THE ROTATION OF RR LYRAE STARS

We report upper limits to rotation from the measurement of line breadt hs via cross-correlation analysis for 27 RR Lyrae variables. The eight best-observed stars of type RRab show the same variation of breadth w ith phase: the breadth peaks sharply during the rise to maximum light, drops gradually, and reaches a broad minimum during the phase of maxi mum radius. During this phase the breadth is always narrow, consistent with instrumental resolution and turbulence alone. For the three well -observed RRc variables, the breadth is this narrow at all phases exce pt for a slight increase during the rise to maximum light. The remaini ng stars also conform to these patterns, albeit with sparse phase cove rage and lower signal-to-noise ratio. We interpret these results as in dicating that lines are broadened in RRab stars by shock-induced plume s or turbulence during the rise to maximum light, and perhaps by other causes as well, but not by rotation in RR Lyrae stars of either type. We estimate an upper limit of nu sin i < 10 km s(-1) in all cases. Th is is in stark contrast to the rotation seen in field blue horizontal branch stars, where nu sin i > 10 km s(-1) in three out of six well-st udied field stars, and to the more rapid rotation, often exceeding 100 km s(-1), of the Population I delta Scuti variables which occupy adja cent regions of the instability strip.