Title: Production of Lithium and Heavy Elements in AGB Stars Experiencing PIEs

Authors: Arthur Choplin, Lionel Siess, Stephane Goriely, Sebastien Martinet

First Author’s Institution: Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, Brussels, Belgium

Status: Published in Galaxies [open access]

The origin story of the heavy elements is one of the universe’s greatest mysteries. The Big Bang produced only hydrogen, helium, and a tiny bit of lithium. Somehow, over billions of years, that initial composition was transformed into an entire periodic table of elements – and astronomers are still trying to figure out how exactly that transformation happened.

Stars are a likely suspect, but nuclear fusion alone can’t create elements heavier than iron. Instead, we think that trans-iron elements are primarily formed through neutron-capture reactions, which occur when a “seed” nucleus captures one or more free neutrons from its environment. Astronomers have identified an entire alphabet of neutron capture processes, but exactly how and where these reactions occur is still very uncertain. Today’s authors seek to shed some light on this issue by investigating one of the lesser-known pathways – the i-process – with theoretical models. 

The neutron-capture alphabet

Neutron capture usually destabilizes the seed nucleus, which can then decay radioactively into a different element. This process, known as beta decay, converts neutrons into protons by emitting a negatively charged (beta) particle, increasing the seed’s atomic number. The availability of free neutrons (also called the neutron flux) controls how massive the seed can grow. If the neutron flux is high, the seed accumulates neutrons more quickly than it experiences beta decay, allowing it to reach higher atomic numbers. If the neutron flux is low, the seed usually captures just one neutron at a time, causing the atomic number to increase more slowly.

Astronomers think two main neutron-capture reactions are responsible for most of the trans-iron elements (Figure 1). The s-process (“slow” process) is associated with low neutron flux and is thought to occur primarily in asymptotic giant branch (AGB) stars. The r-process (“rapid” process) is associated with high neutron flux densities. Though we aren’t yet sure what the primary site for the r-process is, neutron star mergers are a strong candidate.

Despite significant uncertainties, our models of the s- and r-processes produce patterns of heavy elements that match what we see in many – but not all! – stars. To explain the stars that don’t match, astronomers have recently turned their attention to the i-process (“intermediate” process), which is associated with neutron fluxes in between those of the s- and r-processes. 

Eat your protons!

Current theories suggest that the i-process is triggered when protons are mixed into a helium-burning region of a star. Crucially, the helium-burning region must be convective, meaning that energy is transported through the motion of clumps of material rather than by radiation. Convection is able to carry the protons to higher-temperature regions, where a series of reactions that produce neutrons in “intermediate” amounts occur. 

One of the most common sites for convective helium burning is the envelopes of asymptotic giant branch (AGB) stars. AGB stars are low- to intermediate-mass stars that are nearing the end of their lives. They have a carbon-oxygen core surrounded by a thin (often convective!) helium-burning shell, a hydrogen-burning shell, and an outer envelope consisting mostly of hydrogen (Figure 2). 

Most importantly for the i-process, AGB stars experience thermal pulses: sudden bursts of energy caused by instabilities in the helium-burning shell. During a thermal pulse, material from the star’s interior is “dredged up” to the surface, enriching the envelope with heavy elements made during helium burning. Astronomers think that the mixing induced by this process could also cause a “proton ingestion event” (PIE), in which protons are transported down from the star’s outer layers down into the He-burning region. If a PIE occurs, it would quickly create the conditions needed for i-process nucleosynthesis!

Simulating PIEs in AGB stars

In today’s paper, the authors used a stellar evolution code called STAREVOL to compute detailed models of AGB stars experiencing PIEs. They varied two key parameters: the mass of the AGB star (from 1 to 3 solar masses) and the metallicity ([Fe/H], from -3 to 0). To make sure that the PIEs and any resulting i-process nucleosynthesis were accurately represented, they included over 2000 different nuclear reactions and evolved each model through as many thermal pulses as possible. Once their models were complete, the authors searched for broad trends in elemental abundances, finding three main results: 

I. Lithium production. Lithium is quickly destroyed at high temperatures, so stars typically lose all of their lithium by the time they become a red giant. However, the authors found that a PIE results in the ingestion of not just protons, but also 3He, which fuels the production of lithium. As a result, PIEs can substantially increase the lithium abundance of AGB stars (Figure 2), an effect that survives additional thermal pulses. This effect is strongest in lower-mass stars, which synthesize more 3He during core-H burning than higher-mass stars.

II. 13C production. In stars that don’t experience a PIE, the ratio of the two most common carbon isotopes (12C/13C) is expected to increase slightly with each thermal pulse as 12C-rich material is brought to the surface. However, the authors found that 13C is synthesized during PIEs, counteracting this effect and causing the ratio to decrease. Though the ratio may increase again with additional thermal pulses, not all models continue the AGB phase after a PIE, resulting in post-AGB stars with unusually low 12C/13C ratios.

III. i-process nucleosynthesis. As expected, the authors find that the nuclear reactions triggered by PIEs produce enough neutrons to trigger i-process nucleosynthesis, enhancing surface abundances of elements like barium, europium, strontium, and lead (Figure 3). The production of heavy elements is highest for low-metallicity stars ([Fe/H] ≤ 1). At higher metallicities, the star contains more heavy elements to begin with, so there are more seed nuclei even though the PIE produces the same number of neutrons. As a result, the formation of heavier elements is less likely because each seed will capture fewer neutrons on average. 

Excitingly, these results seem to align with odd abundance patterns that have been observed in certain populations of stars. For example, J-type stars are a class of AGB stars that have strong 13C features and unusually high lithium abundances. The origin of these stars is currently unknown, but the results of today’s paper suggest that i-process nucleosynthesis could provide a simple answer to this mystery.

The models from today’s paper are the most detailed models of PIEs and i-process nucleosynthesis in AGB stars to date, but there’s still a lot of work to do! Further investigations are needed to explain why some AGB stars experience a PIE while others do not, and to determine how PIEs are affected by more complex stellar physics (like rotation and magnetism). Nonetheless, these models represent an important step forward in our understanding of the origin of the elements. As we continue to learn more about the i-process, we may soon be able to explain some of the unusual stellar abundance patterns that have puzzled astronomers for years.

Astrobite edited by Sowkhya Shanbhog.

Featured image credit: Background image by Jeremy Thomas on Unsplash.