We report on a global CCD time-series photometric campaign to decode the pulsations of the nucleus of the planetary nebula NGC 1501. The WC4 central star is an extremely hot, hydrogen-deficient, "O VI"-type object, with some spectroscopic characteristics similar to those of the pre-white-dwarf PG 1159−035 stars. NGC 1501 shows pulsational brightness variations of a few percent with numerous individual periods ranging from 19 to 87 minutes. The pulsation amplitudes and periods are highly variable, suggesting a complex pulsation spectrum that requires a long unbroken time series to resolve. To that end, we obtained CCD photometry of the central star over a two-week period in 1991 November, using a network of observatories around the globe. We obtained nearly continuous coverage over an interval of almost one week in the middle of the run. With this data set, we have identified 10 independent pulsation periods, ranging from 5235 s down to 1154 s. The pulsation modes changed amplitude significantly
Abstract. I discuss the advantages of CCD detectors for the type of science done with the Whole Earth Telescope. The advantages are illustrated by my experiences in a collaboration with R. Ciardullo involving photometric monitoring of planetary-nebula nuclei (PNNs). CCDs combine high quantum efficiency with the ability to measure the target, any surrounding nebulosity (or close companions), several comparison stars, and the sky background, all strictly simultaneously. One can therefore monitor faint stars under non-photometric conditions, in bright moonlight, and well into twilight; the total coverage possible in a photometric campaign is extended significantly, and the campaign can be done at full moon in order to compete more successfully for telescope time. Scientific results from our own collaboration include discoveries of close-binary PNNs, a new class of irregularly variable central stars, and seven new nonradially pulsating PNNs. To date we have organized multi-site asteroseismological CCD campaigns on two of the new nonradial pulsatore, NGC 1501 and Sand 3.
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