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

TL;DR

This paper investigates how low-mass substellar companions can eject the envelope of red giant stars during common envelope phases, leading to the formation of hot subdwarf B stars, using hydrodynamical simulations and a semi-analytic model.

Contribution

It demonstrates that low-mass companions, including brown dwarfs, can successfully unbind stellar envelopes if recombination energy is considered, and proposes a new criterion for the minimum companion mass needed.

Findings

Envelope ejection possible with brown dwarf companions when recombination energy is included.

Envelope removal becomes difficult for companions approaching planetary masses.

Results align with observational data on sdB star companions.

Abstract

Common envelope (CE) phases in binary systems where the primary star reaches the tip of the red giant branch are discussed as a formation scenario for hot subluminous B-type (sdB) stars. For some of these objects, observations point to very low-mass companions. In hydrodynamical CE simulations with the moving-mesh code AREPO, we test whether low-mass objects can successfully unbind the envelope. The success of envelope removal in our simulations critically depends on whether or not the ionization energy released by recombination processes in the expanding material is taken into account. If this energy is thermalized locally, envelope ejection eventually leading to the formation of an sdB star is possible with companion masses down to the brown dwarf range. For even lower companion masses approaching the regime of giant planets, however, envelope removal becomes increasingly difficult or impossible to achieve. Our results are consistent with current observational constraints on companion masses of sdB stars. Based on a semianalytic model, we suggest a new criterion for the lowest companion mass that is capable of triggering a dynamical response of the primary star thus potentially facilitating the ejection of a common envelope. This gives an estimate consistent with the findings of our hydrodynamical simulations.