The origin of the heaviest elements in the Universe is a long-standing puzzle. Neutron star mergers, confirmed by the event GW170817, are considered an important but apparently insufficient source. The chemical evolution of the Milky Way's disk shows that r-process elements such as europium appeared too early to be explained only by standard, delayed mergers. This work joins a line of studies that quantitatively test alternative merger channels: fast neutron star mergers and neutron star–black hole mergers, and shows that they too do not solve the problem.
The authors built a unified Bayesian model of the Galaxy's chemical evolution that accounts for uncertainties in merger rates, delay time distributions, ejecta masses, and observational data. The model includes three scenarios: fast mergers visible to gravitational-wave detectors; fast mergers that detectors miss; and neutron star–black hole mergers as an additional channel. For each scenario, they computed the probability of reproducing the observed [Eu/Fe] versus [Fe/H] relation for disk stars.
All three scenarios require extreme parameters. If fast mergers are visible to detectors, they must make up more than 98% of all neutron star mergers. If they are invisible, their rate must be 17 times higher than the rate of delayed mergers. Neutron star–black hole mergers, even under optimistic assumptions, contribute less than 10%. Thus, more than 90% of r-process enrichment must come from an unknown channel that follows the star formation rate.
This work quantitatively rules out two popular candidates for the missing source of r-process elements. Unlike previous studies that used point estimates, here a consistent Bayesian analysis with full uncertainty treatment is applied for the first time. This strengthens the case for non-merger channels, such as magnetorotational supernovae or collapsars.
Future research should focus on non-merger sources of the r-process. Improved models of supernovae with strong magnetic fields and observations of the most metal-poor stars will help narrow down the candidates. More precise measurements of merger rates and delay time distributions with next-generation gravitational-wave observatories are also important.
The results will affect models of galactic chemical evolution, the interpretation of kilonova observations, and the planning of future sky surveys.
Test predictions for specific non-merger channels, such as magnetorotational supernovae, and compare them with data on halo stars and dwarf galaxies.
This work is directly related to the unsolved problem in astrophysics: where exactly in the Universe elements heavier than iron are formed. It narrows the space of possible sources and points to the need to study rare types of supernovae.
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What the work does
The authors built a Bayesian model of the Galaxy's chemical evolution to test whether rapid neutron star mergers or neutron star–black hole mergers can explain the observed europium abundance in the disk. They showed that reproducing the data requires extreme parameters, and even optimistic scenarios contribute less than 10%, pointing to the need for additional r-process sources.
Where its strength is
The main strength lies in the consistent Bayesian approach, which accounts for uncertainties in rates, delays, and ejecta masses, making the conclusion that mergers are insufficient more robust than in previous work.
Where it could go next
Worth trying
Why this may matter
If the proposed unknown channel indeed follows the star formation rate, it changes our understanding of nucleosynthesis in galaxies and can be tested by future observations of stellar chemical compositions across different populations.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.56, against a usual 0.64. It looks like few have drilled nearby.
Europium, used in phosphors and smartphone screens, is produced by the r-process. If its source were only neutron star mergers, our Galaxy would be much poorer in this element than observations show.
In Carl Sagan's novel 'Contact', the protagonist hears a signal encoded in the number π. Here, the Universe itself leaves us a message in the chemical composition of stars, and deciphering that message requires ruling out some sources to find others.