The impact of recombination energy on simulations of the common-envelope binary interaction

Abstract

During the common envelope binary interaction, the expanding layers of the gaseous common envelope recombine and the resulting recombination energy has been suggested as a contributing factor to the ejection of the envelope. In this paper we perform a comparative study between simulations with and without the inclusion of recombination energy. We use two distinct setups, comprising a 0.88-M and a 1.8-M giants, that have been studied before and can serve as benchmarks. In so doing we conclude that (i) the final orbital separation is not affected by the choice of equation of state. In other words, simulations that unbind but a small fraction of the envelope result in similar final separations to those that, thanks to recombination energy, unbind a far larger fractions. (ii) The adoption of a tabulated equation of state results in a much greater fraction of unbound envelope and we demonstrate the cause of this to be the release of recombination energy. (iii) The fraction of hydrogen recombination energy that is allowed to do work should be about half of that which our adiabatic simulations use. (iv) However, for the heavier star simulation we conclude that it is helium and not hydrogen recombination energy that unbinds the gas and we determine that all helium recombination energy is thermalised in the envelope and does work. (v) The outer regions of the expanding common envelope are likely to see the formation of dust. This dust would promote additional unbinding and shaping of the ejected envelope into axisymmetric morphologies.

Common-envelope evolution with an asymptotic giant branch star

“…As for the case of CEE with RG primary stars (Nandez et al 2015;Prust & Chang 2019;Reichardt et al 2020), the question of accounting for the release of recombination energy turns out to be decisive for the success of envelope ejection also with AGB primaries. Our simulations strongly indicate that a complete envelope removal is possible provided this energy is indeed transferred to the envelope material.…”

“…In the CE phase, the stellar envelope expands and recombination processes progressively liberate the ionization energy. This has been proposed as a mechanism leading to successful envelope ejection (Nandez et al 2015;Prust & Chang 2019;Reichardt et al 2020). Sabach et al (2017), Grichener et al (2018) and Soker et al (2018), however, question its relevance because instead of being converted locally into kinetic energy of the gas, the recombination energy might be transported away by convection (Wilson & Nordhaus 2019 or radiation (see Ivanova 2018).…”

“…Sabach et al (2017), Grichener et al (2018) and Soker et al (2018), however, question its relevance because instead of being converted locally into kinetic energy of the gas, the recombination energy might be transported away by convection (Wilson & Nordhaus 2019 or radiation (see Ivanova 2018). Other mechanisms such as dust formation (Glanz & Perets 2018;Iaconi et al 2020;Reichardt et al 2020), accretion onto the in-spiraling star (Chamandy et al 2018), and the formation of jets (Shiber et al 2019) have been proposed to aid envelope ejection.…”

Formation of sdB-stars via common envelope ejection by substellar companions

“…Despite recent progress in numerical techniques and available computational resources, a fundamental question of CEE remains unanswered: How is the envelope ejected? If driven by the release of orbital energy only, the ejection remains incomplete in all published CE simulations (for instance Ricker & Taam 2012;Passy et al 2012;Ohlmann et al 2016a;Reichardt et al 2020;Sand et al 2020). Additional physics seems to be required for a successful envelope removal.…”

“…The ionization energy stored in the envelope will be released by recombination processes provided that the material expands sufficiently. If thermalized locally, this energy leads to further unbinding of material (Nandez et al 2015;Nandez & Ivanova 2016;Prust & Chang 2019;Reichardt et al 2020) and a complete envelope ejection seems possible.…”