Tracking Potassium in the Oldest Stars: What It Tells Us About Stellar Explosions
Miho Ishigaki and colleagues set out to measure potassium in some of the oldest known stars in the universe, called extremely metal-poor (EMP) stars. These stars contain very little of the heavier elements (astronomers call all elements heavier than hydrogen and helium “metals”), making them excellent records of the first generations of stellar explosions. Because potassium is difficult to detect in such faint stars, only a few past studies have tried to measure it. Ishigaki’s team uses the Subaru Telescope’s high-resolution spectrograph to carefully analyze the faint potassium lines in 18 EMP stars.
Why Potassium Matters
Potassium belongs to a group of “odd-Z” elements, whose origins remain mysterious. Theoretical models often predict too little potassium compared to what astronomers observe in stars. One possible explanation is that multidimensional effects in massive stars or special processes during supernova explosions might enhance potassium production. Studying EMP stars offers a way to test these ideas, since their chemical makeup likely reflects only one or a few supernova events from the early universe.
Collecting the Data
The researchers observed stars with brightness suitable for the Subaru Telescope, focusing on the two potassium lines near 766 and 769 nanometers. Because Earth’s atmosphere produces interfering absorption at these same wavelengths, the team had to carefully correct these telluric features by observing comparison stars. Out of the 18 targets, useful potassium measurements could be obtained for seven, while only upper limits could be set for the others.
Analyzing the Abundances
To translate the faint spectral lines into potassium abundances, Ishigaki and collaborators used detailed computer models of stellar atmospheres. They also applied corrections for “non-local thermodynamic equilibrium” (NLTE), a physics effect that significantly changes how potassium lines appear. In addition to potassium, they measured or compared data for elements like sodium, magnesium, calcium, titanium, chromium, and nickel. This multi-element approach allowed them to check whether potassium behaves differently from other elements.
What They Found
The team discovered that potassium-to-iron ([K/Fe]) and potassium-to-calcium ([K/Ca]) ratios were consistently higher than solar values across the sample, with a very small scatter of about 0.1 dex, comparable to the measurement uncertainty. In contrast, sodium-to-magnesium ratios varied much more widely, even after NLTE corrections. This suggests that the processes that make potassium in massive stars or their explosions are more stable and independent compared to the mechanisms that drive sodium variation.
Implications for Stellar Evolution
The low scatter in potassium abundances points to robust physical processes in massive stars and supernovae. Models involving multidimensional mixing inside stars or neutrino-driven processes in explosions may explain why potassium is more predictably produced than other odd-Z elements. By highlighting this stability, Ishigaki’s work strengthens the case that potassium can serve as a sensitive probe of how massive stars end their lives.
Looking Ahead
Although measuring potassium in EMP stars is challenging, Ishigaki and her team show that it is possible to collect reliable data. Expanding such studies with even larger telescopes will sharpen our understanding of how the universe’s first stars exploded and seeded the cosmos with the ingredients for planets and life.
Source: Ishigaki
