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The First Stars in the Universe

The first stars in the Universe, so-called Population III stars, existed a mere few hundred million years after the Big Bang. Research over the past decade led to the consensus that they formed in isolation and were extremely top-heavy, perhaps one hundred times more massive than the Sun. Recently, scientists from the Max Planck Institute for Astrophysics with colleagues in Heidelberg and Texas used a new simulation technique with unprecedented spatial and dynamical range to show that this picture might require revision: The stars that formed might have been much less massive.

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).

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).

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


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

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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