Induced differential rotation and mixing in asynchronous binary stars

¹ Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Ave. Universidad s/n, Chamilpa, Cuernavaca, Mexico
e-mail: gloria@astro.unam.mx
² Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70-264, Ciudad de México 04510, Mexico
e-mail: edmundo@astro.unam.mx
³ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: nlanger@astro.uni-bonn.de

Received: 8 September 2020
Accepted: 28 June 2021

Abstract

Context. Rotation contributes to internal mixing processes and observed variability in massive stars. A significant number of binary stars are not in strict synchronous rotation, including all eccentric systems. This leads to a tidally induced and time-variable differential rotation structure.

Aims. We present a method for exploring the rotation structure of asynchronously rotating binary stars.

Methods. The method consists of solving the equations of motion of a 3D grid of volume elements located above the rigidly rotating core of a binary star in the presence of gravitational, centrifugal, Coriolis, gas pressure and viscous forces to obtain the angular velocity as a function of the three spatial coordinates and time. The method is illustrated for a short period massive binary in a circular orbit and in an eccentric orbit.

Results. We find that for a fixed set of stellar and orbital parameters, the induced rotation structure and its temporal variability depend on the degree of departure from synchronicity. In eccentric systems, the structure changes over the orbital cycle with maximum amplitudes occurring potentially at orbital phases other than periastron passage. We discuss the possible role of the time-dependent tidal flows in enhancing the mixing efficiency and speculate that, in this context, slowly rotating asynchronous binaries could have more efficient mixing than the analogous more rapidly rotating but tidally locked systems. We find that some observed nitrogen abundances depend on the orbital inclination, which, if real, would imply an inhomogeneous chemical distribution over the stellar surface or that tidally induced spectral line variability, which is strongest near the equator, affects the abundance determinations. Our models predict that, neglecting other angular momentum transfer mechanisms, a pronounced initial differential rotation structure converges toward average uniform rotation on the viscous timescale.

Conclusions. A broader perspective of binary star structure, evolution and variability can be gleaned by taking into account the processes that are triggered by asynchronous rotation.