ROTATIONAL AND RADIAL VELOCITIES FOR A SAMPLE OF 761 HIPPARCOS GIANTS AND THE ROLE OF BINARITY

We present rotational and radial velocities for a sample of 761 giants selected from the Hipparcos Catalogue to lie within 100 pc of the Sun. Our original goal was to examine stellar rotation in field giants using spectroscopic line broadening to look for evidence of excess rotation that could be attributed to planets that were engulfed as the parent stars expanded. Thus we were obliged to investigate other sources of line broadening, including tidal coupling in close binaries and macroturbulence. For all the binaries in our sample with periods shorter than 20 days the orbits have been circularized, while about half the orbits with periods in the range 20–100 days still show significant eccentricity. All our primaries in orbits shorter than 30 days show line broadening consistent with synchronized rotation, while about half the primaries with periods in the range 30–120 days are synchronized. To study the dependence of rotation on stellar evolution when tidal effects are not important, we used a subsample of single stars and members in wide binaries. We found evidence to suggest that the first dredge-up may play a role in speeding up the rotation of the observable outer layers of giants and that the rotational velocity of horizontal branch stars is larger by a few km s−1 than that of first-ascent giants with similar mass, effective temperature, and radius. Finally, we found three giants that rotate more rapidly than expected. We conjecture that they acquired their excess angular momentum by ingesting planets.