The formation of long-period eccentric binaries with a helium white dwarf

TL;DR

This paper proposes an evolutionary model explaining the high eccentricity of long-period binaries with helium white dwarfs, emphasizing the role of enhanced stellar wind mass loss on the red giant branch.

Contribution

It introduces a new evolutionary scenario involving enhanced wind mass loss to explain observed eccentricities in long-period binary systems with He-WDs.

Findings

Eccentricity can be preserved or increased with larger initial separations.

Enhanced wind mass loss prevents Roche-lobe overflow and circularization.

The model matches observed properties of the IP Eri system.

Abstract

The recent discovery of long-period eccentric binaries hosting a He-WD or a sdB star has been challenging binary-star modelling. Based on accurate determinations of the stellar and orbital parameters for IP Eri, a K0 + He-WD system, we propose an evolutionary path that is able to explain the observational properties of this system and, in particular, to account for its high eccentricity (0.25). Our scenario invokes an enhanced-wind mass loss on the first red giant branch (RGB) in order to avoid mass transfer by Roche-lobe overflow, where tides systematically circularize the orbit. We explore how the evolution of the orbital parameters depends on the initial conditions and show that eccentricity can be preserved and even increased if the initial separation is large enough. The low spin velocity of the K0 giant implies that accretion of angular momentum from a (tidally-enhanced) RGB wind should not be efficient.