Authors: Alexander L. DeSouza and Shantanu Basu
First Author’s Institution: Department of Physics and Astronomy, University of Western Ontario
Status: Accepted by MNRAS
A major goal for the next generation of telescopes, such as the James Webb Space Telescope (JWST) is to study the first stars and galaxies in the universe. But what would they look like? Would JWST be able to see them? Recent studies have suggested that even the most massive specimens of the very first generation of stars, known as Population III stars, may be undetectable with JWST.
But not all hope is lost–one of the reasons why Population III stars are so hard to detect is that, unlike later generations of stars, they are believed to form in isolation. Later generations of stars (called Population I and Population II stars) usually form in clusters, from the fragmentation of large clouds of molecular gas. On the other hand, cosmological simulations have suggested that Population III stars would form from gas collected in dark matter mini-halos of about a million solar masses in size which would have virialized (reached dynamic equilibrium) by redshifts of about 20-50. Molecular hydrogen acts as a coolant in this scenario, allowing the gas to cool enough to condense down into a star. Early simulations showed that gravitational fragmentation would eventually produce one massive fragment–on the order of about a hundred solar masses–per halo. This molecular hydrogen, however, could easily be destroyed by the UV radiation from the first massive star formed, preventing others from forming from the same parent cloud of gas. While Population III stars in this paradigm are thought to be much more massive than later generations of stars, they would also be isolated from other ancient stars.
However, there is a lot of uncertainty about the masses of these first stars, and recent papers have investigated the possibility that the picture could be more complicated than first thought. The molecular gas in the dark matter mini-halos could experience more fragmentation before it reaches stellar density, which may lead to multiple smaller stars, rather than one large one, forming from the same cloud of gas. These stars could then evolve relatively independently of each other. The authors of today’s paper investigate the idea that Population III stars could have formed in clusters and also study the luminosity of the resulting groups of stars.
The authors of today’s paper begin by arguing that the pristine, mostly atomic gas that collects in these early dark matter mini-halos could fragment by the Jeans criterion in a manner similar to the giant molecular clouds that we see today. This fragmentation would produce small clusters of stars that are relatively isolated from each other, so they are able to model each of the members in the cluster independently. They do this by using numerical hydrodynamical simulations in the thin-disk limit.
Their fiducial model is a gas of 300 solar masses, about 0.5 pc in radius, and at a temperature of 300 K. They find that the disk that forms around the protostars (the large fragments of gas that have contracted out of the original cloud of gas) forms relatively quickly, within about 3 kyr of the formation of the protostar. The disk begins to fragment a few hundred years after it forms. These clumps can then accrete onto the protostar in bursts of accretion or get raised to higher orbits.
Most of the time, however, the protostar is in a quiescent phase and is accreting mass relatively smoothly. The luminosity of the overall star cluster increases during the bursts of accretion, and it also increases as new protostars are formed. The increasing luminosity of the stellar cluster can make it more difficult to detect single accretion events. For clusters of a moderate size of about 16 members, these competing effects result in the star cluster spending about 15% of its time at an elevated luminosity, sometimes even a 1000 times the quiescent luminosity. The star clusters can then have luminosities approaching and occasionally exceeding 108 solar luminosities. Population III stars with masses ranging from 100-500 solar masses on the other hand, are likely to have luminosities of about 106 to 107.
These clusters would be some of the most luminous objects at these redshifts and would make a good target for telescopes such as ALMA and JWST. We have few constraints on the star formation rates at such high redshifts, and a lot of uncertainty in what the earliest stars would look like. So should these exist, even if we couldn’t see massive individual population III stars, we may still be able to detect these clusters of smaller stars and gain insight into what star formation looked like at the beginning of our universe.