4.2 Accreting White Dwarf Pulsators
4.2.2 Speculations
(pushing the theoretical limit).
Are these results relevant to the DAYs, and to GW Librae in particular? The models described above are only simple approximations, but with no self-consistent interacting binary evolution code available, they are currently the best models for accreting white dwarf pulsators. A model for an accreting DAV would need to in- clude surface (longitudinal) differential rotation and surface temperature and density gradients (a disc accreting onto the equator would heat an equatorial band and spin it up). Accretion torque may spin up the star to angular velocities several orders of magnitude greater than those seen in single DAVs (Bildsten 1998)- the m-splittings given by equation (3.9) apply to slow rotation only.
itta and Winget's models have relied on the fact that the AM CVns evolve quite quickly, with Tacc rv 106 to 109 years, so that their primaries receive more heat from accretion than they can radiate away. This may not be the case for the hydrogen-rich CVs, however, for which Tacc rv 108 to 1011 years. In stars like GW Librae, we may not find long-lasting temperature inversions.
hot accreted material is confined to a broad equatorial band for 10 years. Therefore we need to assume that the outer envelope of an accreting white dwarf will have a non-spherically symmetric rotation, temperature, density and chemical composition structure. The analysis of normal modes of oscillation in white dwarfs has thus far been heavily dependent on assumptions of spherical symmetry.
The effects of these non-symmetric structures on the normal modes of a star de- pend on the depth to which they extend. The pulsation driving regions are buried deep in the envelope. If accretion affects only the upper layers of the photosphere, perturbations on the normal modes may be small, but if the effects of accretion ex- tend deeply into the nondegenerate envelope, the normal modes may be fundamen- tally changed. Perhaps the hot DAYs, with their thin, shallow driving zones, may be more strongly affected by accretion than the cool DAYs, which have deep, broad driving zones. The effect of accretion on the pulsations may also differ greatly from star to star; depending on the specific accretion geometry, boundary layer structure, inner disc structure and various other stellar and system parameters.
Normal mode analysis has also assumed slow rotation. Single white dwarfs have rotation periods on the order of"" 1 day (Kepler 1990). For a CY primary accreting angular momentum from a disc for a few x 109 years, the time taken to spin it up to break-up velocity is relatively short, but its actual Prot depends on a number of competing and uncertain factors (Warner 1995). The shortest rotation period for a white dwarf is given by Warner (1995) as
Prot min
=
15.1MwD S' (4.5)
(any faster than this and they break up and fly apart). The general rotation periods for the outer envelopes of non-magnetic CY primaries can be found from the max- imum periods of Dwarf Nova Oscillations (Warner 1995), giving Prot "" 30s! Direct measurements of v sin i for CY primaries give Prot
2:
40s for TT Ari (Shafter et al.1985), Prot "" 11 Os for U Gem ( Sion et al. 1994), and Prot "" 29s for the matriarch of GW Lib's family, WZ Sge (Patterson 1980). Accreted angular momentum may
not necessarily be transferred to the deep interior of the star (King, Regev & Wynn 1991), in which case there may be fearsome radial differential rotation gradients through these stars.
GW Librae may be spinning rv 3000 times faster than a single DAY. The slow rotation model for normal modes may not apply. In addition, rapid rotation distorts the primary: the equatorial radius of a white dwarf spinning at Prot,min is 1.255 times its average radius (Tassoul1978); yet another departure from spherical symmetry.
Because of the low mass transfer rate requirement, a nonmagnetic DAY CY will be a Dwarf Nova. This is potentially the most exciting feature of accreting DAYs:
it may be possible for us to watch these stars move through the instability strip in a matter of weeks or years, far faster than the (5- 10) x 108 years required for single DAYs (Wood 1990; Kleinman 1998a).
The dumping of hot material into the primary's photosphere during a DN out- burst temporarily increases its Teff· For example, Wood et al. (1993) find that Z Cha's primary has a Teff equal to 17 400 K immediately after a normal outburst, cooling to its quiescent temperature of 15 600 Kin rv 16 days; and Long et al. (1994) find that for U Gem's outbursts, its primary has a Tef f equal to 39 400 K after 13 days, and 32 100 K after 70 days. Of greater relevance, WZ Sge takes rv 3000 days (rv 8 years) to cool after its superoutburst (Sion & Szkody 1990).
These cooling time scales can give the depth of heating of the white dwarf outer envelopes; Sion & Szkody (1990) find the heated masses to be (5- 20) x 10-10 M0. Are these masses sufficient to give the temperature inversion effects on normal modes predicted by Nitta & Winget (previous section)?. These masses are for spherically symmetric heating, however, whereas accretion heat is more likely to be deposited in equatorial bands. An equatorial band cutting deeply into the driving zone may have a much more dramatic effect on a star's g-modes than a spherically symmetric temperature inversion in the top (5- 20) x 10-10 M0 layers. With no theoretical work or modelling, it is not possible to say just how outbursts will affect the g-modes, but the possibilities are extremely exciting.
Could accretion be a stochastic or chaotic pulsation driving force? While ac-
creting material may not penetrate the envelope deeply enough to affect the driving region directly, it may drive r-mode oscillations on the surface, which in turn could be a perturbing force on the g-modes. r-modes (Rossby waves) are surface oscilla- tions which are driven by "winds" of a less dense fluid blowing over it (ocean waves are an example of this). Accretion may be seen as a high velocity, low density wind blowing across the surface of the much denser white dwarf photosphere, and it could generate r-mode oscillations in the photosphere. For primaries in the DA instabil- ity strip, these r-modes could excite the deeper, larger amplitude, self-sustaining g-modes, or perturb them in a stochastic or chaotic manner, resulting in continuous phase, frequency or amplitude modulation of each normal mode.