ASTEROSEISMOLOGICAL OBSERVATIONS OF THE CENTRAL STAR OF THE PLANETARY-NEBULA NGC-1501

We report on a global CCD time-series photometric campaign to decode t he pulsations of the nucleus of the planetary nebula NGC 1501. The WC4 central star is an extremely hot, hydrogen-deficient, ''O VI''-type o bject, with some spectroscopic characteristics similar to those of the pre-white-dwarf PG 1159-035 stars. NGC 1501 shows pulsational brightn ess variations of a few percent with numerous individual periods rangi ng from 19 to 87 minutes. The pulsation amplitudes and periods are hig hly variable, suggesting a complex pulsation spectrum that requires a long unbroken time series to resolve. To that end, we obtained CCD pho tometry of the central star over a two-week period in 1991 November, u sing a network of observatories around the globe. We obtained nearly c ontinuous coverage over an interval of almost one week in the middle o f the run. With this data set, we have identified ten independent puls ation periods, ranging from 5235 down to 1154 s. The pulsation modes c hanged amplitude significantly during the course of the run, indicatin g either real amplitude variations, or that the modes are not fully re solved over the two-week interval. We find strong evidence that the mo des we see in this star are indeed nonradial g-modes. The ratios of th e frequencies of the largest-amplitude modes agree closely with those expected for modes that are trapped by a density discontinuity in the outer layers. This conclusion is strengthened by including single-site observations of this star, obtained during previous years, in our ana lysis. We offer a model for the pulsation spectrum that includes a com mon period spacing of 22.30 s and a stellar rotation period of 1.17 da ys; the period spacing allows us to assign a preliminary asteroseismol ogical mass of 0.55+/-0.03M.. However, several factors complicate the analysis. Aside from the proximity of the rotational splitting to 1 cy cle per day, this frequency splitting corresponds closely to period sp acings near 20 seconds near the dominant frequencies of the star, Thus , the period spacing and frequency spacings are nearly degenerate. (C) 1996 American Astronomical Society.