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When carbon is ignited off-centre in a CO core of a super-AGB star, itsburning in a convective shell tends to propagate to the centre. Whether the Cflame will actually be able to reach the centre depends on the efficiency ofextra mixing beneath the C convective shell. Whereas thermohaline mixing is tooinefficient to interfere with the C-flame propagation, convective boundarymixing can prevent the C burning from reaching the centre. As a result, aC-O-Ne white dwarf (WD) is formed, after the star has lost its envelope. Such a"hybrid" WD has a small CO core surrounded by a thick ONe zone. In our 1Dstellar evolution computations, the hybrid WD is allowed to accrete C-richmaterial, as if it were in a close binary system and accreted H-rich materialfrom its companion with a sufficiently high rate at which the accreted H wouldbe processed into He under stationary conditions, assuming that He could thenbe transformed into C. When the mass of the accreting WD approaches theChandrasekhar limit, we find a series of convective Urca shell flashesassociated with high abundances of 23Na and 25Mg. They are followed byoff-centre C ignition leading to convection that occupies almost the entirestar. To model the Urca processes, we use the most recent well-resolved datafor their reaction and neutrino-energy loss rates. Because of the emphasizeduncertainty of the convective Urca process in our hybrid WD models of SN Iaprogenitors, we consider a number of their potentially possible alternativeinstances for different mixing assumptions, all of which reach a phase ofexplosive C ignition, either off or in the centre. Our hybrid SN Ia progenitormodels have much lower C to O abundance ratios at the moment of the explosive Cignition than their pure CO counterparts, which may explain the observeddiversity of the SNe Ia.

Hybrid C-O-Ne white dwarfs as progenitors of type Ia supernovae: dependence on Urca process and mixing assumptions