Outer convection zones of white dwarfs in the range 5800 K ≤ Teff ≤ 30000 K have been studied assuming that they have the same chemical composition as determined by Weidemann (1960) for van Maanen 2. Convection is important in all these stars. In white dwarfs Teff < 8000 K the adiabatic temperature gradient is strongly influenced by the pressure ionization of H, HeI and HeII which occurs within the convection zone. Partial degeneracy is also important.

Convective velocities are very small for cool white dwarfs but they reach considerable values for hotter objects. For a white dwarf of Teff = 30000 K a velocity of 6.05 km/sec and an acoustic flux (generated by the turbulent convection) of 1.5 × 1011 erg cm−2 sec−1 is reached. The formation of white dwarf coronae is briefly discussed.

Zones with μ gradient in stars have been discussed extensively in recent years in relation with the so-called ‘semi-convective zone’ which forms in massive stars when the convective core shrinks.

We have computed a set of models of white dwarfs. The equation of state is the Chandrasekhar (1939) one corrected by Salpeter (1961). It takes into account both the interactions between particles and the effect of non-zero temperature.

The models consist of an isothermal core with 50% of carbon and 50% of oxygen, surrounded by an envelope of population I composition with 70% of hydrogen and 3% of metals by mass. Nuclear energy is liberated by p-p reactions located in a shell at the edge of the isothermal core. Equilibrium models of various masses and luminosities have been built; Table I gives the values of their characteristic parameters. The central temperature-luminosity-mass relation for our models is compared with the Schatzman (1952) one for pure hydrogen envelopes (S), the Hubbard and Wagner (1970) one for Mg core and solar composition envelope models (HW) and the Van Horn (1970) one for carbons models (VH) (Figure 1). At high luminosities there is a sensible departure from the linear relation. Though our models are not evolutionary sequence ones, they fit very well with the sequences calculated by Vila (1966, 1967). The models studied here are of the type expected from the calculation of the evolution of some double systems (Lauterborn, 1970) after the white dwarfs have lost their very diluted envelopes.

Science owes much of its success to the specialized approach, which endeavours to isolate objects and phenomena and to study them under elimination of the disturbing influences of the surroundings and internal contaminations. Thus, for instance, physicists grow pure single crystals of various chemical elements and compounds and study their mechanical, thermal, electrical, magnetic and optical properties under strictly controlled conditions of pressure, temperature and other parameters.