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TOPIC: The first stars


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Primordial stars
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Title: Powerful explosions at Z=0 ?
Authors: S. Ekström, G. Meynet, R. Hirschi, A. Maeder

Metal-free stars are assumed to evolve at constant mass because of the very low stellar winds. This leads to large CO-core mass at the end of the evolution, so primordial stars with an initial mass between 25 and 85 Msol are expected to end as direct black holes, the explosion energy being too weak to remove the full envelope.
We show that when rotation enters into play, some mass is lost because the stars are prone to reach the critical velocity during the main sequence evolution. Contrarily to what happens in the case of very low- but non zero-metallicity stars, the enrichment of the envelope by rotational mixing is very small and the total mass lost remains modest. The compactness of the primordial stars lead to a very inefficient transport of the angular momentum inside the star, so the profile of Omega(r) is close to Omega r^2 = const. As the core contracts, the rotation rate increases, and the star ends its life with a fast spinning core. Such a configuration has been shown to modify substantially the dynamics of the explosion. Where one expected a weak explosion or none at all, rotation might boost the explosion energy and drive a robust supernova. This will have important consequences in the way primordial stars enriched the early Universe.

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RE: The first stars
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The very first stars were indeed giants, according to a new simulation. It shows that huge stars coalesced out of the primordial gas about 300 million years after the big bang, ending the cosmic dark ages and spawning later generations of stars, including our Sun.
Other groups have simulated various parts of the process, but a team led by Naoki Yoshida of Nagoya University in Japan has built a much more complete picture of how the first stars formed.

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Title: Where can we really find the First Stars' Remnants today?
Authors: M. Trenti, M. R. Santos, M. Stiavelli (STScI)

A number of recent numerical investigations concluded that the remnants of rare structures formed at very high redshift, such as the very first stars and bright redshift z~6 QSOs, are preferentially located at the centre of the most massive galaxy clusters at redshift z=0. In this paper we readdress this question using a combination of cosmological simulations of structure formation and extended Press-Schechter formalism and we show that the typical remnants of Population III stars are instead more likely to be found in a group environment, that is in dark matter halos of mass ~2x10^{13} h^{-1}M_sun. Similarly, the descendants of the brightest z~6 QSOs are expected to be in medium-sized clusters (mass of a few 10^{14} h^{-1}M_sun), rather than in the most massive superclusters (M>10^{15} h^{-1}M_sun) found within the typical 1 Gpc^3 cosmic volume where a bright z~6 QSO lives. The origin of past claims that the most massive clusters preferentially host these remnants is rooted in the numerical method used to initialise their numerical simulations: Only a small region of the cosmological volume of interest was simulated with sufficient resolution to identify low-mass halos at early times, and this region was chosen to host the most massive halo in the cosmological volume at late times. The conclusion that the earliest structures formed in the entire cosmological volume evolve into the most massive halo at late times was thus arrived at by construction. We demonstrate that, to the contrary, the first structures to form in a cosmological region evolve into relatively typical objects at later times. We propose alternative numerical methods for simulating the earliest structures in cosmological volumes.

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Population III Stars
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The universe's first stars were the rock stars of the stellar world: they lived fast and died young, burning out in only a few hundred thousand years.
But new research suggests some of them might still be around as a result of interactions with dark matter, which halted their growth and curbed their blazing excess.

"These stars can be frozen for timescales longer than the age of the universe" - Gianfranco Bertone of the Paris Institute of Astrophysics in France.

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RE: The first stars
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Title: The Destruction of Cosmological Minihalos by Primordial Supernovae
Authors: Daniel Whalen, Bob van Veelen, Brian W. O'Shea, Michael L. Norman
(Version v2)

We present numerical simulations of primordial supernovae in cosmological minihalos at z  ~ 20. We consider Type II supernovae, hypernovae, and pair instability supernovae (PISN) in halos from 6.9 x 10^5 - 1.2 x 10^7 \Ms, those in which Population III stars are expected to form via H_2 cooling. The supernovae evolve along two evolutionary paths according to whether they explode in H2 regions or neutral halos. Those in H2 regions first expand adiabatically and then radiate strongly upon collision with baryons ejected from the halo during its photoevaporation by the progenitor. Explosions in neutral halos promptly emit most of their kinetic energy as x-rays, but retain enough momentum to seriously disrupt the halo. We find that the least energetic of the supernovae are capable of destroying halos <~ 10^7 \Ms, while a single PISN can destroy even more massive halos. Blasts in H2 regions disperse heavy elements into the IGM, but neutral halos confine the explosion and its metals. In H2 regions, a prompt second generation of stars may form in the remnant at radii of 100 - 200 pc in the halo. Explosions confined by large halos instead recollapse, with infall rates in excess of 10^{-2} \Ms yr^{-1} that heavily contaminate their interior. This fallback may either fuel massive black hole growth at very high redshifts or create the first globular cluster with a radius of 10 - 20 pc at the centre of the halo. Our findings allow the possibility that the first primitive galaxies formed sooner, with greater numbers of stars and distinct chemical abundance patterns, than in current models.

