Abstract
ABSTRACT We have modelled the multicycle evolution of rapidly accreting CO white dwarfs (RAWDs) with stable H burning intermittent with strong He-shell flashes on their surfaces for 0.7 ≤ MRAWD/M⊙ ≤ 0.75 and [Fe/H] ranging from 0 to −2.6. We have also computed the i-process nucleosynthesis yields for these models. The i process occurs when convection driven by the He-shell flash ingests protons from the accreted H-rich surface layer, which results in maximum neutron densities Nn, max ≈ 1013–1015 cm−3. The H-ingestion rate and the convective boundary mixing (CBM) parameter ftop adopted in the one-dimensional nucleosynthesis and stellar evolution models are constrained through three-dimensional (3D) hydrodynamic simulations. The mass ingestion rate and, for the first time, the scaling laws for the CBM parameter ftop have been determined from 3D hydrodynamic simulations. We confirm our previous result that the high-metallicity RAWDs have a low mass retention efficiency ($\eta \lesssim 10{{\ \rm per\ cent}}$). A new result is that RAWDs with [Fe/H] $\lesssim -2$ have $\eta \gtrsim 20{{\ \rm per\ cent}}$; therefore, their masses may reach the Chandrasekhar limit and they may eventually explode as SNeIa. This result and the good fits of the i-process yields from the metal-poor RAWDs to the observed chemical composition of the CEMP-r/s stars suggest that some of the present-day CEMP-r/s stars could be former distant members of triple systems, orbiting close binary systems with RAWDs that may have later exploded as SNeIa.
Highlights
The type Ia supernovae (SNeIa) are thermonuclear explosions of carbon-oxygen (CO) white dwarfs (WDs) (e.g. Hoyle & Fowler 1960; Hillebrandt & Niemeyer 2000; Hillebrandt et al 2013; Churazov et al 2014; Livio & Mazzali 2018)
4 RESULTS 4.1 The rapidly-accreting WDs (RAWDs) multicycle evolution Using the methods described in Section 2.1, we have computed the evolution of seven RAWD models with the metallicities and WD masses listed in Table 1 along with other model parameters
All of the RAWD models have nearly the same initial central temperature with log10 Tc ≈ 7.2 and use the same WD Roche-lobe radius RRL,WD = 2R in Equation 2 that corresponds to the orbital period P ≈ 1.2 days for a secondary mass of ∼ 2M
Summary
The type Ia supernovae (SNeIa) are thermonuclear explosions of carbon-oxygen (CO) white dwarfs (WDs) (e.g. Hoyle & Fowler 1960; Hillebrandt & Niemeyer 2000; Hillebrandt et al 2013; Churazov et al 2014; Livio & Mazzali 2018). The primary star of such a binary system was an intermediate-mass star with M ≈ 2.5 – 7M , the upper boundary of this mass interval depending on the amount of convective boundary mixing and C-burning rate (Chen et al 2014) It left a core — the CO WD — after having lost the rest of its mass during a common-envelope event, when it arrived at the asymptotic-giant branch (AGB) and filled its Roche lobe. The non-processed H accumulates in an expanding envelope, so that such a rapidly accreting WD would be resembling a red giant
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