Asteroseismological Observations of the Central Star of the Planetary Nebula NGC 1501

We report on a global CCD time-series photometric campaign to decode thepulsations of the nucleus of the planetary nebula NGC1501. The star is hot andhydrogen-deficient, similar to the pre-white-dwarf PG 1159 stars. NGC1501 showspulsational brightness variations of a few percent with periods ranging from 19to 87 minutes. The variations are very complex, suggesting a pulsation spectrumthat requires a long unbroken time series to resolve. Our CCD photometry of thestar covers a two-week period in 1991 November, and used a global network ofobservatories. We obtained nearly continuous coverage over an interval of oneweek in the middle of the run. We have identified 10 pulsation periods, rangingfrom 5235 s down to 1154 s. We find strong evidence that the modes are indeednonradial g-modes. The ratios of the frequencies of the largest-amplitude modesagree with those expected for modes that are trapped by a density discontinuityin the outer layers. We offer a model for the pulsation spectrum that includesa common period spacing of 22.3 s and a rotation period of 1.17 days; theperiod spacing allows us to assign a seismological mass of 0.55+/-0.03 Msun.Comment: 12 pages, AASTEX, 7 tables, 6 EPS figures, to appear in AJ, 12/96 Corrected version repairs table formatting and adds missing Table