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Dark Stars: Begynnelsen
Abstract
The first phase of stellar evolution in the history of the universe may be Dark Stars, powered by dark matter heating rather than by fusion. Weakly interacting massive particles, which are their own antiparticles, can annihilate and provide an important heat source for the first stars in the universe. This and the following contribution present the story of Dark Stars. In this first part, we describe the conditions under which dark stars form in the early universe: 1) high dark matter densities, 2) the annihilation products get stuck inside the star, and 3) dark matter heating wins over all other cooling or heating mechanisms. Comment: 3 pages, 2 figures, and proceeding for IDM 2008
The first phase of stellar evolution in the history of the universe may be Dark Stars, powered by dark matter heating rather than by fusion. Weakly interacting massive particles, which are their own antiparticles, can annihilate and provide an important heat source for the first stars in the universe. This and the previous contribution present the story of Dark Stars. In this second part, we describe the structure of Dark Stars and predict that they are very massive (), cool (6000 K), bright (), long-lived ( years), and probable precursors to (otherwise unexplained) supermassive black holes. Later, once the initial dark matter fuel runs out and fusion sets in, dark matter annihilation can predominate again if the scattering cross section is strong enough, so that a Dark Star is born again. Comment: 3 pages, 1 figure, and conference proceeding for IDM Sweden
The paper analyzes the structure of fast shocks incident upon interstellar gas of ambient density from 10 to the 7th per cu cm, while focusing on the problems of formation and destruction of molecules and infrared emission in the cooling, neutral post shock gas. It is noted that such fast shocks initially dissociate almost all preexisting molecules. Discussion covers the physical processes which determine the post shock structure between 10 to the 4 and 10 to the 2 K. It is shown that the chemistry of important molecular coolants H2, CO, OH, and H2O, as well as HD and CH, is reduced to a relatively small set of gas phase and grain surface reactions. Also, the chemistry follows the slow conversion of atomic hydrogen into H2, which primarily occurs on grain surfaces. The dependence of this H2 formation rate on grain and gas temperatures is examined and the survival of grains behind fast shocks is discussed. Post shock heating and cooling rates are calculated and an appropriate, analytic, universal cooling function is developed for molecules other than hydrogen which includes opacities from both the dust and the lines.
We describe results from a fully self-consistent three-dimensional hydrodynamical simulation of the formation of one of the first stars in the Universe. In current models of structure formation, dark matter initially dominates, and pregalactic objects form because of gravitational instability from small initial density perturbations. As they assemble via hierarchical merging, primordial gas cools through ro-vibrational lines of hydrogen molecules and sinks to the center of the dark matter potential well. The high-redshift analog of a molecular cloud is formed. As the dense, central parts of the cold gas cloud become self-gravitating, a dense core of approximately 100 M (where M is the mass of the Sun) undergoes rapid contraction. At particle number densities greater than 10(9) per cubic centimeter, a 1 M protostellar core becomes fully molecular as a result of three-body H2 formation. Contrary to analytical expectations, this process does not lead to renewed fragmentation and only one star is formed. The calculation is stopped when optical depth effects become important, leaving the final mass of the fully formed star somewhat uncertain. At this stage the protostar is accreting material very rapidly (approximately 10(-2) M year-1). Radiative feedback from the star will not only halt its growth but also inhibit the formation of other stars in the same pregalactic object (at least until the first star ends its life, presumably as a supernova). We conclude that at most one massive (M 1 M) metal-free star forms per pregalactic halo, consistent with recent abundance measurements of metal-poor galactic halo stars.
We use N-body simulations to investigate the structure of dark halos in the standard Cold Dark Matter cosmogony. Halos are excised from simulations of cosmologically representative regions and are resimulated individually at high resolution. We study objects with masses ranging from those of dwarf galaxy halos to those of rich galaxy clusters. The spherically averaged density profiles of all our halos can be fit over two decades in radius by scaling a simple ``universal'' profile. The characteristic overdensity of a halo, or equivalently its concentration, correlates strongly with halo mass in a way which reflects the mass dependence of the epoch of halo formation. Halo profiles are approximately isothermal over a large range in radii, but are significantly shallower than near the center and steeper than near the virial radius. Matching the observed rotation curves of disk galaxies requires disk mass-to-light ratios to increase systematically with luminosity. Further, it suggests that the halos of bright galaxies depend only weakly on galaxy luminosity and have circular velocities significantly lower than the disk rotation speed. This may explain why luminosity and dynamics are uncorrelated in observed samples of binary galaxies and of satellite/spiral systems. For galaxy clusters, our halo models are consistent both with the presence of giant arcs and with the observed structure of the intracluster medium, and they suggest a simple explanation for the disparate estimates of cluster core radii found by previous authors. Our results also highlight two shortcomings of the CDM model. CDM halos are too concentrated to be consistent with the halo parameters inferred for dwarf irregulars, and the predicted abundance of galaxy halos is larger than the observed abundance of galaxies.
