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

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Abstract
We have determined the projected rotational velocities of thirteen 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 ~= 11000 K, and reveals a pronounced difference in stellar rotation on either side of this feature---bluewards of the gap, all the stars show modest rotations, vsini < 10 km/s, while to the red side of the gap, we confirm the more rapidly rotating population (vsini ~= 40 km/s) previously observed by Peterson et al (1995). 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.Comment: 7 pages, 3 figures, accepted in Ap
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