Close-binary evolution

Abstract

Context. Tides are known to play an important role in binary evolution, leading in particular to synchronization of axial and orbital rotations and to binary mass transfer. Aims. We study how tides in a binary system induce some specific internal shear mixing that can substantially modify the evolution of close binaries prior to mass transfer. Methods. We constructed numerical models accounting for tidal interactions, meridional circulation, transport of angular momentum, shears and horizontal turbulence. Furthermore, we considered a variety of orbital periods and initial rotation velocities. Results. Depending on orbital periods and rotation velocities, tidal effects may spin down (spin-down case) or spin up (spin-up case) the axial rotation. In both cases, tides may induce a high internal differential rotation. The resulting tidally induced shear mixing is so efficient that the internal distributions of angular velocity and chemical elements are highly influenced. The evolutionary tracks are modified, and in for spin down as well as for spin up, large amounts of nitrogen can be transported to the stellar surfaces before any binary mass transfer. Meridional circulation, when properly treated as a advection, always tends to counteract the tidal interaction, tending to spin up the surface when it is braked down and vice versa. As a consequence, the times needed for the axial angular velocity to become equal to the orbital angular velocity may be longer than given by typical synchronization timescales. Moreover, because of the meridional circulation some differential rotation remains in tidally locked binary systems.

Chemical abundances of fast-rotating massive stars

“…Such a fast rotation can be produced by several mechanisms: it can be acquired at birth as a result of their formation or develop subsequently during their evolution as they interact with a companion (through tidal forces, mass accretion, or even merging ;Zahn 1975;Hut 1981;Packet 1981;Pols et al 1991;Podsiadlowski et al 1992;Langer et al 2003;Petrovic et al 2005a,b;de Mink et al 2009de Mink et al , 2013Dervişoǧlu et al 2010;Tylenda et al 2011;Song et al 2013). …”

Evolution and Nucleosynthesis of Massive Stars

“…• When braked at the surface either through a process of wind magnetic braking [38], by tidal interaction with a companion [44], or by coupling with a disk during for instance the pre-MS phase [13], these models show low surface rotations together with strong signs of mixing at their surface (e.g., during the main-sequence phase, increased nitrogen abundance and decreased carbon abundance). Stronger mixing is obtained because in these models shear is the main mechanism The cyan and magenta hatched regions correspond to the wind ejected masses when the star is respectively a WN and a WC star.…”

“…2) Such models are able to explain the occurrence of observed non-evolved, nitrogen rich and slowly rotating stars (group 2 stars in the Hunter diagram) in case these stars would have been braked down either by wind magnetic braking or by tidal interactions [38,44].…”

Project VeSElkA: Vertical Stratification of Element Abundances in CP stars

“…Special attention will be paid to the CP stars in binary systems, as due to the tidal interaction its members usually have a slow axial rotation [1,59], which can lead to a hydrodynamically stable atmosphere. Fig.…”