Star-planet interactions II. Is planet engulfment the origin of fast rotating red giants?

Context. Fast rotating red giants in the upper part of the red giant branch have surface velocities that cannot be explained by single star evolution. Aims. We check whether tides between a star and a planet followed by planet engulfment can indeed accelerate the surface rotation of red giants for a sufficiently long time to produce these fast rotating red giants. Methods. We studied how the surface rotation velocity at the stellar surface evolves using rotating stellar models, accounting for the redistribution of the angular momentum inside the star by different transport mechanisms, the exchanges of angular momentum between the planet orbit and the star before the engulfment, and for the deposition of angular momentum inside the star at the engulfment. We considered different situations with masses of stars in the range between 1.5 and 2.5 M-circle dot, masses of the planets between 1 and 15 M-J (Jupiter mass), and initial semimajor axis between 0.5 and 1.5 au. The metallicity Z for our stellar models is 0.02. Results. We show that the surface velocities reached at the end of the orbital decay due to tidal forces and planet engulfment can be similar to values observed for fast rotating red giants. This surface velocity then decreases when the star evolves along the red giant branch but at a sufficiently slow pace to allowing stars to be detected with such a high velocity. More quantitatively, star-planet interaction can produce a rapid acceleration of the surface of the star, above values equal to 8 km s(-1), for periods lasting up to more than 30% the red giant branch phase. As found already by previous works, the changes of the surface carbon isotopic ratios produced by the dilution of the planetary material into the convective envelope is modest. The increase of the lithium abundance due to this effect might be much more important, however lithium may be affected by many different, still uncertain, processes. Thus any lithium measurement can hardly be taken as a support or argument against any star-planet interaction. Conclusions. The acceleration of the stellar surface to rotation velocities above limits that depend on the surface gravity does appear at the moment to be the clearest signature of a star-planet interaction.