Pier-Emmanuel Tremblay

Abstract Atmospheric carbon has been detected in the optical spectra of six hydrogen-rich ultra-massive white dwarfs, revealing large carbon abundances (log(C/H) > −0.5) attributable to the convective dredge-up of internal carbon into thin hydrogen surface layers. These rare white dwarfs likely originate from stellar mergers, making them ‘smoking guns’ for one of the binary evolution channels leading to thermonuclear supernovae.

Abstract Double white dwarf binaries are a leading explanation of the origin of type Ia supernovae, but no system exceeding the Chandrasekhar mass limit (1.4 M⊙) has been found that will explode anywhere close to a Hubble time. Here we present the super-Chandrasekhar mass double white dwarf WDJ181058.67+311940.94 whose merger time (22.6 ± 1.0 Gyr) is of the same order as a Hubble time. The mass of the binary is large, combining to 1.555 ± 0.044 M⊙, while being located only 49 pc away.

Abstract White dwarfs are stellar embers depleted of nuclear energy sources that cool over billions of years1. These stars, which are supported by electron degeneracy pressure, reach densities of 107 grams per cubic centimetre in their cores2.