Abstract
Abstract Based on stellar evolution simulations, we demonstrate that rapidly accreting white dwarfs (WDs) in close binary systems are an astrophysical site for the intermediate neutron-capture process. During recurrent and very strong He-shell flashes in the stable H-burning accretion regime H-rich material enters the He-shell flash convection zone. reactions release enough energy to potentially impact convection, and i process is activated through the reaction. The H-ingestion flash may not cause a split of the convection zone as it was seen in simulations of He-shell flashes in post-AGB and low-Z asymptotic giant branch (AGB) stars. We estimate that for the production of first-peak heavy elements this site can be of similar importance for galactic chemical evolution as the s-process production by low-mass AGB stars. The He-shell flashes result in the expansion and, ultimately, ejection of the accreted and then i-process enriched material, via super-Eddington-luminosity winds or Roche-lobe overflow. The WD models do not retain any significant amount of the accreted mass, with a He retention efficiency of depending on mass and convective boundary mixing assumptions. This makes the evolutionary path of such systems to supernova Ia explosion highly unlikely.
Highlights
Trans-iron elements are produced through n-capture nucleosynthesis, such as the slow (s, with a neutron density Nn 1011 cm−3) and the rapid (r, Nn 1020 cm−3) processes (e.g., Kappeler et al 2011; Thielemann et al 2011). Cowan & Rose (1977) proposed that an i process with Nn ∼ 1015 cm−3 intermediate between s and r processes might be triggered when H is mixed into a convective He-burning shell
Based on stellar evolution simulations, we demonstrate that rapidly accreting white dwarfs in close binary systems are an astrophysical site for the intermediate neutron-capture process
Rate followed by a prolonged period of lower H ingestion, and all of this without split of the He-flash convection zone. If it is confirmed in 3D simulations that no Global Oscillation of Shell H-ingestion (GOSH, Herwig et al 2014) is found in this situation, 1D spherically symmetric simulations of the i-process nucleosynthesis in rapidly accreting WD (RAWD) are probably more realistic compared to the case of postAGB very late thermal pulse (VLTP) models and low-Z AGB models for which 3D stellar hydrodynamics simulations have yet to determine the detailed outcome of the GOSH
Summary
Trans-iron elements are produced through n-capture nucleosynthesis, such as the slow (s, with a neutron density Nn 1011 cm−3) and the rapid (r, Nn 1020 cm−3) processes (e.g., Kappeler et al 2011; Thielemann et al 2011). Cowan & Rose (1977) proposed that an i process with Nn ∼ 1015 cm−3 intermediate between s and r processes might be triggered when H is mixed into a convective He-burning shell. In the VLTP the He-convection zone reaches into the remaining H-rich atmosphere, which will mix protons into He-burning conditions (Fujimoto 1977; Iben 1982) This mixing triggers i-process nucleosynthesis, and simulations of this situation can reproduce the unusual heavy element i-process fingerprint in Sakurai’s object if mixing predictions from a mixing-length theory (MLT) are modified to reflect possible effects of the inhomogeneous nature of convective-reactive burn in Hingestion events (Herwig et al 2011, 2014). Cassisi et al (1998) reported that ≈ 0.6M WDs that accreted solarcomposition material at rates of 4 · 10−8 and 10−7M /yr would eventually experience He-shell flashes with Hingestion Their simulations ended due to the ensuing numerical difficulties, but Cassisi et al (1998) suggested that those He-shell flashes could lead to substantial additional mass loss as they would cause the WD to evolve back toward the giant branch, just as the VLTP model of Sakurai’s object.
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