Sanduleak 3
Sanduleak 3 (hereafter simply Sand 3) represents the rapid evolutionary stage between planetary nebula nuclei (PNNs) and hot pre-white dwarfs. It is one of the hottest known stars, showing strong emission lines of HeII, OVI through OVIII, and CIV and CV. Sand 3 is also a very complex multiperiodic nonradially pulsating star, with typical periods near 900 seconds (Bond, Ciardullo & Kawaler 1993). This time scale is intermediate between the pulsating PNNs and the GW Vir (pulsating PG1159) stars. In 1992, a global CCD network observed Sand 3 intensively for over 1 week. These data (currently in analysis by Kawaler, Bond, et al.) show that Sand 3 changed its pulsation spectrum significantly in just 2 years, and indeed changed during the course of the 1992 run. This complicates the analysis significantly compared to simpler pulsators, and the star is still not fully resolved.
This phenomenon is common amongst pulsating white dwarf stars. In most cases, the periods that are observed at any given season are consistent with a constant underlying mode spectrum. Data from several seasons can provide a large sample of modes. This ensemble approach allows the modes from various seasons to be used to decode the underlying spectrum. For Sand 3, the data already obtained have restricted the possible parameters of the pulsation spectrum, but a few new modes are needed to provide a unique determination of the period spacings and splittings.
Data from this run will be used to generate a Fourier power spectrum and to identify as many individual frequencies as possible. These frequencies will be used, along with those found in prior observing campaigns on this star, to attempt to identify non-radial g-modes of consecutive radial overtone and the same value of l. Such modes should show a nearly uniform spacing in period (Kawaler and Bradley 1994). The period spacing is most sensitive to the mass of the star for hot white dwarfs, so that identification of an underlying uniform period interval allows accurate determination of the mass of the star. Deviations from the mean spacing tell us about the star's internal structure and composition. The study of these PNNVs is important as they illuminate lesser understood aspects of the transition to the white dwarf stage. There is also hope of using our asteroseismological results to help calibrate the use of PNNs as distance indicators.
References:
Bond, Ciardullo, & Kawaler, 1993, Acta Ast, 43, 425
Kawaler & Bradley, 1994, ApJ, 427, 415