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 thirteen bluehorizontal-branch (BHB) stars in the globular cluster M13 via rotationalbroadening of metal absorption lines. Our sample spans the photometric gapobserved in the horizontal branch distribution at Teff ~= 11000 K, and revealsa pronounced difference in stellar rotation on either side of thisfeature---bluewards of the gap, all the stars show modest rotations, vsini < 10km/s, while to the red side of the gap, we confirm the more rapidly rotatingpopulation (vsini ~= 40 km/s) previously observed by Peterson et al (1995).Taken together with these prior results, our measurements indicate that astar's rotation is indeed related to its location along the HB, although themechanism behind this correlation remains unknown. We explore possibleconnections between stellar rotation and mass loss mechanisms which influencethe photometric morphology of globular cluster HBs.Comment: 7 pages, 3 figures, accepted in Ap