Dark stars and boosted dark matter annihilation rates

Abstract

Dark stars (DSs) may constitute the first phase of stellar evolution, powered by dark matter (DM) annihilation. We investigate here the properties of DSs, assuming that the DM particle has the properties required for explaining the excess positron and electron signals in the cosmic rays detected by the PAMELA and FERMI satellites. Any possible DM interpretation of these signals will require exotic DM candidates, with annihilation cross-sections a few orders of magnitude higher than the canonical value required for correct thermal relic abundance for weakly interacting DM candidates; additionally, in most models, the annihilation must be preferentially to leptons. Secondly, we study the dependence of DS properties on the concentration parameter of the initial DM density profile of the halos where the first stars are formed. We restrict our study to the DM in the star due to simple (versus extended) adiabatic contraction and minimal (versus extended) capture; this simple study is sufficient to illustrate dependence on the cross-section and concentration parameter. Our basic results are that the final stellar properties, once the star enters the main sequence, are always roughly the same, regardless of the value of the boosted annihilation or concentration parameter in the range between c=2 and c=5: stellar mass , luminosity and lifetime ∼106 years (for the minimal DM models considered here; additional DM would lead to more massive DSs). However, the lifetime, final mass and final luminosity of the DSs show some dependence on the boost factor and concentration parameter, as discussed in this paper.