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Title:  Dark matter annihilation effects on the first stars
Authors: F. Iocco, A. Bressan, E. Ripamonti, R. Schneider, A. Ferrara, P. Marigo

We study the effects of WIMP 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 (MS) phase of a grid of metal-free stars with masses in the range 5-600 solar mass forming in the centre of a 1e6 solar mass halo at redhisft z=20. DM particles of the parent halo are accreted in the proto-stellar interior by adiabatic contraction and scattering/capture processes, reaching central densities of order 1e12 GeV/cm3 at radii of the order of the AU: energy release from annihilation reactions can effectively counteract the gravitational collapse. This induces a transient stalling phase (i.e. a "dark" star) lasting from 2.1e3 yr (M=600 solar mass) to 1.8e4 yr (M=9 solar mass). 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, (iii) elastic scattering cross section with baryons. For our fiducial set of parameters (10 km/s, 1e11 GeV/cm3, 1e-38 cm2) we find that the evolution of stars of mass lower than 40 solar masses "freezes" on the HR diagram before reaching the ZAMS. Stars with bigger masses manage to ignite nuclear reactions; however, DM "burning" prolongs their lifetimes by a factor 2 (5) for a 600 (40) solar mass star.

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When scientists look at what makes a star and what makes it burn, they turn to fusion. However, according to new research, this may not have been the case for the stars classified as Population 3."

The first stars were different in a lot of ways - Katherine Freese, a theoretical physicist at the University of Michigan.

According to Freese, dark matter annihilation was the source of energy for the earliest stars, rather than fusion, when the universe was only 100 to 200 million years young.

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Title: The First Stars
Authors: Jarrett L. Johnson, Thomas H. Greif, Volker Bromm

The formation of the first generations of stars at redshifts z > 15-20 signalled the transition from the simple initial state of the universe to one of increasing complexity. We here review recent progress in understanding the assembly process of the first galaxies, starting with cosmological initial conditions and modelling the detailed physics of star formation. In particular, we study the role of HD cooling in ionised primordial gas, the impact of UV radiation produced by the first stars, and the propagation of the supernova blast waves triggered at the end of their brief lives. We conclude by discussing how the chemical abundance patterns observed in extremely low-metallicity stars allow us to probe the properties of the first stars.

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HE 1523-0901.kmz
Google Sky file (1kb, kmz)

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Title: The 21cm Signature of the First Stars
Authors: Xuelei Chen, Jordi Miralda-Escude
(Version v2)

We predict the 21-cm signature of the first metal-free stars. The soft X-rays emitted by these stars penetrate the atomic medium around their host halos, generating Lyman alpha photons that couple the spin and kinetic temperatures. These creates a region we call the Lyman alpha sphere, visible in 21-cm against the CMB, which is much larger than the HII region produced by the same star. The spin and kinetic temperatures are strongly coupled before the X-rays can substantially heat the medium, implying that a strong 21-cm absorption signal from the adiabatically cooled gas in Hubble expansion around the star is expected when the medium has not been heated previously. A central region of emission from the gas heated by the soft X-rays is also present although with a weaker signal than the absorption. The Lyman alpha sphere is a universal signature that should be observed around any first star illuminating its vicinity for the first time. The 21-cm radial profile of the Lyman alpha sphere can be calculated as a function of the luminosity, spectrum and age of the star. For a star of a few hundred solar masses and zero metallicity (as expected for the first stars), the physical radius of the Lyman alpha sphere can reach tens of kiloparsecs. The first metal-free stars should be strongly clustered because of high cosmic biasing; this implies that the regions producing a 21-cm absorption signal may contain more than one star and will generally be irregular and not spherical, because of the complex distribution of the gas. We discuss the feasibility of detecting these Lyman alpha spheres, which would be present at redshifts z ~ 30 in the Cold Dark Matter model. Their observation would represent a direct proof of the detection of a first star.

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