K giants in 47 Tucanae: Detection of a new class of variable stars

We report the discovery of variability among K giants in the globular cluster 47 Tucanae, using a time series of U exposures with the Hubble Space Telescope. The variables lie along a narrow band in the color-magnitude diagram, joining the faint end of the asymptotic giant branch to the red giant branch. The variations, if coherent, mostly have periods between similar to 2 and similar to 4 days, consistent with low-overtone radial pulsation or nonradial pulsation, and V amplitudes in the range 5-15 mmag, which explains their nondetection so far in clusters. One of the variables may have a period of 1.1 days and a V amplitude of 5.3 mmag. These stars define a new class of variable stars and probably contain variable field K giants such as alpha Boo as members. An understanding of their variations may have significant ramifications for theories of stellar structure and stellar evolution.