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The mass of the neutron star in Vela X-1 has been found to be more massivethan the canonical 1.5 Mo. This result relies on the assumption that theamplitude of the optical component's measured radial velocity curve is notseriously affected by the interactions in the system. In this paper we explorethe effect on the radial velocity curve caused by surface motions excited bytidal interactions. We use a calculation from first principles that involvessolving the equations of motion of a Lagrangian grid of surface elements. Thevelocities on the visible surface of the star are projected along theline-of-sight to the observer to obtain the absorption-line profile in theobserver's reference frame. The centroid of the line-profiles for differentorbital phases is then measured and a simulated RV curve constructed. Modelsare run for the "standard" (vsini=116 km/s) and "slow" (56 km/s) supergiantrotation velocities. We find that the surface velocity field is complex andincludes fast, small-spatial scale structures. It leads to strong variabilityin the photospheric line profiles which, in turn, causes significant deviationsfrom a Keplerian RV curve. The peak-to-peak amplitudes of model RV curves arein all cases larger than the amplitude of the orbital motion. Keplerian fits toRV curves obtained with the "standard" rotation velocity imply a neutron star>1.7 Mo. However, a similar analysis of the "slow" rotational velocity modelsallows for m_ns ~ 1.5 Mo. Thus, the stellar rotation plays an important role indetermining the characteristics of the perturbed RV curve. Given theobservational uncertainty in GP Vel's projected rotation velocity and thestrong perturbations seen in the published and the model RV curves, we areunable to rule out a small (~1.5 Mo) mass for the neutron star companion.

Tidal effects on the radial velocity curve of HD77581 (Vela X-1)