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Astron. Astrophys. 343, 899-903 (1999)

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5. Conclusions

During the Hubble Deep Field program two guide stars were observed continuously with Fine Guidance Sensors 2 and 3 over a period of 10 days. A correction for enhanced background radiation due to the South Atlantic Anomaly and for stray light due to the illuminated Earth beneath HST resulted in photometric time series which are essentially photon noise limited. For frequencies larger than about 0.1 mHz only the two effects mentioned above influence the FGS photometry and the instrument itself proved to be very stable.

The spectral types of the two guide stars were determined to be G2mG0 IV and K1 V and the noise level in the amplitude spectra found to be 23 ppm and 47 ppm, respectively. According to computations of luminosity amplitudes for solar-type pulsation (Houdek 1997), the expected maximum amplitude for the G-type star (GS-54) should be between 10 and up to 80 ppm (Fig. 6) with an estimated period for the maximum amplitude of about 7 to 30 minutes, using the scaling relation of Kjeldsen & Bedding (1995). No increase of amplitudes exceeding the noise level of 23 ppm is present in the frequency range of interest (Fig. 4), what might indicate a possible overestimation of the luminosity variations computed by Houdek, a fact which may be important for planning future ground based observing campaigns as well as for selecting targets for space experiments.

[FIGURE] Fig. 6. HR-diagram and evolution tracks (Gimenez 1995) of 1 to 2 solar mass stars. The box centered on GS-54 corresponds to the uncertainties of the respective quantities derived from spectroscopy. The cool observed border of the classical instability strip is given as well as lines of constant estimated luminosity amplitudes (Houdek 1997) and of extrapolated period of maximum pulsation amplitude (Kjeldsen & Bedding 1995).

In spite of the lack of an unambiguous pulsation, the data themselves are of extremely high quality, demonstrating the potential of the Hubble Space Telescope Fine Guidance Sensors to provide observations which are well suited for asteroseismic investigations. Considering the importance of a clear detection of solar type pulsation for the first time also in stars other than our Sun, it would be highly desirable to select guide stars accordingly. Slightly evolved bright late F-type stars are prime targets for this project.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999