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Dark Stars: D\"od och \AA teruppst\aa ndelse
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 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
What is the connection between how the dark matter was produced in the early
universe and how we can detect it today? Where does the WIMP miracle come from,
and is it really a "WIMP" miracle? What brackets the mass range for thermal
relics? Where does <\sigma v> come from, and what does it mean? What is the
difference between chemical and kinetic decoupling? Why do some people think
that dark matter cannot be lighter than 40 GeV? Why is b\bar b such a popular
annihilation final state? Why is antimatter a good way to look for dark matter?
Why should the cosmic-ray positron fraction decline with energy, and why does
it not? How does one calculate the flux of neutrinos from dark matter
annihilation in a celestial body, and when is that flux independent of the dark
matter pair-annihilation rate? How does dark matter produce photons? Read these
lecture notes, do the suggested 10 exercises, and you will find answers to all
of these questions (and to many more on what You Always Wanted to Know About
Dark Matter But Were Afraid to Ask).
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 discovery of luminous quasars at redshift z ~ 6 indicates the presence of supermassive black holes (SMBHs) of mass ~109 M☉ when the universe was less than 1 billion years old. This finding presents several challenges for theoretical models because whether such massive objects can form so early in the ΛCDM cosmology, the leading theory for cosmic structure formation, is an open question. Furthermore, whether the formation process requires exotic physics such as super-Eddington accretion remains undecided. Here we present the first multiscale simulations that, together with a self-regulated model for the SMBH growth, produce a luminous quasar at z ~ 6.5 in the ΛCDM paradigm. We follow the hierarchical assembly history of the most massive halo in a ~3 Gpc3 volume and find that this halo of ~8 × 1012 M☉ forming at z ~ 6.5 after several major mergers is able to reproduce a number of observed properties of SDSS J1148+5251, the most distant quasar detected at z = 6.42 (Fan et al. 2003). Moreover, the SMBHs grow through gas accretion below the Eddington limit in a self-regulated manner owing to feedback. We find that the progenitors experience vigorous star formation (up to 104 M☉ yr-1) preceding the major quasar phase such that the stellar mass of the quasar host reaches 1012 M☉ at z ~ 6.5, consistent with observations of significant metal enrichment in SDSS J1148+5251. The merger remnant thus obeys a similar MBH-Mbulge scaling relation observed locally as a consequence of coeval growth and evolution of the SMBH and its host galaxy. Our results provide a viable formation mechanism for z ~ 6 quasars in the standard ΛCDM cosmology and demonstrate a common, merger-driven origin for the rarest quasars and the fundamental MBH-Mbulge correlation in a hierarchical universe.
We study the impact of the capture and annihilation of Weakly Interacting Massive Particles (WIMPs) on the evolution of Pop III stars. With a suitable modification of the Geneva stellar evolution code, we study the evolution of 20 and 200 M stars in Dark Matter Haloes with densities between 10 and GeV/cm during the core H-burning phase, and, for selected cases, until the end of the core He-burning phase. We find that for WIMP densities higher than 5.3 GeV cm the core H-burning lifetime of and stars exceeds the age of the Universe, and stars are sustained only by WIMP annihilations. We determine the observational properties of these `frozen` objects and show that they can be searched for in the local Universe thanks to their anomalous mass-radius relation, which should allow unambiguous discrimination from normal stars. Comment: 4 pages, 4 figures
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
Recent theoretical studies have revealed the possibly important role of the
capture and annihilation process of weakly interacting massive particles
(WIMPs) for the first stars. Using new evolutionary models of metal-free
massive stars, we investigate the impact of such ``dark matter burning'' for
the first stars in different environments of dark matter (DM) halos, in terms
of the ambient WIMP density (rho_chi). We find that, in agreement with existing
literature, stellar life times can be significantly prolonged for a certain
range of rho_\chi (i.e., 10^{10} ~ < \rho_\chi GeV/cm3 ~ < 10^{11} with the
current upper limit for the spin-dependent elastic scattering cross section
sigma = 5*10^{-39} cm2}). This greatly enhances the role of rotationally
induced chemical mixing in rotating stars, in favor of abundant production of
primary nitrogen, massive helium stars and long gamma-ray bursts, from the
first stars. We also find that stars with rho_chi > 2*10^{11} GeV/cm3 may not
undergo nuclear burning stages, confirming the previous work, and that ionizing
photon fluxes from such DM supported stars are very weak. Delayed metal
enrichment and slow reionization in the early universe would have resulted if
most of the first stars had been born in DM halos with such high rho_\chi,
unless it had been lowered significantly below the threshold for efficient DM
burning on a short time scale.
