Mapping the Motion of the Milky Way’s r-Process Stars
Pallavi Saraf and collaborators study how certain stars in our Galaxy, those rich in heavy elements created by the r-process (rapid neutron capture), move through space. The r-process is one of two main ways elements heavier than zinc are formed; the other is the s-process (slow neutron capture). While scientists understand where the s-process happens (mostly in aging stars called AGB stars), the precise locations of the r-process remain uncertain. Possible sites include supernova explosions, neutron star mergers, and other extreme cosmic events. By examining the chemical makeup and orbital motions of these r-process-enhanced (RPE) stars, astronomers hope to uncover how and where these elements were first created and distributed throughout the Milky Way.
Sample and Methods: Gathering the Data and Simulating Orbits
The team combined data from two major studies (Gudin et al. 2021 and Shank et al. 2023), collecting information on nearly 500 RPE stars. Each star’s chemical data, elements like carbon, iron, strontium, barium, and europium, came from previous surveys, while motion data such as distance, velocity, and parallax were obtained from the Gaia mission. Using this information, Saraf and her team calculated the orbits of these stars over the past eight billion years with a computer model of the Milky Way’s gravitational field. This allowed them to determine where each star likely travels in the Galaxy, whether it belongs to the central bulge, the flat disk, or the extended halo. To classify these regions, they used the farthest distance a star reaches from the Galactic center (its apocenter) and how far it moves above or below the Galactic plane (zmax).
Results: Where the r-Process Stars Live
The analysis showed that RPE stars are spread across all major parts of the Milky Way. About half belong to the disk and half to the halo, challenging the older belief that most such stars reside only in the halo. A small number of stars are found in the bulge, the dense central region of the Galaxy. The researchers also tested the traditional Toomre diagram, a tool that plots stellar velocities to separate disk and halo stars. They found that this diagram fails for stars on very eccentric (elongated) or retrograde (backward-moving) orbits, which can make disk stars appear halo-like. Saraf and colleagues argue that orbital characteristics, not instantaneous velocity, give a more accurate picture of where a star truly belongs.
Tracing Origins: Born Here or From Afar?
By analyzing the stars’ orbital angular momenta, essentially the amount and direction of their spin around the Galaxy, the team inferred which stars likely formed within the Milky Way (in-situ) and which were captured from smaller galaxies that merged with it (ex-situ). They discovered that only about 10% of the stars clearly came from outside systems, while around 25% likely formed within the Milky Way. Strikingly, about three-quarters of the stars fall into a “mixed zone,” meaning their origins are ambiguous. All definitively ex-situ stars appear in the halo, consistent with the idea that many halo stars were accreted from ancient, disrupted dwarf galaxies.
Chemical Clues: How Enrichment Varies Across the Galaxy
The study also examined how the abundance of europium (a classic r-process element) and metallicity (overall heavy-element content) vary among Galactic regions. Stars in the inner disk and halo share similar abundance patterns, showing large variations in europium at low metallicity. Outer disk stars, on the other hand, show nearly constant europium levels and tend to have highly eccentric orbits, suggesting they may have been enriched by a single r-process event. The inner disk’s metallicity distribution appears bimodal, possibly reflecting multiple waves of star formation or accretion events in the Milky Way’s history.
Conclusions: Connecting Motion and Chemistry
Saraf and her team conclude that the Milky Way’s RPE stars are not confined to the halo as once thought, but instead populate the disk and halo in nearly equal measure. The familiar Toomre diagram, long used to distinguish these regions, may mislead when dealing with complex orbits. Instead, orbital simulations provide a clearer view of stellar origins. The finding that most RPE stars occupy a mixed region between in-situ and ex-situ formation highlights the Galaxy’s dynamic and blended history. Finally, similar chemical trends across different Galactic components suggest that the r-process operated under comparable conditions in multiple environments, leaving behind a shared chemical fingerprint across the Milky Way.
Source: Saraf
