[Previous] | [Session 6] | [Next]
R.P. Stefanik (CfA), B.W. Carney (UNC), D.W. Latham (CfA), J.A. Morse (CASA), J.B. Laird (Bowling Green)
We have monitored the radial velocities of 93 metal-poor field red giants for intervals of up to 16 years. Fourteen stars have been found to be spectroscopic binaries. Velocity jitter is detected among half the giants with MV \leq\ -1.5. The two best-observed cases, HD~3008 and BD+22~2411, show apparent periodicities of 172 and 186 days, too long to be due to pulsation. Photometric variability seen in HD~3008 and two other stars showing velocity jitter suggests that starspots are the cause. The implied rotational velocities for HD~3008 and BD+22~2411, both with R \approx 50~R\odot, exceed 10~km/s. Eight of the sixteen red giants with MV \leq -1.5 have vrot~sin~i values of this size or larger. In some cases, BD+30~2034, CD-37~14010, and HD~218732, this probably arises from tidal locking between the axial rotation and the orbital motion with close stellar companions. But this cannot explain the other five stars, comprising roughly one-third of the luminous red giants in our sample. There appear to be too few stellar binary systems with short enough periods among field metal-poor main sequence stars to explain such a large frequency of high rotational velocities among field metal-poor luminous red giants. {\em Capture of a planetary-mass companion as a red giant expands in radius could explain the high rotational velocities, and perhaps the high rotational velocities seen in field and cluster blue horizontal branch stars.} We also find that four of the red horiozontal branch stars in our program show comparable rotational velocities. Transport of internal angular momentum to the surface appears unlikely since the high rotation does not appear at luminosities when the stellar convection zone is at its deepest.
We thank the National Science Foundation for support at UNC and BGSU.