Dark matter annihilation effects on the first stars

We study the effects of weakly interacting massive particles (WIMPs) dark matter (DM) on the collapse and evolution of the first stars in the Universe. Using a stellar evolution code, we follow the pre-main-sequence (pre-MS) phase of a grid of metal-free stars with masses in the range 5 ≤M_*≤ 600 M_⊙ forming in the centre of a 10⁶ M_⊙ halo at z= 20. DM particles of the parent halo are accreted in the protostellar interior by adiabatic contraction and scattering/capture processes, reaching central densities of O(10¹²  GeV cm⁻³) at radii of the order of 10 au. Energy release from annihilation reactions can effectively counteract the gravitational collapse, in agreement with results from other groups. We find this stalling phase (known as a dark star) is transient and lasts from 2.1 × 10³ yr (M_*= 600 M_⊙) to 1.8 × 10⁴ yr (M_*= 9 M_⊙). Later in the evolution, DM scattering/capture rate becomes high enough that energy deposition from annihilations significantly alters the pre-MS evolution of the star in a way that depends on DM (i) velocity dispersion, , (ii) density, ρ, (iii) elastic scattering cross-section with baryons, σ₀. For our fiducial set of parameters we find that the evolution of stars of mass M_* < 40 M_⊙‘freezes’ on the HR diagram before reaching the zero-age main sequence (ZAMS). Stars with M_*≥ 40 M_⊙ manage to ignite nuclear reactions; however, DM ‘burning’ prolongs their lifetimes by a factor of 2 (5) for a 600 M_⊙ (40 M_⊙) star. For ρ≳ 10¹² GeV cm⁻³, and same values of the other parameters, we find that all our models are entirely supported by DM annihilation and ‘freeze’ on the HR diagram before igniting nuclear reactions.