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

Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, ULB, CP. 226, Boulevard du Triomphe, 1050 Brussels, Belgium
e-mail: siess@astro.ulb.ac.be

Received: 10 January 2014
Accepted: 18 March 2014

Abstract

The recent discovery of long-period eccentric binaries hosting a He-white dwarf or a subdwarf star has been challenging binary-star modeling. 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.