The formation of ultra-massive carbon-oxygen core white dwarfs and their evolutionary and pulsational properties

TL;DR

This paper investigates the formation, evolution, and pulsational characteristics of ultra-massive carbon-oxygen core white dwarfs, proposing single-star evolution pathways and contrasting them with merger scenarios to distinguish core compositions.

Contribution

It introduces new formation scenarios for ultra-massive CO-core white dwarfs from single stellar evolution and compares their properties with ONe-core and merger-origin white dwarfs.

Findings

Ultra-massive CO-core white dwarfs evolve slower than ONe-core counterparts.

Distinct pulsational properties can differentiate core compositions.

Single-star evolution can produce ultra-massive CO-core white dwarfs.

Abstract

(Abridged abstract) We explore the formation of ultra-massive (M_{\rm WD} \gtrsim 1.05 M_\sun$), carbon-oxygen core white dwarfs resulting from single stellar evolution. We also 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. We consider two single-star evolution scenarios for the formation of ultra-massive carbon-oxygen core white dwarfs that involve rotation of the degenerate core after core helium burning and reduced mass-loss rates in massive asymptotic giant-branch stars. We compare 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. The resulting ultra-massive carbon-oxygen core white dwarfs evolve markedly slower than their oxygen-neon counterparts. Our study strongly suggests the formation of ultra-massive white dwarfs with carbon-oxygen core from 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 core 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.