In the stellar kitchen: how mixing radiation and magnetic fields leads to smaller Population III stars

Title: Magnetic fields limit the mass of Population III stars even before the onset of protostellar radiation feedback

Authors: Piyush Sharda, Shyam H. Menon, Roman Gerasimov, Volker Bromm, Blaskesley Burkhart, Lionel Haemmerlé, Lisanne van Veenen, and Benjamin D. Wibking

First Author’s Institution: Leiden Observatory, Leiden University

Status: Submitted to MNRAS (open access); preprint on arXiv

Population III stars are some of astronomers’ favorite (or, incredibly vexing) mysteries. They are the first stars created in the universe, made just after it became cool enough for gas to clump together due to gravity. They are thought to be a mix of Hydrogen, Deuterium (H2), and a little bit of Helium, aka all the elements formed in Big Bang Nucleosynthesis. Since heavier elements are formed in the deaths of stars, these Pop III stars aren’t old enough to have encountered the chemical relics of dead stars. 

We have yet to see these stars, but as JWST gets closer to the beginning of the universe and closer to these stars, we need to prep our stellar recipes (models) for (hopefully) their future spotlight. The authors of today’s paper look towards this future by starting to build a physics-informed understanding of how these stars form, and taking a guess at just how big and numerous they might be.

If you don’t look too closely, the way stars are born might appear simple enough: cold, molecular gas must clump together from gravity into a dense ball, known as a protostar, until the pressure of the packed gas becomes too intense, heating the protostar until the hydrogen inside starts bumping into one another, beginning nuclear fusion, and beginning their time as a main-sequence star. We accept that stars are beholden primarily to the whims of gravity, but recent research and simulations have started to pry deeper into this simplistic model, moving now to explore the whims of magnetic fields. This treatment is called magnetohydrodynamics (MHD) (fun!), and has been relatively common in many stellar models for the past couple of years, especially when modeling these Pop III stars. 

However, today’s authors take this modeling one step further and consider how radiation feedback also affects stellar births, in a treatment called radiation-magnetohydrodynamics (RMHD). Simply, they consider how photons emitted by the star heat or cool the gas around the star, affecting the delicate scenario of its growth. Radiation has, in the past decade, been proposed to affect the growth of Population III stars, but never tested together with magnetohydrodynamics.

Simulating different recipes for stellar births

The gas in the early universe is incredibly turbulent, so it’s already difficult for gas to clump together, and magnetic fields and radiative feedback makes it all the more difficult. The authors simulate four different scenarios of stellar birth: one with no magnetic fields or radiation (HD), one with only magnetic fields (MHD), one with only radiation (HD), and one with all of the above (RMHD).

The simulations are run with an MHD code called POPSICLE which uses an adaptive mesh refinement method. New for this study, the authors edit the code to include specific treatments for radiative feedback, meaning they consider how photons from the protostars ionize the hydrogen and deuterium gas around it. These considerations can affect the temperature and general thermodynamics of gas around the star, which can affect its formation if the gas gets too cold or too hot. They begin the simulations with a sphere of mostly Hydrogen gas, with some Helium and a bit of deuterium, letting the gas move and coalesce based on gravity and a bit of rotation and turbulence. All simulations are run until 5000 years after the formation of the first star, by which time the young star should have settled into a relatively stable state.

Figure 1. Snapshots of the gas density (left) and gas temperatures (right) around the protostars at the end of each of the simulations: HD (upper left), MHD (upper right), RHD (lower left), and RMHD (lower right). Dots represent where Pop III stars are thought to form. Figures 1 and 2 in original paper.

They find, as expected, that the gas is cooler in the simulations with magnetic fields because magnetic fields push against gravitational collapse, and preventing the gas from heating up. When the gas is cooler, it will split into more separate clouds in a process called fragmentation, which can be seen in both runs with magnetic fields (right-hand panels in Figure 1). This fragmentation leads to more companion stars forming nearby to the central primary star, seen in the MHD and RMHD panels. This fragmentation takes away potential mass from the primary star, making it less massive than it would otherwise be. Even further, they find that magnetic fields also inhibit the stars getting bigger because, as they act against gravity, they don’t let more gas fall onto the accretion disk.

How big are they when they’re taken out of the oven?

Figure 2. Comparisons of accretion rate (top) and stellar mass (bottom) for each simulation. Figure 5 in the original paper.

To understand what to look for with JWST, it’s good to know how big these Pop III stars could get. Because of the effects from magnetic fields and radiation, the masses differ quite a bit depending on the simulated scenario, as shown in Figure 2: the final mass of the primary star in the HD simulations was 127 Solar masses, RHD was 120 Stellar Masses, and MHD and RMHD were 48 Solar masses and 67 Solar masses, respectively. This is quite a large difference, and underlies the importance in understanding how different treatments affect what we expect from observations. Moreover, they find that the RMHD simulations overall create about thirty percent less total stellar mass than the other three simulations.

Are both ingredients needed?

The authors argue that the inclusion of both magnetic fields and radiation feedback is necessary for understanding the growth of Population III stars. They suggest that both of them together produce less stellar mass than the other simulations because the magnetic fields make radiation feedback occur earlier in the protostar system, curtailing its growth earlier on in its life. This radiation feedback further halts accretion onto the primary star, but it takes longer than 5000 years to see these effects.

There is much future work to be done to explore the interplay of magnetic fields and radiation feedback in these stars, and this paper reminds us to consider all the ingredients when building our stellar recipes. Hopefully, when we successfully observe Pop III stars we will be able to taste our creation!

Astrobite edited by Nathalie Korhonen Cuestas

Featured image credit: NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani

Author

  • Caroline von Raesfeld

    I’m a third-year PhD student at Northwestern University. My research explores how we can better understand high-redshift galaxy spectra using observations and modeling. In my free time, I love to read, write, and learn about history.

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