An ultra-massive white dwarf with a mixed hydrogen–carbon atmosphere as a likely merger remnant

White dwarfs are dense, cooling stellar embers consisting mostly of carbon and oxygen1, or oxygen and neon (with a few per cent carbon) at higher initial stellar masses2. These stellar cores are enveloped by a shell of helium, which in turn, is usually surrounded by a layer of hydrogen, generally prohibiting direct observation of the interior composition. However, carbon is observed at the surface of a sizeable fraction of white dwarfs3,4, sometimes with traces of oxygen, and is thought to be dredged up from the core by a deep helium convection zone5,6. In these objects, only traces of hydrogen are found7,8, as large masses of hydrogen are predicted to inhibit hydrogen–helium convective mixing within the envelope9. We report the identification of WD J055134.612+413531.09, an ultra-massive (1.14 solar masses (M⊙)) white dwarf with a unique carbon–hydrogen mixed atmosphere (atomic ratio C∕H = 0.15). Our analysis of the envelope and interior indicates that the total hydrogen and helium mass fractions must be several orders of magnitude lower than predictions of single-star evolution10: less than 10−9.5 and 10−7.0, respectively. Due to the fast kinematics (129 ± 5 km s−1 relative to the local standard of rest), large mass and peculiar envelope composition, we argue that WD J0551+4135 is consistent with formation from the merger of two white dwarfs in a tight binary system11–14. A massive white dwarf (WD) with unusually low hydrogen and helium abundances, undetectable oxygen and high C/H poses a challenge to single-star evolution models. However, this object fits with expectations of a WD–WD merger product.