We consider a simple model for the evolution of a poloidal magnetic field initally trapped in a region containing normal npe matter within the outerliquid core of a neutron star. We have performed numerical computations for neutron stars with masses of 1.4, 1.6, and 1.7 M⊙ that undergo very rapid cooling due to the direct Urca process. Because the timescale for the magnetic field decay is directly proportional to T2, such a cooling history produces a rapid decline in the magnetic-field strength B, even for B as low as ∼ 1012 G. In particular, we show that an initially quasi-homogeneous magnetic field of strength B = 1012 G declines during the first ∼ 1 Myr.

We have investigated the effect on the seismological properties of the giant planet Jupiter of different descriptions of the equation of state (EOS) of the fluid hydrogen helium envelop. Recent Jovian models by Chabrier et al. 1992 use the equation of state of Saumon et al. (1992) which predicts the PPT (plasma phase transition). We show that different hypothesis at the level of the PPT induce large differences in the internal structure of the corresponding models, specially the sound speed. This gives rise to substantial differences of the oscillation frequencies up to 100 μHz, for modes of degrees l up to 20. These results show the great capability of Jovian seismology to test the physics involved in the interior of the giant planets.

We discuss the quality of the electronic screening prescriptions usable in stellar evolution codes during hydrogen burning. The assumption log f = H(0)/kT is compared to a precise formalism where the radial dependence of the screened potential of the 2 ions is introduced.

Coulomb interactions for the Free Helmholtz energy and the pressure are studied in a partial new formulation which described more exactly the numerical evaluation of many body theories.

Model sequences computed with the recently-published OPAL radiative opacities, Itoh et al. conductive opacities, and Itoh et al. neutrino rates are presented. Cooling times for DA model sequences are found to be independent of metallicity for Z ≤ 0.001.

The importance of low temperature opacities in stellar calibrations led us to compute new sets of Rosseland mean opacities for different Z-values. For the solar metallicity, these tables have been compared to those of Alexander (1975), Cox (1983), Sharp (1991) and Kurucz (1992).