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New Study Shows Very First Stars Not Monstrous

The very first stars in our universe were not the behemoths scientists had once thought, according to new simulations performed at NASA's Jet Propulsion Laboratory, Pasadena, Calif.
Astronomers "grew" stars in their computers, mimicking the conditions of our primordial universe. The simulations took weeks. When the scientists' concoctions were finally done, they were shocked by the results -- the full-grown stars were much smaller than expected.

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Universe's first stars not so big after all

The universe's earliest stars may have been less than half as large as previously thought, according to two new simulations. The findings could resolve one of the universe's oldest mysteries: why some elements are more abundant than our theories suggest they should be.
In the first hundreds of millions of years after the big bang, the early universe was composed mainly of atomic hydrogen, helium and tiny amounts of other light elements. Eventually clouds of these gases condensed into the first stars, but without dust, heavy elements, or molecules, these early stars were unable to cool down as quickly as their descendents.

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Spinstars
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The universe's first stars were whirling dervishes

The universe's first stars were fast-rotating "spinstars", suggests a study of the chemical signature they left behind. Their fast spins may have made them especially prone to dying in spectacular explosions called gamma-ray bursts.
Today's telescopes are not powerful enough to directly observe the universe's first stars, which formed and died just a few hundred million years after the big bang. Little is known about them, except that they were probably much heavier than the sun.

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Massive PopIII stars
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Title: SPINSTARS at low metallicities
Authors: G. Meynet, S. Ekstrom, A. Maeder, R. Hirschi, C. Chiappini, C. Georgy
(Submitted on 14 Sep 2007)

The main effect of axial rotation on the evolution of massive PopIII stars is to trigger internal mixing processes which allow stars to produce significant amounts of primary nitrogen 14 and carbon 13. Very metal poor massive stars produce much more primary nitrogen than PopIII stars for a given initial mass and rotation velocity. The very metal poor stars undergo strong mass loss induced by rotation. One can distinguish two types of rotationally enhanced stellar winds:

1) Rotationally mechanical winds occurs when the surface velocity reaches the critical velocity at the equator,  i.e. the velocity at which the centrifugal acceleration is equal to the gravity;

2) Rotationally radiatively line driven winds are a consequence of strong internal mixing which brings large amounts of CNO elements at the surface. This enhances the opacity and may trigger strong line driven winds. These effects are important for an initial value of \upsilon/\upsilon_{crit} of 0.54 for a 60 solar mass at Z=10^{-8},  i.e. for initial values of \upsilon/\upsilon_{crit} higher than the one (~0.4) corresponding to observations at solar Z.

These two effects, strong internal mixing leading to the synthesis of large amounts of primary nitrogen and important mass losses induced by rotation, occur for Z between about 10^{-8} and 0.001. For metallicities above 0.001 and for reasonable choice of the rotation velocities, internal mixing is no longer efficient enough to trigger these effects.

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Spinstars
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Spinstars: The First Polluters of the Universe?

By studying some of the oldest stars in our galaxy, an international team of astronomers led by Cristina Chiappini  has used data from ESO's Very Large Telescope to show that the first massive stars in the Universe were probably very fast rotators, which they have dubbed spinstars. Their findings will be published in an article in Nature on 28 April 2011.
Massive stars live fast and die young, so the first generation of massive stars in the Universe is already dead. However, their chemical imprint, left behind like an incriminating fingerprint, can still be found today in the oldest stars in the Milky Way. These fossil records provide valuable clues about the mysterious first stellar generation to enrich the pristine early Universe.
Soon after the Big Bang, the Universe was made almost entirely of hydrogen and helium. It was only enriched with other elements around 300 million years later with the death of the first generation of massive stars. These polluted the primordial gas with new chemical elements, which were then incorporated into subsequent stellar generations.

