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We investigate the effects of weakly-interacting massive particle (WIMP) dark matter annihilation on the formation of Population III.1 stars, which are theorized to form from the collapse of gas cores at the centers of dark matter minihalos. We consider the relative importance of cooling due to baryonic radiative processes and heating due to WIMP annihilation. We analyze the dark matter and gas profiles of several halos formed in cosmological-scale numerical simulations. The heating rate depends sensitively on the dark matter density profile, which we approximate with a power law rho_chi ~ r^{-alpha_chi}, in the numerically unresolved inner regions of the halo. If we assume a self-similar structure so that alpha_chi ~= 1.5 as measured on the resolved scales ~1pc, then for a fiducial WIMP mass of 100GeV, the heating rate is typically much smaller (<10^{-3}) than the cooling rate for densities up to n_H=10^{17}cm^{-3}. In one case, where alpha_chi=1.65, the heating rate becomes similar to the cooling rate by a density of n_H=10^{15}cm^{-3}. The dark matter density profile is expected to steepen in the central baryon-dominated region <~1pc due to adiabatic contraction, and we observe this effect (though with relatively low resolution) in our numerical models. From these we estimate alpha_chi~=2.0. The heating now dominates cooling above n_H~=10^{14}cm^{-3}, in agreement with the previous study of Spolyar, Freese & Gondolo. We expect this leads to the formation of an equilibrium structure with a baryonic and dark matter density distribution exhibiting a flattened central core. Examining such equilibria, we find total luminosities due to WIMP annihilation are relatively constant and ~10^3 L_sun, set by the radiative luminosity of the baryonic core. We discuss the implications for Pop III.1 star formation... (abridged)
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We study the effects of WIMP dark matter (DM) annihilations on the thermal and chemical evolution of the gaseous clouds where the first generation of stars in the Universe is formed. We follow the collapse of the gas inside a typical halo virializing at very high redshift, from well before virialization until a stage where the heating from DM annihilations exceeds the gas cooling rate. The DM energy input is estimated by inserting the energy released by DM annihilations (as predicted by an adiabatic contraction of the original DM profile) in a spherically symmetric radiative transfer scheme. In addition to the heating effects of the energy absorbed, we include its feedback upon the chemical properties of the gas, which is critical to determine the cooling rate in the halo, and hence the fragmentation scale and Jeans mass of the first stars. We find that DM annihilation does alter the free electron and especially the H2 fraction when the gas density is n>~ 10^4 cm^-3, for our fiducial parameter values. However, even if the change in the H2 abundance and the cooling efficiency of the gas is large (sometimes exceeding a factor 100), the effects on the temperature of the collapsing gas are far smaller (a reduction by a factor <~1.5), since the gas cooling rate depends very strongly on temperature: then, the fragmentation mass scale is reduced only slightly, hinting towards no dramatic change in the initial mass function of the first stars.
Dark matter (DM) decays and annihilations might heat and partially reionize the Universe at high redshift. Although this effect is not important for the cosmic reionization, the gas heating due to DM particles might affect the structure formation. In particular, the critical halo mass for collapse is increased up to a factor of ~2. Also the fraction of gas which collapses inside the smallest halos is substantially reduced with respect to the cosmological value. These effects imply that DM decays and annihilations might delay the formation of the first structures and reduce the total star mass in the smallest halos.
It has been proposed that during the formation of the first generation stars there might be a dark star phase in which the power of the star comes from dark matter annihilation. The adiabatic contraction process to form the dark star would result in a highly concentrated density profile of the host halo at the same time, which may give enhanced indirect detection signals of dark matter. In this work we investigate the extragalactic $gamma$-ray background from dark matter annihilation with such a dark star formation scenario, and employ the isotropic $gamma$-ray data from Fermi-LAT to constrain the model parameters of dark matter. The results suffer from large uncertainties of both the formation rate of the first generation stars and the subsequent evolution effects of the host halos of the dark stars. We find, in the most optimistic case for $gamma$-ray production via dark matter annihilation, the expected extragalactic $gamma$-ray flux will be enhanced by 1-2 orders of magnitude. In such a case, the annihilation cross section of the supersymmetric dark matter can be constrained to the thermal production level, and the leptonic dark matter model which is proposed to explain the positron/electron excesses can be well excluded. Conversely, if the positron/electron excesses are of a dark matter annihilation origin, then the early Universe environment is such that no dark star can form.
DarkCapPy is a Python 3/Jupyter package for calculating rates associated with dark matter capture in the Earth, annihilation into light mediators, and the subsequent observable decay of the light mediators near the surface of the Earth. The package includes a calculation of the Sommerfeld enhancement at the center of the Earth and the timescale for capture--annihilation equilibrium. The code is open source and can be modified for other compact astronomical objects and mediator spins.
The existence of monopoles is a characteristic signature of Kaluza-Klein multidimensional theories. The topology of these solutions is extremely interesting. The existence of a dipole solution, which we have associated to a monople-anti-monopole bound state, is the leitmotiv of this investigation. The dipole in its lowest energy state, which we here call also monopolium, is electromagnetically inert in free space interacting only gravitationally. Monopolium when interacting with time dependent magnetic fields acquires a time dependent induced magnetic moment and radiates. We have analyzed the most favorable astrophysical scenario for radiative monopolium and found that the amount of radiation is so small that is not detectable by conventional equipments. These findings suggest that Kaluza-Klein monopolium, if existent, would be a candidate for a primordial dark matter constituent.
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