Stencel, Robert

Contact Email: rstencel@phoenix.phys.du.edu
Institute: University of Denver Observatories
First Coauthor: A.Phillips, T.Ostrowski, C.Jurgenson
Institute: University of Denver Observatories
Research URL:http://www.du.edu/~rstencel
Subject Area: Stellar Atmospheres, Circumstellar Material
Waveband: Optical
Technique: Photometry
Presentation: Poster Display
Title: Microvariability among Selected Long Period Variable Stars
Abstract: DeLaverny et al. (1998) reported short-term brightness variations in 15 percent of the 250 Long Period Variable (MIRA/AGB) stars surveyed using the HIPPARCOS satellite, with the broadband 340 to 890 nm Hp filter. The abrupt variations ranged 0.2 to 1.1 magnitudes, on timescales between 2 to 100 hours, with a preponderance found nearer Mira minimum light phases. However, the sampling frequency was extremely sparse and requires confirmation because of potentially important dust-formation physics that can be revealed. We report here ground-based photometric observations of several of these objects that support and tend to confirm, the deLaverny et al. findings. According to deLaverny et al., majority of the significant fluctuations (0.2 mag or more) occurred when the variables were at or near minimum light. Their limited sampling suggested changes on timescales of hours and days. In our observations, four out of five Miras sampled were found to have significant fluctuations over these same short time-scales (at 95 confidence level and higher), based on analysis of photometric variance, and F-tests. Interpretation of this microvariation is offered in terms of dust formation episodes in the upper atmosphere of the star, and tests using interferometric visibility curves are proposed.