Line profile variability and tidal flows in eccentric binaries

A number of binary systems display enhanced activity around periastron passage which may be caused by the tidal interactions. We have developed a time-marching numerical calculation from first principles that computes the surface deformations, the perturbed velocity field, the energy dissipation rates and the photospheric line-profiles in a rotating star with a binary companion in an eccentric orbit. The method consists of solving the equations of motion for a grid of elements covering the surface of star nil, subjected to gravitational, centrifugal, Coriolis, gas pressure and viscous shear forces (Moreno et al. 1999, Toledano at al. 2007, Moreno at al. 2011). At selected times during the orbital cycle, the velocities of surface elements on the visible hemisphere of the star are projected along the observer's line of sight and the photospheric line-profile calculation is performed (Moreno at al. 2005). Direct comparison with observational photospheric line profile variability is then possible, showing that the general features are reproduced (Harrington at al. 2009). In this poster we show the example of a highly eccentric system (e = 0.8, P = 15 d). The surface deformation changes rapidly from that of an "equilibrium tide" at periastron to one with smaller-scale structure shortly thereafter. The computed line profiles display the presence of large blue-to-red migrating "bumps" around periastron, with smaller scale structure appearing later in the orbital cycle. Because the growth rate of the surface perturbations increases very abruptly at periastron, instabilities are expected to arise which may cause the observed activity and mass-ejection events around this orbital phase.