Tracing Starlight: How Ultraviolet Observations Reveal the Heavy Elements in HD 196944
Ian Roederer and collaborators set out to answer a long-standing question in astrophysics: how do the heavy elements beyond iron form and find their way into stars? In this paper, the authors focus on HD 196944, a carbon-enhanced metal-poor (CEMP-s) star that contains unusually high amounts of elements made by the slow neutron-capture process, or s-process. This process happens when atomic nuclei slowly absorb neutrons, building up heavier elements over time, unlike the r-process, which does so rapidly in explosive events. By using ultraviolet (UV) observations from the Hubble Space Telescope (HST), the team was able to measure rare heavy elements that had never before been detected in this kind of star.
Observations and Methods: A Deep Look into the Ultraviolet
Roederer and his team used HST’s Space Telescope Imaging Spectrograph (STIS) to collect new UV spectra of HD 196944, extending to wavelengths as short as 2029 Å. The star’s brightness in the UV made this possible, though it required more than 28 hours of telescope time. These new data were combined with earlier optical and UV observations from telescopes in Chile and Texas. The authors then analyzed the data using computer models that simulate how light interacts with atoms in the star’s atmosphere, essentially reverse-engineering the spectrum to determine what elements are present.
By combining data from different instruments, the researchers identified 35 elements heavier than zinc (Z > 30), the most comprehensive list ever compiled for a CEMP-s star. They also established upper limits for nine more, pushing the boundaries of what can be detected in stellar spectra.
Results: A Star Rich in Heavy Elements
The analysis confirmed that HD 196944’s atmosphere contains unusually large amounts of elements such as barium (Ba), lead (Pb), and ytterbium (Yb), a clear signature of the s-process. Many of these elements were likely transferred from a now-dead companion star that once went through the asymptotic giant branch (AGB) phase, when stars produce heavy elements in their interiors before shedding their outer layers. Models of this AGB companion suggest it had a mass of about three times that of the Sun and contributed the s-process elements now seen in HD 196944. The agreement between the model predictions and the observations was remarkably close across most of the periodic table.
Understanding What Was (and Wasn’t) Made by the s-Process
Interestingly, not all heavy elements in HD 196944 seem to come from this process. Elements such as gallium (Ga), germanium (Ge), and arsenic (As) appear in similar proportions in another metal-poor star, HD 222925, which is dominated by the r-process. This similarity suggests that neither the s-process nor the r-process contributed much to these elements; instead, they may have been produced by other kinds of stellar explosions. The authors also explored, but ruled out, the idea that an intermediate “i-process” could explain these abundances.
The Curious Case of Missing Bismuth
While most of the predicted elements were successfully detected, bismuth (Bi) was not. The absence of detectable Bi, even when its lighter neighbor Pb was abundant, challenges existing models of nucleosynthesis. The team suggests that the Bi lines may be difficult to see due to “non-local thermodynamic equilibrium” effects, physical conditions that cause atoms to absorb light differently than expected. More advanced models or future observations could help resolve this puzzle.
Classification and Broader Significance
With its high carbon and barium abundances, and a moderate europium level, HD 196944 fits the definition of a CEMP-s star, a carbon-enhanced, s-process-rich star. The precise match between the data and the AGB models strengthens the interpretation that its unusual chemistry originated from mass transfer in a binary system. Beyond classifying one star, this study shows that UV spectroscopy can reveal elements that are invisible at optical wavelengths, vastly expanding our ability to study how stars make heavy elements.
Looking Ahead
Roederer and his coauthors conclude by emphasizing the importance of UV observations for stellar archaeology. Future missions like NASA’s Habitable Worlds Observatory (HWO) could collect even more sensitive UV spectra, allowing astronomers to study these same processes in fainter stars. By continuing this work, scientists will get closer to understanding how the universe built the rich chemical diversity found in stars, and, eventually, in us.
Source: Roederer
