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Title: SIRIUS: Identifying Metal-Poor Stars Enriched by a Single Supernova in a Dwarf Galaxy Cosmological Zoom-In Simulation Resolving Individual Massive Stars
Authors: Yutaka Hirai et al.
First Author’s Institution: Tohoku University
Status: Published in ApJL

All of the elements in stars come from, well, other stars. Each star has its own unique makeup of elements, kind of like our own DNA. Just like genes passing down generation after generation, elements in stars can be passed down to newly forming stars after the previous generation has died. So, if you’re looking at the chemical makeup of a particular star, what you’re really seeing is all the elements that were created after another star in the region exploded (went supernova) and enriched the gas (that the new star formed from) around it with elements like oxygen, nitrogen, iron, and more.

Like genealogy for stars, astronomers can use stellar enrichment to work their way back from a star they observe to its potential predecessors. It’s a bit trickier for stars that have a lot of elements, dubbed “metal-rich” stars, because there’s no way to tell which star contributed what mixture of elements at what point in time. However, scientists have recently been observing “extremely metal-poor” stars, which are stars with only small amounts of elements like oxygen, nitrogen, or carbon. Because there are so few of these elements, it’s safe to assume that these stars formed out of gas that didn’t have many predecessor stars enriching it — in fact, astronomers are hoping to find stars that only ever had one ancestor, calling them “mono-enriched” stars. Finding a mono-enriched star would tell us exactly what elements were produced in the supernova — aka in supernova nucleosynthesis.

However, how many of these metal-poor stars are mono-enriched is unknown. Estimating this fraction could help researchers identify more of them. The authors of this study tackle this problem by examining how many might exist in dwarf galaxies and what kind of chemical signatures they could produce.

Simulating Mono-Enriched Stars

To do this, they simulate a dwarf galaxy with the hydrodynamic code ASURA and model the gas physics in the galaxy with CLOUDY. In this method, stars form once the density of hydrogen gas in a region exceeds a certain threshold at a temperature that is cool enough for the gas to clump together. Pockets of this simulated region are then randomly assigned a stellar mass from the initial mass function, and eventually explode, diffusing elements into the gas particles around it.

Finding Mono-Enriched Stars in the Simulated Stellar Haystack

The authors define a mono-enriched star as one with a carbon-to-iron ratio (denoted [C/Fe]) similar to that expected of a core-collapse supernova in their model. They identify a portion of the stars created as being mono-enriched, shown in orange in Figure 1. In total, this makes up about 1.5% of stars in this simulated dwarf galaxy.

Once these data points are collected, the authors investigate the fraction of mono-enriched stars at different metallicities. The authors essentially ask, are mono-enriched stars more likely to occur at lower metallicities? They find that, indeed, stars that are lower metallicity (and thus have lower [Fe/H]) are more likely to be mono-enriched (Figure 2). For stars with exceedingly low metallicities, [Fe/H] < −5, about 11% of them in this simulated dwarf galaxy have formed out of gas enriched by only one supernova.

Finding Mono-Enriched Stars in the Observed Stellar Haystack

These theoretical predictions help observers prepare what to look for when they want to find a mono-enriched star. These predictions seem to suggest that astronomers should keep looking for these extremely metal-poor stars if we want to find these unique stars that tell us about what elements are made in core-collapse supernovae. There is currently a decently large database of extremely metal-poor stars found with photometry, and if observers could follow up with the most ideal candidates spectroscopically we could really test what kind of elements are in these stars. Further, the upcoming Subaru Prime Focus Spectrograph will take many spectroscopic observations of close-by dwarf galaxies, which may contain more extremely metal-poor stars.

The authors note there is more work to be done in simulations, as they are interested in testing how extremely metal-poor stars, specifically ones with enhanced carbon, form. All of this will hopefully lead to a better understanding of how elements are made when stars explode, perhaps leading us to how some of the earliest elements formed in the early universe.

Original astrobite edited by Diana Solano-Oropeza.

About the 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.

The post Looking for a Single Stellar Ancestor: Mono-Enriched Stars in Zoom-In Simulations appeared first on AAS Nova.

Do neutron stars form solely through core-collapse supernovae, or is there another way? New research explores theorized processes in which a white dwarf shrinks down to become a neutron star.

Alternate Pathways

Neutron stars are known to form when high-mass stars explode in core-collapse supernovae. Theory suggests these stars can also form through accretion-induced collapse and merger-induced collapse. These scenarios involve a white dwarf, the remnant core of a low- to intermediate-mass star. If a white dwarf becomes too massive to support itself, either by accreting gas from a companion or colliding with another white dwarf, it could, under certain conditions, collapse into a neutron star.

So far, researchers have yet to definitively associate any transient signal with the creation of a neutron star from a collapsing white dwarf, though there are several candidates. To advance the search for these signals, researchers must understand the details of the collapse and predict its electromagnetic signature.

Collapsing Further

Eirini Batziou (Max Planck Institute for Astrophysics and Technical University of Munich) and collaborators investigated the burst of nucleosynthesis that might follow a white dwarf’s collapse, potentially powering an electromagnetic transient. The team performed two-dimensional hydrodynamic simulations of six white dwarfs on the cusp of collapsing into neutron stars. Each simulation run varied the mass, central density, rotation rate, and angular momentum profile of the white dwarf. Two of the modeled white dwarfs had masses very close to the Chandrasekhar limit, the mass above which a white dwarf can hypothetically no longer support itself through electron degeneracy pressure. The remaining synthetic stars had higher masses that were bolstered by rapid rotation.

Each of the modeled white dwarfs began to collapse when the crushing pressure of gravity forced the star’s sea of electrons to invade atomic nuclei, removing the support of electron degeneracy pressure. The white dwarfs’ cores shrank and condensed into protoneutron stars, while their outer layers fell inward and rebounded.

Setting the Stage for Element Creation

The simulations show that rotating and non-rotating white dwarfs have different outcomes when they collapse. Non-rotating white dwarfs cast off material in all directions, while rapidly rotating white dwarfs tend to lose material through wide outflows at their poles. The equator of the rapidly rotating white dwarf is ringed by a torus of material that feeds the neutron star as it forms.

These divergent outcomes affect the amount of ejected material and the production of neutrinos. Ultimately, these differences result in opposite behavior: in the non-rotating case, the ejected material is initially rich in neutrons before transitioning to a proton-rich outflow, while the rotating case starts out with proton-rich ejecta that transitions to a neutron-rich outflow. This suggests that elements created through the rapid capture of neutrons, like gold, can form when white dwarfs collapse into neutron stars, potentially powering an observable electromagnetic signal.

These simulations by Batziou and coauthors are the first to extend substantially beyond the moment the outer layers of the collapsing white dwarf bounce off the central protoneutron star, illuminating the ejection of material, production of neutrinos, and nucleosynthesis. Future work will dive deeper into the details of element creation, predict the luminosity of these events, and explore the role of magnetic fields.

Citation

“Nucleosynthesis Conditions in Outflows of White Dwarfs Collapsing to Neutron Stars,” Eirini Batziou et al 2025 ApJ 984 197. doi:10.3847/1538-4357/adc300

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