The intermediate neutron capture process. I. Development of the i-process in low-metallicity low-mass AGB stars. (arXiv:2102.08840v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Choplin_A/0/1/0/all/0/1">A. Choplin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Siess_L/0/1/0/all/0/1">L. Siess</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Goriely_S/0/1/0/all/0/1">S. Goriely</a>
Gathering observations report a growing number of metal-poor stars showing an
abundance pattern midway between the s- and r-processes. These so called
r/s-stars raise the need for an intermediate neutron capture process
(i-process), which is thought to result from the ingestion of protons in a
convective helium-burning region, but whose astrophysical site is still largely
debated. We investigate whether an i-process during the asymptotic giant branch
(AGB) phase of low-metallicity low-mass stars can develop and whether it can
explain the abundances of observed r/s-stars. At the beginning of the AGB
phase, during the third thermal pulse, the helium driven convection zone is
able to penetrate in the hydrogen rich layers. The subsequent proton ingestion
leads to a strong neutron burst with neutron densities of $approx 4.3 times
10^{14}$ cm$^{-3}$ at the origin of the synthesis of i-process elements. The
nuclear energy released by proton burning in the helium-burning convective
shell strongly affects the internal structure: the thermal pulse splits and
after $approx 10$ yr the upper part of the convection zone merges with the
convective envelope. The surface carbon abundance is enhanced by more than 3
dex, leading to an increase of the opacity which triggers a strong mass loss
and prevents any further thermal pulse. We show that specific isotopic ratios
of Ba, Nd, Sm and Eu can represent good tracers of i-process nucleosynthesis.
Finally, an extended comparison with 14 selected r/s-stars show that the
observed composition patterns can be well reproduced by our i-process AGB
model. However, such AGB models cannot account for the high level of enrichment
of the giant r/s-stars in a scenario involving pollution by a former AGB
companion.
Gathering observations report a growing number of metal-poor stars showing an
abundance pattern midway between the s- and r-processes. These so called
r/s-stars raise the need for an intermediate neutron capture process
(i-process), which is thought to result from the ingestion of protons in a
convective helium-burning region, but whose astrophysical site is still largely
debated. We investigate whether an i-process during the asymptotic giant branch
(AGB) phase of low-metallicity low-mass stars can develop and whether it can
explain the abundances of observed r/s-stars. At the beginning of the AGB
phase, during the third thermal pulse, the helium driven convection zone is
able to penetrate in the hydrogen rich layers. The subsequent proton ingestion
leads to a strong neutron burst with neutron densities of $approx 4.3 times
10^{14}$ cm$^{-3}$ at the origin of the synthesis of i-process elements. The
nuclear energy released by proton burning in the helium-burning convective
shell strongly affects the internal structure: the thermal pulse splits and
after $approx 10$ yr the upper part of the convection zone merges with the
convective envelope. The surface carbon abundance is enhanced by more than 3
dex, leading to an increase of the opacity which triggers a strong mass loss
and prevents any further thermal pulse. We show that specific isotopic ratios
of Ba, Nd, Sm and Eu can represent good tracers of i-process nucleosynthesis.
Finally, an extended comparison with 14 selected r/s-stars show that the
observed composition patterns can be well reproduced by our i-process AGB
model. However, such AGB models cannot account for the high level of enrichment
of the giant r/s-stars in a scenario involving pollution by a former AGB
companion.
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