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Variable HST guide stars (I)
We have performed time series analyses on more than 4500 sets of HubbleSpace Telescope (HST) Fine Guidance Sensor (FGS) data, obtained fromabout 3600 guide stars in the period from August 1992 to December 1995.We discuss here a subset of 20 stars that were found to be variable at ahigh confidence level. This subset contains all definite variables inour sample for which at least spectral type, B-V, or Strömgrencolors, respectively, are available. Our variability survey ischaracterized by comparably high photometric accuracy and high timeresolution. The time base of a data set for a given guide star can rangefrom half an hour to several days and even up to years in some cases.The elimination of scintillation noise in a space-borne experiment andthe usage of the identical instrument throughout the survey results invery homogeneous data with a typical photometric precision of about 500ppm, and with a best case of 50 ppm. Half of the cases discussed hereare probably delta Sct variables, and two are eclipsing binaries. Wehave also found four K-type stars with a photometric period of a fewhours, which is difficult to interpret as rotation. Currently, severalspace missions are in preparation, dedicated to high precisionphotometry with the goal of applying asteroseismic techniques to asignificant sample of stars. The HST-FGS data provide an excellenttesting ground for modelling the photometric characteristics of suchspace experiments and for gaining experience in handling the issuesrelated to the respective data and algorithms. Based on observationsmade with the NASA/ESA Hubble Space Telescope, obtained from the dataarchive at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.
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