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TOPIC: The first stars


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Astronomy Talk: Fingerprints of the First Stars

Our current story for the origin of the heavy elements has at its core a recycling program on the grandest of scales. Using Keck, we have traced atoms as they flow from the intergalactic medium into galaxies, where they are incorporated into stars, undergo fusion, and are returned in supernovae and other types of stellar death.
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Planck telescope puts new datestamp on first stars

Scientists working on Europe's Planck satellite say the first stars in the Universe lit up later than was previously thought.
The team has made the most precise map of the "oldest light" in the cosmos.
Earlier observations of this radiation had suggested that the first generation of stars burst into life about 420 million years after the Big Bang.

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Title: Are there any first-generation stars in globular clusters today?
Author: Corinne Charbonnel, William Chantereau, Martin Krause, Francesca Primas, Yue Wang

Several models compete to explain the abundance properties of stellar populations in globular clusters. One of the main constraints is the present-day ratio of first- and second-generation stars that are currently identified based on their sodium content. We propose an alternative interpretation of the observed sodium distribution, and suggest that stars with low sodium abundance that are counted as members of the first stellar generation could actually be second-generation stars. We compute the number ratio of second-generation stars along the Na distribution following the fast rotating massive star model using the same constraints from the well-documented case of NGC 6752 as in our previous developments. We reproduce the typical percentage of low-sodium stars usually classified as first-generation stars by invoking only secondary star formation from material ejected by massive stars and mixed with original globular cluster material in proportions that account for the Li-Na anti-correlation in this cluster. Globular clusters could be totally devoid of first-generation low-mass stars today. This can be tested with the determination of the carbon isotopic ratio and nitrogen abundance in turn-off globular cluster stars. Consequences and related issues are briefly discussed.

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Population III Stars
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Title: Dust Production Factories in the Early Universe: Formation of Carbon Grains in Red-supergiant Winds of Very Massive Population III Stars
Author: Takaya Nozawa, Sung-Chul Yoon, Keiichi Maeda, Takashi Kozasa, Ken'ichi Nomoto, Norbert Langer

We investigate the formation of dust in a stellar wind during the red-supergiant (RSG) phase of a very massive Population III star with the zero-age main sequence mass of 500 M_sun. We show that, in a carbon-rich wind with a constant velocity, carbon grains can form with a lognormal-like size distribution, and that all of the carbon available for dust formation finally condense into dust for wide ranges of the mass-loss rate ((0.1-3) x 10^{-3} solar masses yr^{-1}) and wind velocity (1-100 km s^{-1}). We also find that the acceleration of the wind driven by newly formed dust suppresses the grain growth but still allows more than half of gas-phase carbon to be finally locked up in dust grains. These results indicate that at most 1.7 solar masses of carbon grains can form in total during the RSG phase of 500 solar masses Population III stars. Such a high dust yield could place very massive primordial stars as important sources of dust at the very early epoch of the universe if the initial mass function of Population III stars was top-heavy. We also briefly discuss a new formation scenario of carbon-rich ultra-metal-poor stars considering the feedback from very massive Population III stars.

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Robert Simcoe peers deep into the universes past to identify the first stars.

The question of stellar origin is the focus of Robert Simcoe, an associate professor of physics at MIT. Using telescopes on Earth and in space, Simcoe is peering far into the universes past, searching for a period when the first stars blinked on.
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Title: Formation of the First Stars
Authors: Volker Bromm

Understanding the formation of the first stars is one of the frontier topics in modern astrophysics and cosmology. Their emergence signalled the end of the cosmic dark ages, a few hundred million years after the Big Bang, leading to a fundamental transformation of the early Universe through the production of ionising photons and the initial enrichment with heavy chemical elements. We here review the state of our knowledge, separating the well understood elements of our emerging picture from those where more work is required. Primordial star formation is unique in that its initial conditions can be directly inferred from the Lambda Cold Dark Matter (LCDM) model of cosmological structure formation. Combined with gas cooling that is mediated via molecular hydrogen, one can robustly identify the regions of primordial star formation, the so-called minihalos, having total masses of ~10^6 solar masses and collapsing at redshifts z~20-30. Within this framework, a number of studies have defined a preliminary standard model, with the main result that the first stars were predominantly massive. This model has recently been modified to include a ubiquitous mode of fragmentation in the protostellar disks, such that the typical outcome of primordial star formation may be the formation of a binary or small multiple stellar system. We will also discuss extensions to this standard picture due to the presence of dynamically significant magnetic fields, of heating from self-annihilating WIMP dark matter, or cosmic rays. We conclude by discussing possible strategies to empirically test our theoretical models.

