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 12 and 14 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 Boo as members. An understanding of their variations may have significant ramifications for theories of stellar structure and stellar evolution.