We derive the effects of DM decays and annihilations on structure formation. We consider moderately massive DM particles (sterile neutrinos and light DM), as they are expected to give the maximum contribution to heating and reionization. The energy injection from DM decays and annihilations produces both an enhancement in the abundance of coolants (H2 and HD) and an increase of gas temperature. We find that for all the considered DM models the critical halo mass for collapse, m_crit, is generally higher than in the unperturbed case. However, the variation of m_crit is small. In the most extreme case, i.e. considering light DM annihilations (decays) and halos virializing at redshift z_vir>10 (z_vir~10), m_crit increases by a factor ~4 (~2). In the case of annihilations the variations of m_crit are also sensitive to the assumed profile of the DM halo. Furthermore, we note that the fraction of gas which is retained inside the halo is substantially reduced (to ~40 per cent of the cosmic value), especially in the smallest halos, as a consequence of the energy injection by DM decays and annihilations.
In these lecture notes we review the current knowledge about the formation of
the first luminous objects. We start from the cosmological context of
hierarchical models of structure formation, and discuss the main physical
processes which are believed to lead to primordial star formation, i.e. the
cooling processes and the chemistry of molecules (especially H2) in a
metal-free gas. We then describe the techniques and results of numerical
simulations, which indicate that the masses of the first luminous objects are
likely to be much larger than that of present-day stars. Finally, we discuss
the scenario presented above, exposing some of the most interesting problems
which are currently being investigated, such as that of the feedback effects of
these objects.
We investigate the effects of weakly interacting massive particle (WIMP) dark matter annihilation on the formation of Population III.1 (Pop III.1) stars. 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 , in the numerically unresolved inner regions of the halo. If we assume a self-similar structure so that αχ 1.5 as measured on the resolved scales ~1 pc, then for a fiducial WIMP mass of 100 GeV, the heating rate is typically much smaller (<10–3) than the cooling rate for densities up to n
H = 1017 cm–3. In one case, where αχ = 1.65, the heating rate becomes similar to the cooling rate by a density of n
H = 1015 cm–3. The dark matter density profile is expected to steepen in the central baryon-dominated region 1 pc due to adiabatic contraction, and we observe this effect, though with low resolution, in our numerical models. From these we estimate αχ 2.0. Heating now dominates cooling above n
H 1014 cm–3, in agreement with the study of Spolyar, Freese, & Gondolo. We expect that this leads to the formation of an equilibrium structure with baryonic and dark matter density distributions exhibiting a flattened central core. Examining such equilibria, we find that the total luminosities due to WIMP annihilation are relatively constant and ~103L
☉, set by the radiative luminosity of the baryonic core. We discuss the implications for Pop III.1 star formation, particularly the subsequent growth of the protostar. Even if the initial protostar fails to accumulate any additional dark matter, its contraction to the main sequence could be significantly delayed by WIMP annihilation heating, potentially raising the mass scale of Pop III.1 stars to masses 100 M
☉.
We study the formation of the first generation of stars in the standard cold dark matter model. We use a very high resolution cosmological hydrodynamic simulation that achieves a dynamic range of ~1010 in length scale. With accurate treatment of atomic and molecular physics, including the effect of molecular line opacities and cooling by collision-induced continuum emission, it allows us to study the chemothermal evolution of primordial gas clouds to densities up to ρ ~ 2 × 10-8 g cm-3 (nH ~ 1016 cm-3) without assuming any a priori equation of state, an improvement of 6 orders of magnitude over previous three-dimensional calculations. We study the evolution of a primordial star-forming gas cloud in the cosmological simulation in detail. The cloud core becomes marginally unstable against chemothermal instability when the gas cooling rate is increased owing to three-body molecule formation. However, since the core is already compact at that point, runaway cooling simply leads to fast condensation to form a single protostellar seed. During the final dynamical collapse, small angular momentum material collapses faster than the rest of the gas and selectively sinks inward. Consequently, the central regions have little specific angular momentum, and rotation does not halt collapse. We, for the first time, obtain an accurate gas mass accretion rate within a 10 M☉ innermost region around the protostar. We carry out protostellar evolution calculations using the obtained accretion rate. The resulting mass of the first star when it reaches the zero-age main sequence is MZAMS ~ 100 M☉, and less (60 M☉) for substantially reduced accretion rates.