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Title: The mass of first stars
Authors: Hajime Susa (Konan University)

We investigate the final mass of the first stars by simulating the gravitational collapse of a primordial gas cloud until ~ 0.1 Myrs after the formation of the primary proto-first-star. According to our numerical experiment using the radiative hydrodynamics calculations, we find that the mass accretions onto the proto-first-stars are significantly suppressed by the radiative feedback from themselves. As a result, we find five stars formed in this particular simulation, and that the final mass of the stars are less than 60 solar masses, including a star of 4.4 solar masses. Although it is larger than 0.8 solar masses, presence of such a low mass star infer the existence of first stars in the local universe.

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Supermassive primordial stars
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Title: The Biggest Explosions in the Universe
Authors: Jarrett L. Johnson (LANL), Daniel J. Whalen (LANL, Heidelberg ITA), Wesley Even (LANL), Chris L. Fryer (LANL), Alex Heger (Monash), Joseph Smidt (LANL), Ke-Jung Chen (Minnesota)

Supermassive primordial stars are expected to form in a small fraction of massive protogalaxies in the early universe, and are generally conceived of as the progenitors of the seeds of supermassive black holes (BHs) at high redshift. Supermassive stars with masses of ~ 55,000 solar masses, however, have been found to explode and completely disrupt in a supernova (SN) with an energy of up to ~ 10^55 erg, instead of collapsing to a BH. Such events, roughly 10,000 times more energetic than typical SNe today, would be among the biggest explosions in the history of the universe. We carry out a simulation of such a supermassive star SN in two stages. Using the RAGE radiation hydrodynamics code we first evolve the explosion from the earliest stages, through the breakout of the shock from the surface of the star until the blast wave has propagated out to several parsecs from the explosion site, which lies deep within an atomic cooling dark matter (DM) halo at z ~ 15. Then, using the GADGET cosmological hydrodynamics code we evolve the explosion out to several kiloparsecs from the explosion site, far into the low-density intergalactic medium. The host DM halo, with a total mass of 4 x 10^7 solar masses, much more massive than typical primordial star-forming halos, is completely evacuated of high density gas after < 10 Myr, although dense metal-enriched gas recollapses into the halo, where it will likely form second-generation stars after > 70 Myr. The ~ 20,000 solar masses in metals that are released in the explosion are widely distributed, and enrich the dense recollapsing gas to an average metallicity of ~ 0.05 Z_Sun. Such a high level of enrichment suggests that the chemical signature of these supermassive star explosions may have been missed in previous surveys of metal-poor stars.

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Population III stars
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Title: The First Stars
Authors: Simon C. O. Glover

The first stars to form in the Universe -- the so-called Population III stars -- bring an end to the cosmological Dark Ages, and exert an important influence on the formation of subsequent generations of stars and on the assembly of the first galaxies. Developing an understanding of how and when the first Population III stars formed and what their properties were is an important goal of modern astrophysical research. In this review, I discuss our current understanding of the physical processes involved in the formation of Population III stars. I show how we can identify the mass scale of the first dark matter halos to host Population III star formation, and discuss how gas undergoes gravitational collapse within these halos, eventually reaching protostellar densities. I highlight some of the most important physical processes occurring during this collapse, and indicate the areas where our current understanding remains incomplete. Finally, I discuss in some detail the behaviour of the gas after the formation of the first Population III protostar. I discuss both the conventional picture, where the gas does not undergo further fragmentation and the final stellar mass is set by the interplay between protostellar accretion and protostellar feedback, and also the recently advanced picture in which the gas does fragment and where dynamical interactions between fragments have an important influence on the final distribution of stellar masses.

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Title: The Radio Signatures of the First Supernovae
Authors: Avery Meiksin (IfA, University of Edinburgh), Daniel J. Whalen (Carnegie-Mellon University)

Primordial stars are key to primeval structure formation as the first stellar components of primeval galaxies, the sources of cosmic chemical enrichment and likely cosmic reionisation, and they possibly gave rise to the supermassive black holes residing at the centres of galaxies today. While the direct detection of individual Pop III stars will likely remain beyond reach for decades to come, we show their supernova remnants may soon be detectable in the radio. We calculate radio synchrotron signatures between 0.5 - 35 GHz from hydrodynamical computations of the supernova remnants of Pop III stars in minihaloes. We find that hypernovae yield the brightest systems, with observed radio fluxes as high as 1 - 10 muJy. Less energetic Type II supernovae yield remnants about a factor of 30 dimmer and pair-instability supernova remnants are dimmer by a factor of more than 10,000. Hypernovae radio remnants should be detectable by existing radio facilities like eVLA and eMERLIN while Type II supernova remnants will require the Square Kilometre Array. The number counts of hypernova remnants at z > 20 with fluxes above 1 muJy are expected to be one per fifty square degree field, increasing to a few per square degree if they form down to z = 10. The detection of a z > 20 Type II supernova remnant brighter than 1 nJy would require a 200 - 300 square degree field, although only a 1 - 2 square degree field for those forming down to z = 10. Hypernova and Type II supernova remnants are easily distinguishable from one another by their light curves, which will enable future surveys to use them to constrain the initial mass function of Pop III stars.

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