The notion that heavy element formation predominantly stems from supernovae or neutron star mergers is frequently relayed, albeit inaccurately. Indeed, the full story is more complicated.

Are we children of the stars? Is gold formed in supernovae? Where have heavy elements originated from? These are fascinating questions that have been raised frequently. I contemplate them frequently as well. Over the past twelve years, I have conducted pioneering research in nuclear astrophysics, seeking to understand the origin of chemical elements in the cosmos by bridging two scientific fields: nuclear physics and stellar physics. However, widespread public misconceptions often surround it. Stellar nucleosynthesis, the process by which chemical elements are formed in stars, is a complex and captivating field of research. Here, I aim to clarify the topic by identifying and discussing a few of its most confounding aspects. Specifically, I have chosen to explore three such points in a concise and non-exhaustive manner.

Beyond iron

The origin of elements heavier than iron is frequently misunderstood. While it most people know that stars can produce energy through nuclear fusion stages up to the production of iron, much fewer have a clear idea about the production of elements with a higher atomic number. Yet these elements clearly exist. The notion that heavy elements predominantly stem from supernovae or neutron star mergers is frequently relayed, albeit inaccurately.  While neutron star mergers undoubtedly play a critical part in generating elements beyond iron, their contribution does not surpass 50% of the quantities detected in the solar system. The remaining 50% originates primarily from a commonly overlooked source: AGB stars. These stars are the advanced evolutionary phase for stars with similar mass to the Sun (up to 8 solar masses), and those with masses between 2 and 3 solar masses are particularly noteworthy. These stars feature a carbon-13-rich area (referred to as the ‘carbon-13 pocket’) located at the boundary between their convective envelope and the upper portion of the helium shell. In such a region, high temperatures (about 90 million degrees) facilitate a highly efficient reaction:

carbon-13 + helium-4 –> oxygen-16 + neutron.

This process leads to the emission of a significant amount of neutrons (up to 10 million neutrons per cubic centimetre), which initiates the capture of these particles by iron nuclei in combination with beta decays. As a result, the neutrons are transformed into protons, leading to the creation of increasingly heavier nuclei. This marks the beginning of the s-process, accountable for around half of the elements beyond iron’s abundance. Then, it should be noted that there is also a “weak” component of the s-process that takes place in massive stars, primarily during the central burning of helium and carbon in the shell. This process subsequently generates the elements ranging from iron to zirconium. Roughly speaking, elements beyond iron (excluding minor processes like the p-process), are formed through two major neutron capture processes; the r-process (occurring in neutron star mergers and possibly in certain types of supernovae from massive stars) and the s-process (predominantly in small-mass AGB stars).

Elements abundance distribution on the Sun. Credits: Maria Lugaro

Observational evidence

It is worth noting that what just described is not simply a theoretical construct; there is solid observational evidence to back it up. As with the r-process in neutron star mergers, there is direct observational evidence for the s-process in AGB stars. Technetium, which has no stable isotopes, was first observed on the surface of the AGB star R Andromedae by Paul W. Merrill as early as 1952. Because the average lifetime of the technetium isotopes involved is orders of magnitude shorter than the lifetime of such stars, it is logical to conclude that the observed technetium was produced in situ by the very star under investigation. This marked the initial piece of solid evidence that AGB stars are the authentic factories of heavy elements.

Matching observations

Lastly, there have been numerous mentions and readings indicating that the composition of heavier-than-iron elements in the Sun matches that of nucleosynthesis from neutron stars. This is only true for a few elements including europium, gold, and the actinoids. However, the peaks observed at atomic numbers corresponding to strontium, barium, and lead align with the s-process in AGB stars. Conversely, there is a strong correlation between the element distribution on the surface of ancient, metal-poor stars, like the CEMP stars, and the r-process, being the first in chronological order to create elements beyond iron during the history of the Cosmos.



Galactic chemical evolution:

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