AAS 205th Meeting, 9-13 January 2005
Session 54 Pulsating Stars: RR Lyraes, Miras, Cepheids, etc.
Poster, Tuesday, January 11, 2005, 9:20am-6:30pm, Exhibit Hall

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

I.U. Roederer, K.J. Mighell (NOAO)

We present the analysis of two epochs of archived {\sl{HST}} WFPC2 observations of the Ursa Minor dwarf spheroidal galaxy. Reductions were performed using the {\sl{HSTphot}} stellar photometry package. We report the detection of nine faint (MV\,\gtrsim\,0.0 mag) variable stars on the red giant branch which exhibit low-amplitude brightness fluctuations on ten-minute timescales with amplitudes ranging from 36 to 130 mmags. These variables account for 14% of all red giants that were observed. The timing of the observations does not yield sufficient information to determine whether the variability is periodic. If low-amplitude variability of red giants on ten-minute timescales can be verified and should their numbers prove to be \geq\,10% of all red giants in some ancient Population II stellar systems, then the length of star formation bursts in nearby dwarf spheroidal systems could be overestimated in short snapshot observations due to color-spreads caused by flickering red giants. I.U.R. was supported by the NOAO/KPNO Research Experiences for Undergraduates (REU) Program, which is funded by the National Science Foundation through Scientific Program Order No.\ 3 (AST-0243875) of the Cooperative Agreement No.\ AST-0132798 between the Association of Universities for Research in Astronomy (AURA) and the NSF.

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© 2004. The American Astronomical Society.