2. White dwarf rotation periods
Three methods exist for measuring the rotation rates of white dwarfs. The largest number of determinations comes from spectral and/or polarization variations in magnetic WDs. Fig. 1 shows these rotation periods, as compiled by Schmidt & Norsworthy (1991, see also Schmidt & Smith, 1995). Added to this compilation was G 158-45 (Putney, 1996) with a period of 4.44 hr. A second group of determinations comes from ZZ Ceti and other oscillating stars. In a number of these, sufficiently detailed observations exist to identify the oscillation modes, allowing determination the period splittings due to rotation. The rotation periods for 7 oscillating white dwarfs collected from the literature (Table 1) are also shown in Fig. 1. The spike in the figure at d represents the (magnetic) stars whose periods are inferred to be longer than a century, on the basis of the absence of variations in the polarization on time scales of decades. These stars were put at their approximate lower limits of 100 yrs.
Table 1. Asteroseismologically determined WD rotation periods
The widths of the narrow NLTE line cores have been used to set limits on rotation velocities of stars for which the magnetic and seismological methods can not be used (Wesemael et al. 1980, Koester & Herrero 1988, Koester et al., 1998, in prep.). The detection limit, apparently around 20 km/s, is not sensitive enough to determine the rotation of stars in the d main peak in Fig. 1, but may be useful in setting limits on the number of rapidly rotating ( hr) stars. Reid (1996, his Sect. 3.1) and Heber et al. (1997) infer upper limits from 8 to 40 km/s from Keck spectra of some 25 single white dwarfs.
The distribution of periods in the main hump around 1d looks the same for the magnetic and the oscillating stars, given the limited statistics. Very long periods are absent from the sample of seismologically determined periods, but this may be due to observational limitations. No stars have had their oscillations followed long enough to detect period splittings of a decade. One concludes that with the (limited) data available, there does not seem to be a significant difference in the distribution of rotation rates of magnetic ( G) and nonmagnetic ( G) stars. There may, however, be other differences between the magnetic and nonmagnetic WD, apart from the field strength. Sion et al. (1988) and Liebert (1995) for example, argue that the magnetic stars are more massive than the nonmagnetic ones, and derive from more massive progenitors.
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998