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R.E. Stencel, A. Phillips, C. Jurgenson, T. Ostrowski-Fukuda (U of Denver Observatories)
De Laverny et al. (1998 A& A) reported short-term brightness variations in 15 percent of the 250 Mira or Long Period Variable 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 time-scales 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 de Laverny et al. findings. Initially, our observations indicated that four out of five Miras sampled (XZ Her, HO Lyr, AU Cyg and AM Cyg) were found to have low level but significant fluctuations over these same short time-scales (at 95% confidence level and higher), based on analysis of photometric variance, and F-tests (see Stencel et al. 2001 in Proceedings of Cool Stars 12). However, more careful observations at Mt. Evans observatory during summer 2001 failed to detect comparable behavior in similar stars near minimum light. Further observations are needed to confirm and delineate this effect among long period variables. Interpretation of micro-variation is offered in terms of dust formation episodes in the upper atmosphere of the star, and tests using interferometric visibility curves are proposed. We are pleased to acknowledge support for this work from the estate of William Herschel Womble and the University of Denver.
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The author(s) of this abstract have provided an email address for comments about the abstract: rstencel@phoenix.phys.du.edu