Spinning the First Stars

Title: Rotation and Internal Structure of Population III Protostars
Authors: Athena Stacy, Thomas H. Greif, Ralf S. Klessen, Volker Bromm, and Abraham Loeb
First Author’s Institution: NASA Goddard Space Flight Center, Greenbelt, MD

Although the first stars lived more than 13 billion years ago, way back near the beginning of the Universe, the heavy elements they returned to their surroundings altered the nature of star formation in the Universe forever.  Despite their importance, these so-called Population III (Pop III) stars are much too faint for us to see directly during their short lifetimes.  Theorists use numerical simulations to determine what Pop III stars would have been like, but still need to develop a more complete picture of Pop III stellar evolution before observers can test their predictions.  In this paper, Stacy et al. reveal the detailed internal structure of the seeds of four of the first stars, and demonstrate for the first time that they are rapidly spinning throughout.  Their results bring us one step closer to a coherent story of the lives and deaths of Population III stars.

The birth of Pop III stars

There are a couple of key differences between Pop III star formation and star formation today.  First off, there were no galaxies when the first stars were forming; instead, these stars were born within spheres of dark matter called minihalos.  Secondly, Pop III stars were extremely overweight.  As explained in this astrobite, gas must cool down and remove thermal pressure before gravity win out and cause it to collapse into a star.  Pop III stars formed out of material that was almost entirely hydrogen and helium, with slight traces of the lightest “heavy” elements left over from Big Bang nucleosynthesis.  While this may not seem like it big deal, the lack of heavy elements means that the gas loses its most efficient cooling processes, for instance forming molecular hydrogen on dust grains.  Instead, it has to cool through alternative mechanisms that use only hydrogen, and these mechanisms are much slower than those available today.  Because the gas remained quite hot, a lot of it needed to accumulate before there was enough gravity to cause it to collapse, and hence massive stars were born.

How Pop III stars die: cue the explosions?

Because Pop III stars can be so much larger than the stars we are used to, they can also die in exotic ways.  Early, simplified theories predicted that Pop III stars did not rotate, and could end their lives through either core-collapse supernovae, direct collapse into black holes, or pair-instability supernovae.  Pair-instability supernovae occur for the most massive stars, and are instigated by the creation of electron-positron pairs out of the most energetic photons, which remove pressure from the core of the star.  This change in pressure causes a rapid infall and subsequent recoil that blows the star apart, similar to a core-collapse supernova.  For a pair-instability supernova, the explosion is so violent that it throws almost all of the material from within the star back into its surroundings, efficiently enriching the intergalactic medium with heavy elements.  This comes in stark contrast to direct collapse to a black hole, when the former star keeps everything for itself.

Adding stellar rotation to the mix can change these end states, with the net result that more material – of different composition – is added to the intergalactic medium.  At the lower mass end, normal, core-collapse supernovae may be replaced by hypernovae, their more energetic cousins.  Furthermore, pair-instability supernovae may be possible at lower minimum masses, when a black hole would have formed if the star was not spinning.  Rotationally induced mixing, discussed in this astrobite about stellar rotation, could also create a pathway for long gamma-ray bursts (GRBs) to occur – an exciting idea for observers since these bursts are extraordinarily bright.  Additionally, mixing might be responsible for enhancements of heavy elements seen in some low metallicity, low-mass stars, and for the depletion of lithium found in the strange star discussed in these astrobites.  Tantalizing clues like this motivate detailed simulations of the internal structure of Pop III stars.

Let’s make some protostars

Figure 1: Pop III protostars form out of a disk of material within a dark matter minihalo. The disk fragments into several smaller protostars, some of which are incorporated into the central protostars that are the focus of this paper. The final spatial resolution is a mere 5% of the Sun's radius, while the initial size of the fragments is about 1 astronomical unit (AU) - the distance from the Sun to the Earth. Snapshots are from Greif et al. 2012.

The authors analyze the end result of a numerical simulation of four minihalos by Greif et al. 2012.  The simulation, which uses the moving-mesh code AREPO (see this astrobite), begins on a cosmological scale with a huge box of particles that is 250 by 500 kiloparsecs on a side.  The authors progressively zoom in on the interesting parts of the simulation over time, first pulling out the minihalos and then focusing in on the central parsec of these halos where most of the baryonic matter will settle.

As Figure 1 shows, a rotating disk of baryonic material forms in each minihalo.  In addition to the central protostar, many smaller protostars also collapse out of the disk when they become cool enough and are massive enough for gravity to pull them together, and these often merge with the quickly growing central object.  Stacy et al. go through a detailed analysis of the internal rotation of the main protostar in each halo, and find that they behave roughly as solid bodies in their interiors, while at the surface the protostars rotate at at least 80% of the Keplerian velocity.  This means that the protostars are rotationally supported: their rotation rate does not significantly drag behind the rate at which they would be spinning in the presence of gravity alone, and this helps them to resist gravitational collapse.  Note that a star doesn’t need to rotate to hold itself up – thermal pressure plays a big role here as well. Nonetheless, the rapid rotation is sustained throughout the simulation, even after minor mergers with other protostars (see Figure 2).


Figure 2: Rotational velocity of each protostar (a, b, c, d) over time. The upper solid lines shows the velocity of the protostar itself, the dashed line shows the Keplerian velocity, and the lower solid line shows the cumulative number of minor mergers, for reference.

It is important to note that the protostar formation discussed here takes place over only 10 years – an extremely short amount of time by astronomical standards.  Already the protostars have grown to about the mass of our Sun, but they have a long way to go before they reach the huge masses (tens of times larger, at minimum) that we ultimately expect for the first stars.  While longer timescales and more detailed physics would be needed to characterize the Pop III stars as they grow to adulthood, the results so far suggest that rotation could play an important role throughout their lives.  This paper represents an exciting step towards painting a complete portrait of these ancient massive stars that we can compare to what we see in the Universe today.

About Alice Olmstead

I am a fourth-year graduate student at the University of Maryland, College Park. I currently do astronomy education research with Chandra Turpen, Ed Prather, Joe Redish and Colin Wallace, focusing on how professional development workshops help faculty to grow as educators. Prior to that, I studied distant, highly magnified, gravitationally lensed galaxies to investigate where they formed their stars and why. Outside of academics, I love travel, hiking, music, and vegan food. 🙂

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