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Astron. Astrophys. 356, 585-589 (2000)

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3. High speed photometry

Recently, multi-periodic pulsations have been discovered in a number of subdwarf sdB stars (the EC14026 stars, Kilkenny et al., 1997). Both radial and non-radial modes are present, although the cause of these pulsations is not fully understood. Theoretical studies have shown that these oscillations may be excited by an opacity bump due to heavy element ionization, giving rise to a metal-enrichment in this driving region (Charpinet et al., 1996). However, why pulsations are observed in some sdBs and not in others remains a mystery.

We observed the SM Star on 1999 September 4th with the South African Astronomical Observatory's 0.75m telescope, together with the University of Cape Town's CCD photometer in high speed mode, in order to search for pulsations. A [FORMULA]2600 second light curve was obtained, consisting of 20 second exposures separated by essentially zero seconds of dead time. Four comparison stars were also observed at the same time. The differential light curve is shown in Fig. 2. The SM star (star #8 in Fig. 2) shows no evidence of pulsations; the fluctuations in Fig. 2 are merely random noise. The amplitude spectrum (Fig. 3), which has been calculated out to the Nyquist frequency, also shows no evidence for pulsations. However, at V[FORMULA]16.7 we are clearly unable to detect fluctuations below [FORMULA]0.05 mags. with this telescope. Many of the known sdB pulsators vary at the level of 0.001-0.05 mags., and so clearly we cannot rule out low level pulsations in this object. We suggest that it should be re-observed on a larger telescope.

[FIGURE] Fig. 2. Differential light curve for the SM Star (#8) and four comparison stars in the same field.

[FIGURE] Fig. 3. Amplitude spectrum determined from the SM Star's light curve.

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© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000