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 COWDs accreting CO-rich matter. We find that rotation induces the cooling of theaccreting star so that the delivered gravitational energy causes a greaterexpansion with respect to the standard non-rotating case. The increase in thesurface radius produces a decrease in the surface value of the critical angularvelocity and, therefore, the accreting WD becomes gravitationally unbound(Roche instability). This occurrence is due to an increase in the total angularmomentum of the accreting WD and depends critically on the amount of specificangular momentum deposited by the accreted matter. If the specific angularmomentum of the accreted matter is equal to that of the outer layers of theaccreting structure, the Roche instability occurs well before the accreting WDcan attain the physical conditions for C-burning. If the values of both initialangular velocity and accretion rate are small, we find that the accreting WDundergoes a secular instability when its total mass approaches 1.4 Msun. Atthis stage, the ratio between the rotational and the gravitational bindingenergy of the WD becomes of the order of 0.1, so that the star must deform byadopting an elliptical shape. In this case, since the angular velocity of theWD is as large as 1 rad/s, the anisotropic mass distribution induces the lossof rotational energy and angular momentum via GWR. We find that, independent ofthe braking efficiency, the WD contracts and achieves the physical conditionssuitable for explosive C-burning at the center so that a type Ia supernovaevent is produced.Comment: 39 pages, 22 eps-figures; accepted for publication in Astrophysical Journa