Testing eccentricity pumping mechanisms to model eccentric long period sdB binaries with MESA

Abstract

Hot subdwarf-B stars in long-period binaries are found to be on eccentric orbits, even though current binary-evolution theory predicts those objects to be circularised before the onset of Roche-lobe overflow (RLOF). We aim to find binary-evolution mechanisms that can explain these eccentric long-period orbits, and reproduce the currently observed period-eccentricity diagram. Three different processes are considered; tidally-enhanced wind mass-loss, phase-dependent RLOF on eccentric orbits and the interaction between a circumbinary disk and the binary. The binary module of the stellar-evolution code MESA (Modules for Experiments in Stellar Astrophysics) is extended to include the eccentricity-pumping processes. The effects of different input parameters on the final period and eccentricity of a binary-evolution model are tested with MESA. The end products of models with only tidally-enhanced wind mass-loss can indeed be eccentric, but these models need to lose too much mass, and invariably end up with a helium white dwarf that is too light to ignite helium. Within the tested parameter space, no sdBs in eccentric systems are formed. Phase-dependent RLOF can reintroduce eccentricity during RLOF, and could help to populate the short-period part of the period-eccentricity diagram. When phase-dependent RLOF is combined with eccentricity pumping via a circumbinary disk, the higher eccentricities can be reached as well. A remaining problem is that these models favour a distribution of higher eccentricities at lower periods, while the observed systems show the opposite. The models presented here are potentially capable of explaining the period-eccentricity distribution of long-period sdB binaries, but further theoretical work on the physical mechanisms is necessary.

18 pages, 9 figures, accepted for publication in A&A

References in corpus (13)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Keplerian discs around post-AGB stars: a common phenomenon?
• Optically visible post-AGB/RGB stars and young stellar objects in the Small Magellanic Cloud: candidate selection, spectral energy distributions and spectroscopic examination
• Orbital eccentricities of binary systems with a former AGB star
• Close stellar binary systems by grazing envelope evolution
• SPITZER survey of dust grain processing in stable discs around binary post-AGB stars
• The orbits of subdwarf B + main-sequence binaries. I: The sdB+G0 system PG 1104+243
• The nebula around the post-AGB star 89 Her
• Detection of Keplerian dynamics in a disk around the post-AGB star AC Her
• The evolved circumbinary disk of AC Her: a radiative transfer, interferometric and mineralogical study
• KIC 1718290: A Helium-rich V1093-Her-like Pulsator on the Blue Horizontal Branch
• An interferometric study of the post-AGB binary 89 Herculis. II Radiative transfer models of the circumbinary disk
• The formation of long-period eccentric binaries with a helium white dwarf

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