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Photometry of Hipparcos Variable Stars
Among the discoveries reported by the Hipparcos team was the detectionof short-term photometric fluctuations in several long period, or Mira,variables (de Laverny et al. 1998). Nearly 15 percent of the 250 Mirassurveyed in the broadband 380 to 800 nm filter showed variation of 0.2to 1.1 magnitudes on timescales of 2 to 100 hours, preferentially aroundminimum light phases. We have begun an observational effort to confirmthese variations. We seek to correlate the behavior with ancillaryinformation, such as optical and infrared spectra plus maser data, todetermine whether the fluctuations can be understood as thermalinstabilities or so-called molecular catastrophe (cf. Muchmore et al1987; Stencel et al. 1990). A progress report and call for observationcoordination are given.
| Detection of short-term variations in Mira-type variables from HIPPARCOS photometry
The Hipparcos photometric observations carried out over 37 months havebeen investigated in detail for ~ 250 Miras. This leads to an unexpectedby-product of the mission that we already suspected from ESOC real-timereductions: the first detection from space of photometric short-termbrightness variations in Mira-type variables. Altogether, 51 events in39 M-type Miras are detected and no similar variations are found for Sand C-type Miras. Their amplitude ranges from 0.23 mag up to 1.11 magand their duration extends from 2 hours up to almost 6 days. Theseevents seem to occur preferentially in late spectral types. We suggestthat they might be related to molecular opacity effects. Based on datafrom the \HIP astrometry ESA satellite
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