In this first catalog we present 20 variable guide stars discovered in the HST-FGS archive with large to sub-mmag amplitudes. This group is a small subset of a potentially much larger set of variables, because it is restricted to those stars for which spectral types from the SIMBAD archive and/or B-V values from the TYCHO catalog are available.
Our current program consists of a survey for microvariability among guide
stars with an accuracy of typically
and in the best cases
even of less than 50 ppm. A severe limitation for a sound
statistical analysis of the occurence of variability in the HR
diagram is the small number of guide stars with known spectral
classification; this is caused by the relative faintness of the guide stars
used by HST. Intermediate band photometric data (e.g., Strömgren,
Geneva), that would allow to estimate 
and
,
are very rare
for such faint stars and we therefore had to arrange for dedicated
Strömgren photometry at ESO, as included in this paper.
Future activities will be devoted to better modeling of the photometric properties of HST, in particular with regard to stray light effects, which cause intensity variations modulated by the spacecraft orbit. Such an improved model will allow us to reduce the number of stars classified as "o'' considerably. Furthermore, data sets which presently are classified as "t'' will also be explored in more detail in order to separate intrinsic long term variability from instrumental or guiding effects. In summary, we expect the percentage of variable stars discovered in our survey to increase.
Currently, several space missions are in preparation, that are dedicated to high-precision photometry with the goal to apply asteroseismic techniques to a significant sample of stars. The HST-FGS data provide an excellent testing ground for modelling the photometric characteristics of such space experiments. Our extensive usage of the FGS archive (Kuschnig et al. 1997; Weiss et al. 1999; Zwintz et al. 1999, and this paper) is motivated in part by this argument.
Follow-up ground-based spectroscopic and/or photometric observations of guide stars are definitely needed and will provide essential information for better and more complete astrophysical interpretation of the effects seen in the data.
Acknowledgements
The authors thank Ed Nelan of the Space Telescope Science Institute for enlightening discussions about the technical intricacies of the FGSs. In addition, we would like to thank Martin Sperl for providing us with his program Period98. This project received funding from the Österreichische Nationalbank (project 8014) and the Fonds zur Förderung der wissenschaftlichen Forschung (project S-7303 AST).
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