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