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Fig. 1:
The collapse of gas in a dark matter `minihalo', shown in a box with 30,000
lightyears on a side. The gas heats due to the release of gravitational
potential energy. Only at the centre the gas cools due to molecular
hydrogen. The temperature is color-coded from black (coolest) to white
(hottest).
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Fig. 2:
The gas at the centre of the minihalo forms a disk that fragments into a
small cluster of protostars. The size of the box is only 200 astronomical
units on a side. The density is color-coded from black (most underdense) to
yellow (densest).
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Structure formation in the Universe started with the contraction of
the smallest dark matter halos. These so-called `minihalos' with about
one million solar masses confined the intergalactic gas within their
gravitational potential wells, where it became gradually denser and
hotter. At the centre of these minihalos, at one point the gas was
dense enough to form molecular hydrogen — the simplest molecule in
the Universe — which allowed the gas to cool by activating
internal degrees of freedom. This cooling then resulted in the runaway
gravitational collapse of the gas to densities comparable to that of
the Sun. Finally, a protostar formed, which had only a thousandth of a
solar mass initially, but which rapidly accreted gas from the
surrounding envelope. The newborn star continued to grow until it
entered the main sequence of hydrogen burning after about one hundred
thousand years.
Numerical simulations performed over the past decade found little
evidence for fragmentation during the initial collapse phase,
indicating that the first stars formed in isolation. Assuming that all
the gas in a minihalo accretes onto the only protostar, simple
one-dimensional calculations show that Population III stars will grow
to about one hundred times the mass of the Sun. As they are extremely
massive, they emit many more ionizing photons than normal, present-day
stars and could leave a distinct imprint on the 21-cm background
radiation. They influence the reionization of the Universe and,
furthermore, give rise to extremely energetic supernova explosions -
perhaps even the so-called pair-instability supernova, which disrupts
the entire progenitor star and leaves no compact remnant behind.
In recent work, Thomas Greif and his colleagues used a new simulation
technique to investigate the evolution of gas up to one thousand years
after the formation of the first protostar. In the large spatial and
dynamical range covered by their simulation, they found that the gas
fragmented quite vigorously into about ten individual protostars
instead of forming a single object. Since all individual stars
accreted from the same, common gas reservoir, the typical mass of
their Population III stars is reduced to about ten solar masses. In
the further stellar evolution, this would severely limit the
stars’ ability to explode as pair-instability
supernovae. Instead, these stars could end their lives as more
conventional core-collapse supernovae and gamma-ray bursts, which have
a very distinct observational signature.
A second intriguing result of the same simulation is the ejection of
protostars from the central gas cloud by gravitational slingshot
effects, well before they have accreted even one solar mass. Such
low-mass stars are extremely long-lived and could survive over cosmic
time to the present day. They might even be observable in our own
Galaxy. Discovering these stars would provide a tell-tale signature of
the revised formation scenario proposed by Thomas Greif and his
collaborators.
Thomas Greif, Volker Springel, Simon White, Simon Glover, Paul Clark, Rowan Smith, Ralf Klessen, Volker Bromm
References:
Thomas H. Greif, Volker Springel, Simon D. M. White, Simon C. O. Glover,
Paul C. Clark, Rowan J. Smith, Ralf S. Klessen, Volker Bromm:
"Simulations
on a Moving Mesh: The Clustered Formation of Population III Protostars",
submitted to ApJ
http://de.arxiv.org/abs/1101.5491
Paul C. Clark, Simon C. O. Glover, Rowan J. Smith, Thomas H. Greif, Ralf
S. Klessen, Volker Bromm:
"The Formation and Fragmentation of Disks around
Primordial Protostars". Science Express, 3 February 2011, doi:
10.1126/science.1198027
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