"Let there be light." The dark ages of the Universe ended when the first stars began their nuclear fusion of hydrogen, giving off heat and light about 100 million years after the Big Bang. Current understanding of these Population III—or low metal—stars is that they evolved and lived a solitary life or, at most, as part of a distant binary system. New simulations reported in this week's edition of Sciencexpress, however, suggest that the gas clouds that birthed these early stars were gravitationally unstable and produced tight clusters of stars that would live their short lives together.

No direct observational evidence of Pop III stars has been found to date. The information we have about them and their behavior has come exclusively from simulation and theoretical work. Previous work has found that, as atomic hydrogen was pulled into dark matter minihalos, it would form molecular hydrogen and coalesce into a gas cloud—ripe for the formation of primordial stars—with a mass of around 1000 solar masses.

Previous simulations of these precursor gas clouds have revealed that the clouds didn't fragment during the formation of the first protostars. From this, it was inferred that a single star would be born out of each cloud of gas. Using this assumption as a starting point, others worked out the balance of materials and forces that would exist during the formation of the star; it has been predicted that these earliest stars would weigh somewhere between 30 and 300 solar masses.

Complicating matters, however, it was also discovered that the collapsing hydrogen could break off into two chunks, each forming its own star; however, numerical simulations put the probability of this happening as only one out of every five instances.

In the Sciencexpress paper, an international team of researchers from the US and Germany used numerical simulations with some enhancements that allowed them to study the formation of these protostars in much greater detail—and for longer periods of time—than was previously possible. Their model of stellar formation began as a traditional simulation of a cosmological volume. When they first identified a dark matter minihalo that contained a cooling, gravitationally collapsing gas region, they "re-zoomed" the calculation and focused on that region alone.

Traditionally, simulations of such regions will see the gas condense and collapse to very high densities. But these simulations often become computationally intractable at gas densities above 1014 cm-3. To sidestep this issue, the team used the idea of transforming regions of very high density gas into gravitational 'sink' particles. These simple sink particles represent a young protostar and its physical properties—such as accretion rate, luminosity, mass, and energy radiated back into the gas cloud—consistent with the entire star. This removed the most computationally intense regions from the simulation and allowed it to proceed well past the point where others would need to stop.

As the authors point out, prior to the formation of the first protostar, their results are in good agreement with previously published work—a good test of their methodology, but not very interesting science. The interesting part comes after the first protostar/sink particle has formed. The team was able to then continue to model the accretion disk that formed around it.

Ninety years after the first star formed, the accretion disk around it had doubled in size. For the first 60 years, it existed in a stable, two-arm spiral pattern. Shortly thereafter, the disk became gravitationally unstable and another collapse began to occur, forming a second protostar about 20 AU from the first. After this, material was added to the disk much faster then it was added to either of the protostars, and further fragmentation occurred. All told, less than 120 years after the first star formed, the simulation had four new stars form, all within a region of a few dozen AU on a side.

The authors do not claim this work to be exact for all Pop III star formations, but believe that it may be a representative case. If such fragmenting systems do describe a significant amount of Pop III star formation, then a plausible outcome would be the formation of numerous Pop III binary systems where the stars are of nearly equal mass. The existence of systems of this type "would strengthen the case for high-redshift gamma-ray bursts origination from the first stars." It would also be possible for some of these Pop III stars to have been ejected from their initial birth ground and possibly exist today, waiting to be detected for the first time.

Sciencexpress, 2010. DOI: 10.1126/science.1198027

Listing image by R. Klessen, University of Heidelberg Center for Astronomy