When neutron stars merge, they create a powerful explosion called a kilonova that flings out neutron-rich material, some of which decays into heavy elements through a process called the r-process. Recent observations of kilonovae revealed unexpected signatures that could not be explained by existing models.
Astronomers may now have solved this mystery. The paper explaining a new phenomenon was posted to the arXiv preprint server on July 1.
Gold factories
Kilonovae produce some of the heaviest elements. Scientists got their first direct look at this in 2017, when the gravitational-wave detection of neutron star merger GW170817 was matched with a visible explosion, AT2017gfo, confirming that these cosmic collisions really do produce heavy elements like gold and platinum. More recently, another candidate kilonova, AT2023vfi, linked to gamma-ray burst GRB 230307A, was observed with the James Webb Space Telescope weeks after its initial burst.
Its spectrum showed a strong infrared feature whose shape matched that of simple blackbody radiation—the kind of smooth glow any warm object emits—at a temperature of about 660 K (387°C, 728°F). This temperature is too “cool” to be associated with an extremely hot explosion. A similar late-time reddening was also seen in the original kilonova, AT2017gfo, tens of days after that merger. This suggests that the infrared feature could arise from a recurring phase of these events. However, this puzzling infrared glow has not yet been explained by standard models of atomic emission.
Exotic dust
In the new study, led by Nanae Domoto of the University of Tokyo, astronomers present a model describing how heavy elements can naturally account for this strange late-time glow in the explosion. The model shows that as the ejected material cools, r-process elements condense into solid dust grains—and that this dust naturally reproduces the observed infrared glow.
The model predicts dust should start forming roughly 10–20 days after the merger. “When dust is included, grain formation begins after ∼10 days, first in the cooler outer layers of the ejecta,” they write in the paper. “By day 20, dust formation has largely saturated throughout the inner regions.”
If astronomers watch closely during that window, they should see the spectrum transition from being dominated by sharp atomic emission lines early on to being dominated by a smooth, thermal “glow” from dust later. This is similar to a well-known pattern already seen in ordinary supernovae.
The models predict that the dust forms best in the slower components of the merger ejecta. Because different mergers will have different masses, speeds and compositions in these components, the team expects real diversity in late-time kilonova behavior—some may show signatures of dust, while others may not. “Future optical and infrared observations will provide a direct test of this scenario, both through time-resolved spectroscopy around the dust-formation epoch and through the diversity of late-time kilonova emission,” they conclude.
Researchers say that this exotic dust could become a powerful new way to study how neutron star mergers and other astrophysical events, such as collapsing massive stars, or collapsars, certain unusual supernovae, and the collapse of white dwarfs into neutron stars, forge the universe’s heaviest elements.
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Publication details
Nanae Domoto et al, Heavy element dust explains the late-time spectra of kilonovae, arXiv (2026). DOI: 10.48550/arxiv.2607.00433
Journal information:
arXiv
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Heavy-element exotic dust may solve a neutron star merger mystery (2026, July 14)
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