How an odd star in the 'Gaia Sausage' could help solve one of astronomy's most enduring mysteries

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Astronomers have discovered an exceptionally rare star that may help to solve one of astronomy's enduring mysteries: where the universe's heaviest elements came from.
The star, named LAMOST J0804+5740, resides in the Gaia Sausage (also called Gaia Enceladus), the ancient remnants of a dwarf galaxy that merged with the Milky Way roughly 8 billion to 11 billion years ago. This type of star, known as an actinide-boost star, has a high abundance of radioactive elements known as actinides.
"The actinides are the heaviest group of elements on the periodic table," Anirudh Patel, a doctoral candidate in the Theoretical High Energy Astrophysics group at Columbia University who was not involved in the study, told Space.com. "They include thorium and uranium, for example, and are produced by the r-process."
The r-process, short for "rapid neutron capture process," is a series of nuclear reactions that occur in extreme astronomical environments, like during neutron star mergers or certain types of supernovas, where atomic nuclei rapidly absorb neutrons before they have a chance to decay, becoming heavier elements.
"This is how approximately half of the elements in our universe heavier than iron are synthesized," Patel explained. "The r-process requires more extreme astrophysical environments than your typical nuclear fusion reactions that take place in the cores of massive stars. However, a complete understanding of the astrophysical origin of the r-process has remained elusive for decades."
Direct observations of the r-process in action are rare. So far, astronomers have identified two likely cosmic sites where it occurs. One is the merger of neutron stars, and more recently, Patel and colleagues reported evidence that it may also occur within extremely magnetized neutron stars called magnetars.
Although these discoveries are steps in the right direction, they still fall short of explaining how most of the universe's heavy elements came to be. Known r-process sites alone can't fully account for the observed abundance of heavy elements like uranium, thorium and gold. This is because they are too rare or infrequent to produce the sheer quantity of heavy elements we observe today, suggesting that other, yet-undiscovered sources must be contributing.
Observations of J0804+5740, the first actinide-boost star identified in the Gaia Sausage, provide an exciting new piece of the puzzle. "The study reports a comprehensive set of chemical data, identifying a new r-process enriched star," Patel said. "This, along with other data and theoretical models, will play a role in [pinning down] the origin of the r-process elements in the universe."
To determine which elements are present in a star and uncover the processes that produced them, astronomers use a technique known as atomic spectroscopy.
"The idea is that electrons occupy different energy levels in atoms," Patel said. "The spacing between these energy levels can be different inside atoms of different elements. If an atom is sitting in the atmosphere of a star, it can absorb the light from the star, and its electrons can transition between the internal energy levels. Different elements have different atomic structure[s], so they will absorb different frequencies of light during these transitions."
Using specialized instruments, scientists observe less light at specific colors where elements absorb it due to their internal energy levels. This helps them measure how much of those elements are in the star's atmosphere.
The study's scientific team was excited that, after conducting an elemental analysis, they found that J0804+5740 represents a rare example of an actinide-boost star at relatively high metallicity — the overall abundance of elements heavier than helium in a star.
Actinide-boost stars typically only show an unusually high number of actinides, like thorium and uranium, relative to other heavy elements produced by the r-process, but this boost usually appears in stars that are metal-poor or have low metallicity.
This makes J0804+5740 a bit of an oddball. "Like most r-process enhanced stars, the abundance pattern of most heavy neutron-capture elements in J0804+5740 agrees well with the solar r-process, indicating that the main r-process produced these elements in the early universe," the team wrote in a paper published in The Astrophysical Journal Letters. "However, some elements exhibit deviations from [a typical] solar r-process abundance pattern."
It follows the expected pattern for very heavy elements, but it also shows an unexpectedly high abundance of lighter r-process elements, like barium, lanthanum and cerium. "[Their abundance] in the star J0804+5740 is a few times larger than in our own solar system," Patel said. "This implies that there exist multiple types of r-process sites — in particular, one that could produce a relatively high abundance of these light r-process elements."
To better understand the odd star's origins, the team analyzed the motions of J0804+5740 and similar stars. They found that actinide-boost stars are twice as likely to have come from outside the Milky Way, suggesting they were born in smaller galaxies that were later pulled into ours. This points to an important clue: The actinide-boost phenomenon may be more common in older, smaller galaxies.
Theoretical models indicate that one possible source could be a rare and powerful explosion known as a magneto-rotationally driven supernova. These extreme events could create the kind of neutron-rich environments needed to produce actinides, particularly in galaxies like the Gaia Sausage.
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"The theoretical models are promising, but they are not without their uncertainties," Patel said. "More observational constraints are needed to assess how well these models reproduce what actually happens in nature."
Regardless, these elemental deviations in J0804+5740 suggest a more complex nucleosynthetic origin — possibly involving multiple types of r-process events or contributions from other processes beyond the main r-process.
"This means we don't yet have the complete picture," Patel said. "Future observations […] will reveal more about the nature of r-process sites in the universe," Patel said. "Along with new and advanced theoretical models, this will be essential to resolving the r-process mystery and completing our understanding of the origin of the elements."