REPRODUCING THE OBSERVED ABUNDANCES IN RCB AND HdC STARS WITH POST-DOUBLE-DEGENERATE MERGER MODELS—CONSTRAINTS ON MERGER AND POST-MERGER SIMULATIONS AND PHYSICS PROCESSES

Abstract

The R Coronae Borealis (RCB) stars are hydrogen-deficient, variable stars that are most likely the result of He-CO WD mergers. They display extremely low oxygen isotopic ratios, 16 O/ 18 O ≃ 1 − 10, 12 C/ 13 C ≥ 100, and enhancements up to 2.6dex in F and in s-process elements from Zn to La, compared to solar. These abundances provide stringent constraints on the physical processes during and after the double-degenerate merger. As shown before O-isotopic ratios observed in RCB stars cannot result from the dynamic double-degenerate merger phase, and we investigate now the role of the long-term 1D spherical post-merger evolution and nucleosynthesis based on realistic hydrodynamic merger progenitor models. We adopt a model for extra envelope mixing to represent processes driven by rotation originating in the dynamical merger. Comprehensive nucleosynthesis 1 NuGrid collaboration, http://www.nugridstars.org 2 The Joint Institute for Nuclear Astrophysics post-processing simulations for these stellar evolution models reproduce, for the first time, the full range of the observed abundances for almost all the elements measured in RCB stars: 16 O/ 18 O ratios between 9 and 15, C-isotopic ratios above 100, and ∼ 1.4 -2.35 dex F enhancements, along with enrichments in s-process elements. The nucleosynthesis processes in our models constrain the length and temperature in the dynamic merger shell-of-fire feature as well as the envelope mixing in the post-merger phase. s-process elements originate either in the shellof-fire merger feature or during the post-merger evolution, but the contribution from the AGB progenitors is negligible. The post-merger envelope mixing must eventually cease ∼ 10 6 yr after the dynamic merger phase, before the star enters the RCB phase.

Evolutionary Models for R Coronae Borealis Stars

“…Note that this is 10 times greater than typical carbon-rich AGB wind velocities (e.g., Groenewegen et al 1998). Recent evolutionary models of R CrB stars have adopted the AGB wind prescription of Bloecker (1995), with efficiency factors η ≈ 0.02 (Menon et al 2013;Zhang et al 2014;Lauer et al 2019). Typically mass loss rates are then ∼ 10 −6 − 10 −5 M yr −1 .…”

“…A later series of hydrodynamic merger calculations including hybrid He-CO WDs confirmed the idea that this He layer can prevent the dredge-up of 16 O-rich material during the merger (Staff et al 2018). Menon et al (2013) construct parameterized composition profiles that schematically represent the outcome of the merger simulations of Staff et al (2012). They use abundances from more detailed stellar models and use an amount of dredge-up less than that seen in the hydrodynamic simulations.…”

The Evolution of Compact Binary Star Systems

“…If one of the DD components fills its Roche lobe, the outcome of the contact depends on the chemical composition of the stars and their masses. There are several possible endpoints: merger leading to a supernova (the left and the right-most branches at the bottom of Figure 9, see Section 7.3), stable mass exchange with the formation of an AM CVn system (as indicated in the figure, see Section 9), direct formation of a single massive WD (the central branch, see Section 7.3.2 and discussion of Figure 18) or the formation of an R CrB type star with an extended helium envelope [825, 306, 885, 480], which will also end its evolution as a single WD (not shown in Figure 9). White dwarfs produced by the merger process may be hidden among single WDs without traces of H or He, like those with oxygen-rich atmospheres [222].…”

Dawes Review 6: The Impact of Companions on Stellar Evolution

“…They are thought to be merger products primarily because of the presence of elevated quantities of 18 O (Clayton et al, 2007), which can be made under merger conditions. Models of the mergers have been carried out with 3D hydrodynamic codes (Staff et al, 2012), which in turn were used as inputs to 1D stellar structure codes (Menon et al, 2013) to determine the nucleosynthetic properties of the merger. The relationship between the dust activity and the merger past remains unclear (Bright et al, 2011).…”