Carbon-oxygen white dwarfs accreting CO-rich matter.: I.: A comparison between rotating and nonrotating models

We investigate the lifting effect of rotation on the thermal evolution of CO white dwarfs accreting CO-rich matter. We find that rotation induces the cooling of the accreting structure so that the delivered gravitational energy causes a greater expansion with respect to the standard nonrotating case. The increase in the surface radius produces a decrease in the surface value of the critical angular velocity and, therefore, the accreting white dwarf becomes gravitationally unbound (Roche instability). This occurrence is due to an increase in the total angular momentum of the accreting white dwarf and depends critically on the amount of specific angular momentum deposited by the accreted matter. If the specific angular momentum of the accreted matter is equal to that of the outer layers of the accreting structure, the Roche instability occurs well before the accreting white dwarf can attain the physical conditions for carbon burning. If the values of both initial angular velocity and accretion rate are small, we find that the accreting white dwarf undergoes a secular instability when its total mass approaches 1.4 M.. At this stage, the ratio between the rotational energy and the gravitational binding energy of the white dwarf becomes of the order of 0.1, so that the star must deform by adopting an elliptical shape. In this case, since the angular velocity of the white dwarf is as large as similar to1 rad s(-1), the anisotropic mass distribution induces the loss of rotational energy and angular momentum via gravitational wave radiation. We find that, independent of the braking efficiency, the white dwarf contracts and achieves the physical conditions suitable for explosive carbon burning at the center so that a Type Ia supernova event is produced.