by Daegene Koh
Often in the world of astronomy and astrophysics, unexpected observations lead to new ideas and understanding. However, there are occasionally some models that are built up more traditionally from theories to observational predictions. This is a story of one such model—that of the very first stars in the universe, called, somewhat counterintuitively, Population III (Pop III) stars. We haven’t seen Pop III stars yet because of how long ago they first formed—and then rapidly died.
(A note about the counterintuitive terminology: Astronomers grouped stars in the order they were observed, so Pop I stars are present-day stars, with Pop II stars being one generation older. Pop III stars are the hypothesized oldest stars in existence.)
We know from models of Big Bang nucleosynthesis (which explain how the first atomic nuclei beyond hydrogen were formed and in what percentages), that for the first few hundred million years of its life, the Universe contained only hydrogen, helium, and a scant fraction of lithium. This means the abundance of metals (what astronomers term all elements heavier than helium) found in stars decreases as one looks farther back in time and examines objects that formed earlier in the Universe's life. Since there were virtually no metals in the early Universe, the very first stars must have also been formed without them at all. These are the hard-to-find Pop III stars.
Within this framework, models of these first objects have become further refined and sophisticated, particularly with the aid of numerical simulations. This particular arena of study is well suited for tackling with computational models because the early Universe is significantly less complex than the present one. We now know that Pop III stars were much more massive than present-day stars, ranging up to over a hundred times the mass of the Sun. They were also relatively short-lived due to their large masses (stellar lifetimes become shorter as stars become more massive: they burn bright, live fast, and die young, just like some of our most notorious pop stars). The very first metals were expelled from the supernovae of these objects and the remnant black holes they leave behind may have been the seeds for the supermassive black holes found in the centers of galaxy clusters today. Under the current paradigm of hierarchical structure formation as dictated by Lambda-Cold Dark Matter cosmology, smaller objects build up to form larger ones. Thus, understanding the details of Pop III formation is crucial for piecing together present-day galaxy formation. Below are just a few of the aspects of Pop III stars currently under investigation.
Magnetic fields in Pop III stars
One of the details in the models of Pop III stars is the production and amplification of magnetic fields. Magnetic fields of varying magnitudes, from the magnetosphere that shields the Earth from cosmic radiation to the absurdly powerful fields found on a magnetar (a spinning neutron star), are absolutely unavoidable in the present-day universe. Most models of the early universe only have room for very weak magnetic fields. It is not yet well understood exactly how such weak fields were able to grow through the processes of stellar, galactic, and galaxy cluster formation. Furthermore, these magnetic fields, once amplified, can also influence the structure-formation processes in return. We want to explore this somewhat symbiotic relationship by tracing the growth of magnetic fields starting from the very first objects.
By using the high resolution computer simulation code Enzo, I studied the growth of magnetic fields through the lifetime of a Pop III star, from birth, through its explosive death, and then during its aftermath. It includes the latest models to keep track of the chemical reactions, gravitational dynamics of dark matter, thermodynamics of gas, radiation feedback, and of course coupling of the magnetic fields to the gas dynamics (some of these aspects of astrophysical simulations were previously discussed in this KIPAC Research Highlight). The code also employs a computational technique known as adaptive mesh refinement, which selectively increases the resolution in regions of interest in order to gain high degrees of accuracy with minimal computational time.
These simulations show phenomenal growth of magnetic fields that can be associated with two distinct periods. The first period is during the gravitational collapse of the gas prior to the formation of the star. As the gas piles upon itself due to its own gravitational pull, the magnetic field lines also are tightly wound up, thus becoming greatly amplified. The second phase occurs shortly after the star explodes in a supernova, as the resulting shockwave expands outwards and begins to cool. This temperature difference between the cooling shock front and the heated shockwave itself results in the formation of turbulent motion which once again begins to twist the magnetic fields and amplify them. The figure below shows snapshots of the surrounding region before star formation, after supernova, and the aftermath.
After millions of years pass, the gas that was expelled outwards by the radiation emitted by the star and the ensuing supernova will collapse back on itself again. However, this time around some things will have changed. First, the heavier elements that were formed and released as a result of the supernova will have been mixed into the gas. Secondly, the previously non-magnetized gas will have become highly magnetized in the nearby vicinity of the affected region. Future studies will address exactly how this latter effect impacts the next generation of star formation.
Pop III stars during reionization
Beyond the detailed modeling of the objects themselves, Pop III stars can also be interesting objects in the context of reionization. Reionization is the transition from the neutral universe to the completely ionized universe that is the present. This transition starts off as a result of the formation of stars and the ionizing radiation they emit. At first, these stars would simply form pockets of ionized gas around them which would then shrink back as the atoms recombined. Eventually, however, with the continued formation of stars and radiation emitted from active galactic nuclei, the universe has become fully ionized.
I studied the exact role of Pop III stars during this period using a semi-numeric simulation code 21cmFAST. Instead of doing a full numerical simulation, which would be computationally prohibitive for such large-scale problems, this code combines analytic models to speed up the calculation. In particular, I added the functionality for the models to be dependent on the mass of the galaxies, whereas previously it assumed a single value for the ionization ability for all galaxies. The figure below shows a snapshot of the ionized fraction in the simulated box comparing a model including Pop III stars against a model with only canonical galaxies.
Observations of quasars, measurements of the cosmic microwave background (CMB), and other constraints all point toward a common ending point to the "Epoch of Reionization" at around a redshift of 6, or when the universe was about a billion years old. These models are all tuned to agree with these constraints. However, the key difference is in showing the relative importance of the Pop III stars in dictating the start of the reionization process. They provide a significant fraction of the ionizing radiation during the early universe and are thus be an important component in understanding this rather turbulent period in the history of the universe.
This is a particularly exciting period to be looking into Pop III stars and the first galaxies. Because these stars are relatively short-lived, it would be difficult to expect any to survive to the present-day. Thus, the best chance at observing these objects is by looking deeper into the past. Until now, observational tools have lacked the technical capability to look far enough back into the past to observe Pop III stars. However, the next generation of telescopes, such as the James Webb Space Telescope (JWST), should be inching ever closer to the realm of Pop III stars to further constrain our models. Giant radiotelescope arrays soon to be constructed, such as Square Kilometer Array (SKA), can pierce through beyond the Reionization Era into the cosmic dark ages and will surely provide more clues about the early universe. Also, the advent of gravitational wave astronomy, opened up by the Laser Interferometer Gravitational-Wave Observatory (LIGO), has found evidence of heavy black holes potentially being connected to remnants from Pop III supernova. I look forward to building ever more sophisticated models to compare to the observations we know are ahead in the coming decade.
And—perhaps eventually—someone with a very powerful camera (on a telescope) will get a good clear shot of one of these most reclusive pop stars.
Amplification of Magnetic Fields in a Primordial HII Region and Supernova (published in the Monthly Notices of the Royal Astronomical Society, July 28, 2016)
Extending Semi-numeric Reionisation Models to the First Stars and Galaxies (published in the Monthly Notices of the Royal Astronomical Society, Nov. 23, 2017)