Flickering Red Giants in the Ursa Minor Dwarf Spheroidal Galaxy: Detection of Low-Amplitude Variability in Faint Red Giant Branch Stars on 10 Minute Timescales

Abstract

We have analyzed two epochs of Hubble Space Telescope Wide Field Planetary Camera 2 observations of the Ursa Minor dwarf spheroidal galaxy using the HSTphot photometric reduction package. We report the detection of nine faint (MV 0.0 mag) red giant variable stars that exhibit low-amplitude brightness fluctuations on 10 minute timescales with amplitudes ranging from 36 to 130 mmag. We have found variability in 14% of the red giants we have observed. If low-amplitude variability of red giants on 10 minute timescales can be verified and should their numbers prove to be at the 10% level or greater of all red giants in some ancient Population II stellar systems, then the observed color spread of the red giant branch of such systems would be broadened by flickering red giants in color-magnitude diagrams based on short (snapshot) observations of a single pair of 10 minute timescale observations in two different filters.