The evolution of protostars powered by dark matter annihilation. I. Fiducial model and first results

Abstract

The existence of billion-solar-mass quasars at redshifts $z\gtrsim 7$ poses a formidable challenge to theories of black hole formation, requiring pathways for the rapid growth of massive seeds. One such pathway arises from primordial stars powered by dark matter (DM) self-annihilation rather than conventional fusion, which could form massive black hole seeds in ordinary cosmological mini-haloes. Here we present a suite of stellar evolution models for DM-powered protostars, computed with the \textsc{GENEC} code. We explored a wide parameter space, spanning ambient WIMP densities of $ρ_χ\sim 10^{12}\mbox{--}10^{16}\,\mathrm{GeV\,cm^{-3}}$ and gas accretion rates of $10^{-3}\mbox{--}10^{-1}\,M_\odot\,\mathrm{yr^{-1}}$, to quantify the effects of DM annihilation. A central finding is that for a protostar to grow to supermassive scales ($\gtrsim 10^5{M}_{\odot }$), the ambient DM density in the vicinity of the star must exceed a critical threshold of ${\rho }_{\chi }\gtrsim 5\times 10^{14}{\text{GeV cm}}^{-3}$. The energy injected by WIMP annihilation inflates the protostar, lowering its surface temperature, which suppresses the ionising feedback that would otherwise halt accretion and delays the onset of hydrogen fusion. In dense halos (${\rho }_{\chi }\gtrsim 10^{15}\mathrm{GeV}\mathrm{c}{\mathrm{m}}^{-3}$), stars remain stable against general relativistic instability beyond $10^6{M}_{\odot }$, whereas at lower densities (${\rho }_{\chi }\lesssim 10^{13}\mathrm{GeV}\mathrm{c}{\mathrm{m}}^{-3}$), they collapse at masses of $\sim 5\times 10^5{M}_{\odot }$. Once the DM fuel is exhausted and core burning commences, the protostar contracts and its ionising photon output can reach high levels $\sim 10^{53}{\mathrm{s}}^{-1}$. These distinct evolutionary phases offer clear observational signatures for the \textsc{JWST}, providing a robust, physically grounded pathway for forming heavy black hole seeds in the early Universe.

11 pages, 7 figures, accepted in A&A