Most of my research aims at better understanding the origin of chemical elements in the universe. The main questions driving my work are : how (by which process), where (in which astrophysical sites), and when (at which stage of the Universe’s history) were the chemical elements synthesized ?
Fig. 1 : The main i-process path (red line) in a 1 M⊙ , [Fe/H] = −2.3, AGB model, at the bottom of the second thermal pulse, at the time of maximum neutron density. The chemical abundances are shown by the red colour scale in mass fraction. The black squares are stable nuclei. Typical s- and r-process paths (in blue and green, respectively) are shown for comparison (from Choplin et al. 2023)
Other secondary neutron capture processes were identified. One of them is the so-called intermediate neutron-capture process, which is associated with neutron densities in between the s- and r-processes. The existence of the i-process is supported by the observation of stars with chemical overabundances that are not compatible with the s- or the r-processes alone. Instead these stars can be explained by considering an intermediate neutron irradiation, as modeled by i-process calculations. Some pre-solar
Neutron capture processes
Elements heavier than iron are mainly produced by neutron capture reactions. Two main neutron capture processes are known to be responsible for about half of the trans-iron elements in the Universe: the slow and rapid neutron-capture process (s- and r-process, respectively).
Fig. 2 : Abundances in mass fraction during an i-process (red) and r-process (blue). The black squares are stable nuclei.
grains may also bear the signature of i-process nucleosynthesis. The i-process nucleosynthesis can develop if protons are mixed in a convective helium-burning zone (proton ingestion event or PIE). This could take place in different astrophysical sites, including Asymptotic Giant Branch Stars (see next section). Another neutron capture process is the n-process, which can take place in the helium-burning shell of massive stars during the supernova (see next sections).
Nucleosynthesis in AGB stars : the s- and i-processes
Before crossing the planetary nebula phase and dying as white dwarfs, stars with initial masses between 0.8 and 8 M⊙ go through the asymptotic giant branch (AGB) phase. AGB stars are made of a carbon/oxygen (CO) core, surrounded by thin He- and H-burning shells and large convective envelope (Fig. 4). Thermal instabilities in the helium-burning shell produce recurrent convective thermal pulses (TPs). During the interpulse period and if some extra mixing (e.g. overshoot) is present below the convective envelope at the time of the third dredge up, protons can diffuse in the 12C-rich layers left by the TP. This leads to the formation of a 13C-pocket and the production of trans-iron elements via the radiative s-process (Fig. 5). Typical temperature, timescale and neutron density during the radiative s-process are T = 100 MK, τ = 10^4 yr and Nn = 10^7 cm−3 respectively. The convective s-process can develop at the bottom of the convective TP if the temperature exceeds T ≃ 350 MK, which is the case in AGB stars with Mini ≳ 3M⊙ (Fig. 5). The s-process products are later transported to the surface by third dredge up events.​​
Fig. 3 : Ring Nebula captured by JWST. The central star is a 0.6 M☉ post-AGB star that lost its envelope (credits : ESA/Webb, NASA, CSA, M. Barlow, N. Cox, R. Wesson)
Fig. 4 : Schematic (not to scale) image of the structure of an asymptotic giant branch star (figure from Lugaro et al. 2023)
Fig. 5 : Schematic view of the internal structure of a standard AGB star (without a PIE). Grey zones show convective zones. The yellow area corresponds to the CO core. The green (blue) dashed lines represent the location of the H-burning (He-burning) shell. The red shaded areas show where the radiative and convective s-processes operate (figure from Choplin et al. 2025)
Fig. 6 : Schematic view of the internal structure of an AGB star that experienced a PIE. Grey zones show convective zones. The yellow area corresponds to the CO core. The green (blue) dashed lines show where the energy generated by H burning (He burning) is maximal. The red (magenta) shaded areas show where the s-process (i-process) operates. The numbers indicate different important events: (1) start of PIE, (2) split of the TP, (3) merging of pulse with envelope, and (4) weak PIE (figure from Choplin et al. 2025)
When fully developed, the top of the convective TP can encroach on the bottom of the hydrogen-rich layer, leading to a proton ingestion event (PIE). The PIE mechanism is illustrated in Fig. 6 and 7. During a PIE, protons are mixed into the convective thermal pulse, captured through the 12C(p,g)13N reaction, and rapidly converted to 13C via beta-decay. The subsequent 13C(a,n)16O reaction at roughly 250 MK produces neutron densities up to Nn = 10^15 cm-3, triggering i-process nucleosynthesis and potentially leading to actinide production (Fig. 8). The processed material is ultimately dredged into the envelope and expelled through stellar winds.
Since 2021, I have conducted various modeling studies to better understand the i-process in AGB stars. In particular, I have investigated the effects of mass and metallicity variations, convective overshooting, and nuclear uncertainties.​
Fig. 7 : Kippenhahn diagram highlighting the important aspects during a PIE in a 1 M⊙, [Fe/H] = - 2.5 AGB model, computed with the STAREVOL code. Grey zones are convective regions. The location of the maximal energy generated by H-burning (He-burning) is indicated by the green (blue) dashed line. The colormap represents the neutron density (from ...)
Fig. 8 : Synthesis of actinides during i-process nucleosynthesis in an AGB star model
Nucleosynthesis in massive stars, the s-, n- and p-processes
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Section
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