Varied evidence suggests that galaxies consist of roughly 10 percent baryonic matter by mass and that baryons sink dissipatively by about a factor of 10 in. radius during galaxy formation. It is shown that such infall strongly perturbs the underlying dark matter distribution, pulling it inward and creating cores that are considerably smaller and denser than would have evolved without dissipation. Any discontinuity between the baryonic and dark matter mass distributions is smoothed out by the coupled motions of the two components. If dark halos have large core radii in the absence of dissipation, the above infall scenario yields rotation curves that are flat over large distances, in agreement with observations of spiral galaxies. Such large dissipationless cores may plausibly result from large internal kinetic energy in protogalaxies at maximum expansion, perhaps as a result of subclustering, tidal effects, or anisotropic collapse.
We have performed a large set of high-resolution cosmological simulations using smoothed particle hydrodynamics to study the formation of the first luminous objects in the Lambda cold dark matter cosmology. We follow the collapse of primordial gas clouds in eight early structures and document the scatter in the properties of the first star-forming clouds. Our first objects span formation redshifts from z∼ 10 to ∼50 and cover an order of magnitude in halo mass. We find that the physical properties of the central star-forming clouds are very similar in all of the simulated objects despite significant differences in formation redshift and environment. This suggests that the formation path of the first stars is largely independent of the collapse redshift; the physical properties of the clouds have little correlation with spin, mass or assembly history of the host halo. The collapse of protostellar objects at higher redshifts progresses much more rapidly due to the higher densities, which accelerates the formation of molecular hydrogen, enhances initial cooling and shortens the dynamical time-scales. The mass of the star-forming clouds cover a broad range, from a few hundred to a few thousand solar masses, and exhibit various morphologies: some have disc-like structures which are nearly rotational supported; others form flattened spheroids; still others form bars. All of them develop a single protostellar ‘seed’ which does not fragment into multiple objects up to the moment that the central gas becomes optically thick to H2 cooling lines. At this time, the instantaneous mass accretion rate on to the centre varies significantly from object to object, with disc-like structures having the smallest mass accretion rates. The formation epoch and properties of the star-forming clouds are sensitive to the values of cosmological parameters.
The first phase of stellar evolution in the history of the universe may be Dark Stars, powered by dark matter heating rather than by fusion. Weakly interacting massive particles, which are their own antiparticles, can annihilate and provide an important heat source for the first stars in the universe. This and the previous contribution present the story of Dark Stars. In this second part, we describe the structure of Dark Stars and predict that they are very massive (), cool (6000 K), bright (), long-lived ( years), and probable precursors to (otherwise unexplained) supermassive black holes. Later, once the initial dark matter fuel runs out and fusion sets in, dark matter annihilation can predominate again if the scattering cross section is strong enough, so that a Dark Star is born again. Comment: 3 pages, 1 figure, and conference proceeding for IDM Sweden
The formation of the first stars and quasars marks the transformation of the
universe from its smooth initial state to its clumpy current state. In popular
cosmological models, the first sources of light began to form at redshift 30
and reionized most of the hydrogen in the universe by redshift 7. Current
observations are at the threshold of probing the hydrogen reionization epoch.
The study of high-redshift sources is likely to attract major attention in
observational and theoretical cosmology over the next decade.
The first stars in the universe form inside dark matter
(DM) haloes whose initial density profiles are laid down by gravitational
collapse in hierarchical structure formation scenarios. During the formation of
the first stars in the universe, the baryonic infall compresses the dark matter
further. The resultant dark matter density is presented here, using an
algorithm originally developed by Young to calculate changes to the profile as
the result of adiabatic infall in a spherical halo model; the Young
prescription takes into account the non-circular motions of halo particles. The
density profiles obtained in this way are found to be within a factor of two of
those obtained using the simple adiabatic contraction prescription of
Blumenthal et al. Our results hold regardless of the nature of the dark matter
or its interactions and rely merely on gravity. If the dark matter consists of
weakly interacting massive particles, which are their own antiparticles, their
densities are high enough that their annihilation in the first protostars can
indeed provide an important heat source and prevent the collapse all the way to
fusion. In short, a ``Dark Star'' phase of stellar evolution, powered by DM
annihilation, may indeed describe the first stars in the universe.
Gondolo astro-ph/0705
- D Spolyar
- K Freese
D. Spolyar, K. Freese, & P. Gondolo astro-ph/0705.0521, 2008, Phys. Rev. Lett., 100, 051101