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Title: Imprints of fast-rotating massive stars in the Galactic Bulge
Authors: Cristina Chiappini, Urs Frischknecht, Georges Meynet, Raphael Hirschi, Beatriz Barbuy, Marco Pignatari, Thibaut Decressin & André Maeder

The first stars that formed after the Big Bang were probably extraordinarily fast spinners. These 'spinstars' are prime candidates for recognition as the 'first stars'.
A 12-billion-year-old globular cluster of stars known as NGC 6522 provided the basis for the proposal of spinstars.

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First Stars in Universe Were Not Alone

The first stars in the universe were not as solitary as previously thought. In fact, they could have formed alongside numerous companions when the gas disks that surrounded them broke up during formation, giving birth to sibling stars in the fragments. These are the findings of studies performed with the aid of computer simulations by researchers at Heidelberg Universitys Centre for Astronomy together with colleagues at the Max Planck Institute for Astrophysics in Garching and the University of Texas at Austin (USA). The groups findings, being published in Science magazine, cast an entirely new light on the formation of the first stars after the Big Bang.
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Title: Simulations on a Moving Mesh: The Clustered Formation of Population III Protostars
Authors: Thomas Greif, Volker Springel, Simon White, Simon Glover, Paul Clark, Rowan Smith, Ralf Klessen, Volker Bromm
(Version v2)

The cosmic dark ages ended a few hundred million years after the Big Bang, when the first stars began to fill the universe with new light. It has generally been argued that these stars formed in isolation and were extremely massive - perhaps 100 times as massive as the Sun. We here use a series of high-resolution hydrodynamical simulations performed with the moving mesh code AREPO to show that this view may require revision. We follow the collapse of five independent minihalos from cosmological initial conditions, through the runaway condensation of their central gas clouds, to the formation of the first protostar, and beyond for a further 1000 years. During this latter accretion phase, we represent the optically thick regions of protostars by sink particles. Gas accumulates rapidly in the circumstellar disk around the first protostar, fragmenting vigorously to produce a small group of protostars. After an initial burst, gravitational instability recurs periodically, forming additional protostars with masses ranging from ~0.1 to 10 M_sun. Although the shape, multiplicity, and normalization of the protostellar mass function depend on the details of the sink-particle algorithm, fragmentation into protostars with diverse masses occurs in all cases, with low-mass objects occasionally being ejected through gravitational slingshot effects. Depending on the efficiency of later accretion and merging, Population III stars may enter the main sequence in clusters and with much more diverse masses than is commonly assumed. A few may even survive to the present day.

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Title: The Formation and Fragmentation of Disks around Primordial Protostars
Authors: Paul C. Clark, Simon C.O. Glover, Rowan J. Smith, Thomas H. Greif, Ralf S. Klessen, Volker Bromm

The very first stars to form in the Universe heralded an end to the cosmic dark ages and introduced new physical processes that shaped early cosmic evolution. Until now, it was thought that these stars lived short, solitary lives, with only one extremely massive star, or possibly a very wide binary system, forming in each dark matter minihalo. Here we describe numerical simulations that show that these stars were, to the contrary, often members of tight multiple systems. Our results show that the disks that formed around the first young stars were unstable to gravitational fragmentation, possibly producing small binary and higher-order systems that had separations as small as the distance between the Earth and the Sun.

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Hubble Astronomers Uncover an Overheated Early Universe

If you think global warming is bad, 11 billion years ago the entire universe underwent, well, universal warming. The consequence was that fierce blasts of radiation from voracious black holes stunted the growth of some small galaxies for a stretch of 500 million years. Astronomers used the Hubble Space Telescope's Cosmic Origins Spectrograph (COS) to identify an era, from 11.7 to 11.3 billion years ago, when the universe burned off a fog of primeval helium. This heated intergalactic gas was inhibited from gravitationally collapsing to form new generations of stars in some small galaxies.
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HE1327-2326
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HE1327-2326, discovered in 2005 by Anna Frebel and collaborators, is the star with the lowest known iron abundance to date. The star is a member of Population II, with an iron to hydrogen ratio ([Fe/H]), or metallicity, of -5.6. This number indicates that its iron content is 300,000 times less than that of the Earth's sun.
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