The rotation of horizontal-branch stars. I. Members of the field.

Multiple echelle spectra of eight field horizontal-branch stars at a resolution of 2.4 x 10 to the 4th are obtained. At least two stars exhibit intrinsically broadened lines. The line profiles of the broadest lined star are well resolved, and rotation is found unambiguously to be the cause of the broadening. It is pointed out that the inference that rotation is a rather common phenomenon in horizontal-branch stars has several ramifications for the normal stellar evolution of metal-poor stars. The rotation rates of the main-sequence stars implied if the angular momentum is primordial are discussed, together with the possibility that rotation was acquired during the giant-branch phase by angular momentum transfer from a companion.