Counteracting Tidal Circularization with the Grazing Envelope Evolution

TL;DR

This paper demonstrates that enhanced mass loss during periastron in grazing envelope evolution can counteract tidal circularization, maintaining high eccentricity in AGB binary systems and explaining observed eccentricities in post-AGB binaries.

Contribution

It introduces a model where increased mass loss at periastron offsets tidal effects, allowing high eccentricity to persist in evolving AGB binary systems.

Findings

High eccentricity can be maintained in AGB binaries through enhanced periastron mass loss.

Grazing envelope evolution can explain the eccentricities of some post-AGB intermediate binaries.

Mass accretion onto the secondary energizes outflows, supporting high eccentricity.

Abstract

We show that substantially enhanced mass loss at periastron passages, as is expected in the grazing envelope evolution (GEE), can compensate for the circularization effect of the tidal interaction in binary systems composed of an asymptotic giant branch (AGB) star and a main sequence secondary star. By numerically integrating the equations of motion we show that under our assumptions the binary system can maintain its high eccentricity as the AGB star evolves toward the post-AGB phase. Our results can explain the high eccentricity of some post-AGB intermediate binaries (post-AGBIBs), i.e., those with an orbital periods in the range of several months to few years. In the framework of the GEE, the extra energy to sustain a high mass loss rate comes from the accretion of mass from the giant envelope or its slow wind onto a more compact secondary star. The secondary star energizes the outflow from the AGB outer envelope by launching jets from the accretion disk.