The outcomes of carbon-oxygen white dwarfs accreting CO-rich material

The double-degenerate model, involving the merger of double carbon-oxygen white dwarfs (COWDs), is one of the two classic models for the progenitors of Type Ia supernovae (SNe Ia). Previous studies suggested that off-centre carbon burning would occur if the mass accretion rate ((M) over dot(acc)) is relatively high during the merging process, leading to the formation of oxygen-neon (ONe) cores that may collapse into neutron stars. However, the off-centre carbon burning is still incompletely understood, especially when the inwardly propagating burning wave reaches the centre. In this paper, we aim to investigate the propagating characteristics of burning waves and the subsequently evolutionary outcomes of these CO cores. We simulated the long-term evolution of CO WDs that accrete CO-rich material by employing the stellar evolution code MESA on the basis of the thick-disc assumption (slow merger). We found that the final outcomes of CO WDs strongly depend on (M) over dot(acc) (M-circle dot yr(-1)) based on the thick-disc assumption, which can be divided into four regions: (1) explosive carbon ignition in the centre, then SNe Ia ((M) over dot(acc) < 2.45 x 10(-6)); (2) OSi cores, then neutron stars (2.45 x 10(-6) less than or similar to (M) over dot(acc) < 4.5 x 10(-6)); (3) ONe cores, then e-capture SNe (4.5 x 10(-6) less than or similar to (M) over dot(acc) < 1.05 x 10(-5)); (4) off-centre oxygen and neon ignition, then off-centre explosion or Si-Fe cores ((M) over dot(acc) greater than or similar to 1.05 x 10(-5)). Our results indicate that the final fates of double CO WD mergers are strongly dependent on the merging processes (e.g. slow merger, fast merger, composite merger, violent merger, etc.).