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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 Teff ≃ 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.