Dark stars Implications and constraints from cosmic reionization and the extragalactic X-ra.pdfVIP

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Dark stars Implications and constraints from cosmic reionization and the extragalactic X-ra.pdf

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Dark stars Implications and constraints from cosmic reionization and the extragalactic X-ra

a r X i v : 0 8 0 9 .1 5 1 9 v 1 [ a s t r o - p h ] 9 S e p 2 0 0 8 Dark stars: Implications and constraints from cosmic reionization and the extragalactic X-ray background Dominik R. G. Schleicher, Robi Banerjee, Ralf S. Klessen? Institute of Theoretical Astrophysics / ZAH, Albert-Ueberle-Str. 2, D-69120 Heidelberg, Germany Dark stars powered by dark matter annihilation have been proposed as the first luminous sources in the universe. These stars are believed to form on the dark matter cusp of low-mass minihalos. Different models predict them to be even more luminous than conventional Population III stars, either due to higher mass or modifications to stellar evolution. Here we show that such dark star models would require a somewhat artificial reionization history, based on a double-reionization phase and a late star-burst near redshift z ～ 6, in order to fulfill the WMAP constraint on the optical depth as well as the Gunn-Peterson constraint at z ～ 6. This suggests that, if dark stars were common in the early universe, then models are preferred which predict a number of UV photons similar to conventional Pop. III stars. This excludes 800 M⊙ dark stars that enter a main-sequence phase and other models that lead to a strong increase in the number of UV photons. We also derive constraints from the observed X-ray and gamma-ray background, considering dark matter profiles which have been steepened during the formation of dark stars. This increases the clumping factor at high redshift and gives rise to a higher dark matter annihilation rate in the early universe. We calculate the X-ray background due to 511 keV emission and the gamma- ray background due to internal bremsstrahlung. Compared to standard NFW halo profiles, the background due to 511 keV emission is enhanced below frequencies of 100 keV in the observers frame of reference and may provide a dominant contribution to the measured background. The contribution from internal bremsstrahlung is almost unch