The intermediate neutron capture process. I. Development of the i-process in low-metallicity low-mass AGB stars
arXiv:2102.08840 · doi:10.1051/0004-6361/202040170e
Abstract
Results from 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 the hydrogen-rich layers. The subsequent proton ingestion leads to a strong neutron burst with neutron densities of cm 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 approximately ten years the upper part of the convection zone merges with the convective envelope. The surface carbon abundance is enhanced by more than 3 dex. This leads to an increase in 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.
arXiv commentPublished in A&A. This version of the preprint has been corrected (especially Table 2 and Section 6.5) with the modifications appearing in the A&A corrigendum (which is included at the end of the paper)
References in corpus (19)
- Carbon Enhanced Metal-Poor Stars. I. Chemical Compositions of 26 Stars
- A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-Poor Stars
- A holistic approach to carbon-enhanced metal-poor stars
- HE1327-2326, an unevolved star with [Fe/H]<-5.0. I. A Comprehensive Abundance Analysis
- Normal and Outlying Populations of the Milky Way Stellar Halo at [Fe/H] < -2
- The Hamburg/ESO R-process Enhanced Star survey (HERES) III. HE 0338-3945 and the formation of the r+s stars
- An Elemental Assay of Very, Extremely, and Ultra Metal-Poor Stars
- The Chemical Composition of Carbon-Rich, Very Metal-Poor Stars: A New Class of Mildly Carbon-Rich Objects Without Excess of Neutron-Capture Elements
- Abundance Analysis of HE2148-1247, A Star With Extremely Enhanced Neutron Capture Elements
- First stars IV.CS 29497-030: Evidence for operation of the s-process at very low metallicity
- New analysis of the two carbon-rich stars CS 22948-27 and CS 29497-34 : binarity and neutron-capture elements
- Near-UV Observations of CS29497-030: New Constraints on Neutron-Capture Nucleosynthesis Processes
- The s-process in metal-poor stars: Abundances for 22 neutron-capture elements in CS 31062-050
- Metal-Poor Stars Observed with the Magellan Telescope I. Constraints on Progenitor Mass and Metallicity of AGB Stars Undergoing s-Process Nucleosynthesis
- Carbon and Strontium Abundances of Metal-Poor Stars
- Slowly, slowly in the wind: 3D hydrodynamical simulations of wind mass transfer and angular-momentum loss in AGB binary systems
- Low-mass low-metallicity AGB stars as an efficient i-process site explaining CEMP-rs stars
- Abundances of Extremely Metal-Poor Star Candidates
- The study of s-process nucleosynthesis based on barium stars, CEMP-s and CEMP-r/s stars