The annihilation of weakly interacting massive particles can provide an important heat
source for the first (Pop III, 'Pop' standing for 'population') stars, potentially leading to a
new phase of stellar evolution known as a 'dark star'. When dark matter (DM)
capture via scattering off baryons is included, the luminosity from DM annihilation
may dominate over the luminosity due to fusion, depending on the DM density
and scattering cross section. The influx of DM due to capture may thus prolong
the dark star phase of stellar evolution as long as the ambient DM density is
high enough. Comparison of DM luminosity with the Eddington luminosity for
the star may constrain the stellar mass of zero-metallicity stars. Alternatively, if
sufficiently massive Pop III stars are found, they might be used to bound dark matter
properties.
Assuming that Dark Matter is dominated by WIMPs, it accretes by gravitational attraction and scattering over baryonic material and annihilates inside celestial objects, giving rise to a "Dark Luminosity" which may potentially affect the evolution of stars. We estimate the Dark Luminosity achieved by different kinds of stars in a halo with DM properties characteristic of the ones where the first star formation episode occurs. We find that either massive, metal-free and small, galactic-like stars can achieve Dark Luminosities comparable or exceeding their nuclear ones. This might have dramatic effects over the evolution of the very first stars, known as Population III. Comment: One table (with data for actual ZAMS metal-free stars) added with respect to published version
A variety of host galaxy (bulge) parameters are examined in order to determine their predictive power in ascertaining the masses of the supermassive black holes (SMBH) at the centers of the galaxies. Based on a sample of 23 nearby galaxies, comprised of both elliptical galaxies and spiral/lenticular bulges, we identify a strong correlation between the bulge gravitational binding energy (), as traced by the stellar light profile, and the SMBH mass (), such that . The scatter about the relationship indicates that this is as strong a predictor of as the velocity dispersion (), for the elliptical galaxy subsample. Improved mass-to-light ratios, obtained with IFU spectroscopy and I-band photometry by the SAURON group, were used for those sample galaxies where available, resulting in an energy predictor with the same slope, but with reduced scatter. Alternative predictors such as the gravitational potential and the bulge mass are also explored, but these are found to be inferior when compared with both the bulge gravitational binding energy and bulge velocity dispersion predictors, for the full galaxy sample. Comment: accepted for publication in ApJ
We study the effects of weakly interacting massive particles (WIMPs) dark matter (DM) on the collapse and evolution of the
first stars in the Universe. Using a stellar evolution code, we follow the pre-main-sequence (pre-MS) phase of a grid of metal-free
stars with masses in the range 5 ≤M*≤ 600 M⊙ forming in the centre of a 106M⊙ halo at z= 20. DM particles of the parent halo are accreted in the protostellar interior by adiabatic contraction and scattering/capture
processes, reaching central densities of O(1012 GeV cm−3) at radii of the order of 10 au. Energy release from annihilation reactions can effectively counteract the gravitational
collapse, in agreement with results from other groups. We find this stalling phase (known as a dark star) is transient and lasts from 2.1 × 103yr (M*= 600 M⊙) to 1.8 × 104yr (M*= 9 M⊙). Later in the evolution, DM scattering/capture rate becomes high enough that energy deposition from annihilations significantly
alters the pre-MS evolution of the star in a way that depends on DM (i) velocity dispersion, , (ii) density, ρ, (iii) elastic scattering cross-section with baryons, σ0. For our fiducial set of parameters we find that the evolution of stars of mass M* < 40 M⊙‘freezes’ on the HR diagram before reaching the zero-age main sequence (ZAMS). Stars with M*≥ 40 M⊙ manage to ignite nuclear reactions; however, DM ‘burning’ prolongs their lifetimes by a factor of 2 (5) for a 600 M⊙ (40 M⊙) star. For ρ≳ 1012GeV cm−3, and same values of the other parameters, we find that all our models are entirely supported by DM annihilation and ‘freeze’
on the HR diagram before igniting nuclear reactions.
- F Iocco
F. Iocco 2008, ApJ Letters, 677, 1
- Y X Li
Y. X. Li et al. 2007, ApJ, 665, 187