [128], the best fit of Cas A within the FSU2H EoS is obtained for the 1.88 M ⊙ star considering the proton and neutron single pairing together with the triplet neutron pairing characterized by the maximum critical temperature T c ∼ 1.41 • 10 9 K. For the same EoS, we also see a better agreement with Cas A data while varying the neutron 3 P 2 pairing.…”

“…We show that the rapid cooling of the CCO in Cas A could be explained by the DMadmixed star of 1.2 M ⊙ described by the FSU2H EoS. In [90] the best fit of the Cas A cooling rate was obtained for the M = 1.6 M ⊙ star modeled within the FSU2R EoS with f DM = 4% and light-elements envelope. Thus, the accrued DM helps to reconcile the star mass at which the DU process is kinematically allowed with the observational data on the mass of the CCO in Cas A by lowering the mass compared to a pure BM star.…”

“…As was discussed in Ref. [87], another consequence of the presence of the DM core is related to the reduction in the stellar radius due to the stronger gravitational pull compared to a pure BM configuration. This effect is presented in Table 4.…”

“…As a result, it is triggered in stars of lower mass. In this work, we extend our previ-ous analysis [87] by considering hyperons. We investigate an interplay between hyperons and DM in the inner NS region and the modification of the stars' thermal evolution.…”

“…To model the thermal evolution of NSs we adopted the public code NSCool [123], using the implementation method described by Ávila et al [90]. The specific heat is calculated as the sum of the contributions from its constituent particles: neutrons, protons, electrons, and hyperons (if present) using the standard NSCool implementation.…”