Very low-mass white dwarfs with a C-O core

Context. The lower limit for the mass of white dwarfs (WDs) with a C-O core is commonly assumed to be roughly 0.5 M(circle dot). As a consequence, WDs of lower masses are usually identified as He-core remnants. Aims. When the initial mass of the progenitor star is between 1.8 and 3 M(circle dot), which corresponds to the so-called red giant (RGB) phase transition, the mass of the H-exhausted core at the tip of the RGB is 0.3 < M(H)/M(circle dot) < 0.5. Prompted by this well known result of stellar evolution theory, we investigate the possibility to form C-O WDs with mass M < 0.5 M(circle dot). Methods. The pre-WD evolution of stars was computed with initial mass of about 2.3 M(circle dot), undergoing anomalous mass-loss episodes during the RGB phase and leading to the formation of WDs with He-rich or CO-rich cores. The cooling sequences of the resulting WDs are also described. Results. We show that the minimum mass for a C-O WD is about 0.33 M(circle dot), so that both He and C-O core WDs can exist in the mass range 0.33-0.5 M(circle dot). The models computed for the present paper provide the theoretical tools for indentifying the observational counterpart of very low-mass remnants with a C-O core among those commonly ascribed to the He-core WD population in the progressively growing sample of observed WDs of low mass. Moreover, we show that the central He-burning phase of the stripped progeny of the 2.3 M(circle dot) star lasts longer and longer as the total mass decreases. In particular, the M = 0.33 M(circle dot) model takes about 800 Myr to exhaust its central helium, which is more than three times longer than the value for the standard 2.3 M(circle dot) star: it is, by far, the longest core-He burning lifetime. Finally, we find the occurrence of gravonuclear instabilities during the He-burning shell phase.