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

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1. Introduction

With the success of helioseismology in testing solar models and in investigating hitherto speculative issues, such as internal solar rotation, observational evidence for stochastically driven oscillation in stars other than our Sun has gained increasing interest. So far, results have been disappointing (Kjeldsen & Bedding 1995) and controversial (for example: [FORMULA] Cen, Procyon, [FORMULA] Boo).

The question of how solar luminosity variations scale to stars with different [FORMULA] and log g is important when selecting target stars for expensive observing programs and it touches on theoretical areas, such as convection, that are only poorly understood. The most recent investigation of this scaling problem (Houdek 1997) predicts luminosity amplitudes of up to 80 ppm for stars evolved from the ZAMS and close to the cool border of the classical instability strip.

Low-noise photometric time series are needed to detect the largest amplitude frequencies and even more so to determine the characteristic frequency spacings which are important diagnostic tools in asteroseismology. Space-borne experiments, taking advantage of the lack of atmospheric noise, can provide such high quality data. After two ESA Phase-A studies (PRISMA and STARS), the CNES-lead experiment COROT is an approved space experiment devoted to asteroseismology (and the detection of exoplanets), after its smaller precursor, EVRIS, was a victim of the MARS-96 disaster. Other similar projects, such as MONS (Denmark) and MOST (Canada), hopefully will receive funding.

The High Speed Photometer of the Hubble Space Telescope (HST) could have been used to detect solar-type pulsation in stars, but not surprisingly, the large amount of HST time needed for the project to observe even very bright stars has not been granted. However, HST Fine Guidance Sensors (FGS) can contribute to asteroseismology of solar type stars (Kuschnig et al. 1997, referenced in the following as Paper I). To reduce the enormous amount of photometric data points we average the counts typically over 10 sec. As only half of the actually executed FGS integrations (40 samples per second with 25 msec integration time each) are recorded in the telemetry stream which we are forced to use, every other sample is lost for our analysis and consequently the associated S/N ratio is that one would get from a telescope with a collecting area equivalent to a 1.7m primary mirror.

A special observing program was performed in December 1995 with the HST called the "Hubble Deep Field" (HDF) program. A region in the Northern Continuous Viewing Zone was observed for ten consecutive days. This field is located high above the galactic plane at a right ascension of [FORMULA] and a declination of [FORMULA] (J2000.0) and contains no bright stars or other previously studied objects. Therefore it is one of the "darkest" regions in the sky.

During this period the HST was guided with two of the three Fine Guidance Sensors, and hence photometric time series of the two Guide Stars, GS0416200054 (GS-54, using FGS 3) and GS0416200075 (GS-75, using FGS 2), were obtained. Their position is given in Table 1 as well as their V magnitude. With 10 days of nearly continuous data it is possible to investigate systematic effects occuring in the FGS photometry and to reach a very low noise level in the frequency spectra.


[TABLE]

Table 1. Position (Epoch J2000) and brightness of the two HDF guide stars (Guide Star Catalog 1992) and their spectral type


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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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