Seeing the Hidden Differences: Neutron-Capture Elements in Stellar Doppelgängers
Catherine Manea and collaborators set out to answer a tricky question in Galactic astronomy: how similar are stars that look chemically identical in large surveys? The APOGEE survey (Apache Point Observatory Galactic Evolution Experiment) measures the chemical fingerprints of hundreds of thousands of stars. These fingerprints are usually enough to group stars into families that reveal the Milky Way’s history. However, APOGEE mostly measures lighter elements, and struggles with heavier ones, especially those created through the neutron-capture process, which involves nuclei absorbing neutrons inside stars. Manea and colleagues ask whether two stars that appear chemically the same in APOGEE might still differ once the heavier elements are considered.
Selecting and Observing Doppelgängers
The team focused on “chemical doppelgängers,” stars that appear nearly identical in their APOGEE chemical data but are not physically related. Using APOGEE’s most recent data release (DR17), they selected 25 such pairs of giant stars. They then followed up with high-resolution optical spectroscopy at the McDonald Observatory in Texas, using the Tull spectrograph. Unlike APOGEE’s infrared view, optical spectra give access to the strong lines of neutron-capture elements like yttrium (Y), barium (Ba), and europium (Eu). For comparison, the authors also observed stars in the well-studied open cluster M67, known to be chemically uniform.
Methods: Looking Line by Line
To measure abundances as precisely as possible, the team used a “line-by-line differential” approach. Instead of averaging many measurements, they compared the same spectral lines between paired stars. This technique cancels out many sources of error, allowing the researchers to detect very small abundance differences, down to a few hundredths of a dex, which translates to about 10% in element ratios.
Results: A Hidden Layer of Differences
When comparing the doppelgängers, the authors found that most were indeed nearly identical in the lighter elements measured by APOGEE, just as expected. But the story changed for the neutron-capture elements. None of the 25 pairs were perfect matches in all the heavy elements. Instead, nearly every pair differed in at least one neutron-capture element, with differences ranging from 0.02 to as much as 0.38 dex (about 4–140%). These differences were often larger than what the team measured within the open cluster M67, where stars were expected to be homogeneous.
Discussion: Why Do These Differences Exist?
The authors emphasize that neutron-capture elements follow different evolutionary paths in the Galaxy. Some, like barium and lanthanum, are produced in asymptotic giant branch (AGB) stars through the slow neutron-capture process (s-process). Others, like europium, come from rare but powerful events such as neutron star mergers (the rapid neutron-capture, or r-process). Because these processes are localized and sometimes rare, two stars born at roughly the same time and place can still end up with very different amounts of these heavy elements. The differences Manea’s team observed suggest that APOGEE’s lighter-element data alone cannot always reveal the full chemical story.
Implications and Conclusions
This study shows that even the most carefully selected chemical doppelgängers can hide important differences in their neutron-capture elements. For astronomers trying to trace stars back to their birthplaces, an approach called chemical tagging, this finding is critical. It means that optical spectroscopy of neutron-capture elements will remain essential, even as surveys like APOGEE map vast regions of the Milky Way. By revealing these subtle differences, Manea and collaborators highlight the complexity of Galactic chemical evolution and the need to look beyond the lighter elements when grouping stars by chemistry.
Source: Manea
