A New Spin on the Problem of Horizontal‐Branch Gaps: Stellar Rotation along the Blue Horizontal Branch of Globular Cluster M13

We have determined the projected rotational velocities of 13 blue horizontal-branch (BHB) stars in the globular cluster M13 via rotational broadening of metal absorption lines. Our sample spans the photometric gap observed in the horizontal-branch distribution at T eff ≃ 11,000 K and reveals a pronounced difference in stellar rotation on either side of this feature—blueward of the gap, all the stars show modest rotations, v sin i < 10 km s -1 , while to the red side of the gap, we confirm the more rapidly rotating population ( v sin i ≃ 40 km s -1 ) previously observed by R. C. Peterson and coworkers. Taken together with these prior results, our measurements indicate that a star's rotation is indeed related to its location along the HB, although the mechanism behind this correlation remains unknown. We explore possible connections between stellar rotation and mass-loss mechanisms which influence the photometric morphology of globular cluster HBs.