Context. The existence of ultra-massive white dwarf stars, MWD ≳ 1.05 M⊙, has been reported in several studies. These white dwarfs are relevant for the role they play in type Ia supernova explosions, the occurrence of physical processes in the asymptotic giant-branch phase, the existence of high-field magnetic white dwarfs, and the occurrence of double-white-dwarf mergers. Aims. We aim to explore the formation of ultra-massive, carbon-oxygen core white dwarfs resulting from single stellar evolution. We also intend to study their evolutionary and pulsational properties and compare them with those of the ultra-massive white dwarfs with oxygen-neon cores resulting from carbon burning in single progenitor stars, and with binary merger predictions. The aim is to provide a theoretical basis that can eventually help to discern the core composition of ultra-massive white dwarfs and the circumstances of their formation. Methods. We considered two single-star evolution scenarios for the formation of ultra-massive carbon-oxygen core white dwarfs, which involve the rotation of the degenerate core after core helium burning and reduced mass-loss rates in massive asymptotic giant-branch stars. We find that reducing standard mass-loss rates by a factor larger than 5−20 yields the formation of carbon-oxygen cores more massive than 1.05 M⊙ as a result of the slow growth of carbon-oxygen core mass during the thermal pulses. We also performed a series of evolutionary tests of solar-metallicity models with initial masses between 4 and 9.5 M⊙ and with different core rotation rates. We find that ultra-massive carbon-oxygen core white dwarfs are formed even for the lowest rotation rates we analyzed, and that the range of initial masses leading to these white dwarfs widens as the rotation rate of the core increases, whereas the initial mass range for the formation of oxygen-neon core white dwarfs decreases significantly. Finally, we compared our findings with the predictions from ultra-massive white dwarfs resulting from the merger of two equal-mass carbon-oxygen core white dwarfs, by assuming complete mixing between them and a carbon-oxygen core for the merged remnant. Results. These two single-evolution scenarios produce ultra-massive white dwarfs with different carbon-oxygen profiles and different helium contents, thus leading to distinctive signatures in the period spectrum and mode-trapping properties of pulsating hydrogen-rich white dwarfs. The resulting ultra-massive carbon-oxygen core white dwarfs evolve markedly slower than their oxygen-neon counterparts. Conclusions. Our study strongly suggests the formation of ultra-massive white dwarfs with carbon-oxygen cores from a single stellar evolution. We find that both the evolutionary and pulsation properties of these white dwarfs are markedly different from those of their oxygen-neon core counterparts and from those white dwarfs with carbon-oxygen cores that might result from double-degenerate mergers. This can eventually be used to discern the core composition of ultra-massive white dwarfs and their formation scenario.

It has been shown that the shape of the luminosity function of white dwarfs can be a powerful tool to check for the possible existence of DFSZ-axions. In particular, Isern et al. (2008) showed that, if the axion mass is of the order of a few meV, then the white dwarf luminosity function is sensitive enough to detect their existence. For axion masses of about m a > 5 meV the axion emission can be a primary cooling mechanism for the white dwarf and the feedback of the axion emission into the thermal structure of the white dwarf needs to be considered. Here we present computations of white dwarf cooling sequences that take into account the effect of axion emission in a self consistent way by means of full stellar evolution computations. Then, we study and discuss the impact of the axion emission in the white dwarf luminosity function.

Context. Recent computations of the interior composition of ultra-massive white dwarfs (WDs) have suggested that some WDs could be composed of neon (Ne)-dominated cores. This result is at variance with our previous understanding of the chemical structure of massive WDs, where oxygen is the predominant element. In addition, it is not clear whether some hybrid carbon (C) oxygen (O)-Ne WDs might form when convective boundary mixing is accounted for during the propagation of the C-flame in the C-burning stage. Both the Ne-dominated and hybrid CO-Ne core would have measurable consequences for asteroseismological studies based on evolutionary models. Aims. In this work, we explore in detail to which extent differences in the adopted micro- and macro-physics can explain the different final WD compositions that have been found by different authors. Additionally, we explore the impact of such differences on the cooling times, crystallization, and pulsational properties of pulsating WDs. Methods. We performed numerical simulations of the evolution of intermediate massive stars from the zero age main sequence to the WD stage varying the adopted physics in the modeling. In particular, we explored the impact of the intensity of convective boundary mixing during the C-flash, extreme mass-loss rates, and the size of the adopted nuclear networks on the final composition, age, as well crystallization and pulsational properties of WDs. Results. In agreement with previous authors, we find that the inclusion of convective boundary mixing quenches the carbon flame leading to the formation of hybrid CO-Ne cores. Based on the insight coming from 3D hydro-dynamical simulations, we expect that the very slow propagation of the carbon flame will be altered by turbulent entrainment affecting the inward propagation of the flame. Also, we find that Ne-dominated chemical profiles of massive WDs recently reported appear in their modeling due to a key nuclear reaction being overlooked. We find that the inaccuracies in the chemical composition of the ultra-massive WDs recently reported lead to differences of 10% in the cooling times and degree of crystallization and about 8% in the period spacing of the models once they reach the ZZ Ceti instability strip.

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