Survivors of white dwarf binary interactions

Unlike objects in our day-to-day lives, white dwarfs become physically smaller as they become more massive. This leads to a limit known as the Chandrasekhar-Mass (1.4 Solar Masses), at which point the electron-degeneracy pressure can no longer support the mass of the star, and so runaway nuclear fusion occurs resulting in the explosion of the white dwarf as a supernova called a type Ia. For this kind of supernova to occur a white dwarf must grow in mass until it reaches the Chandrasekhar-limit. This is possible for white dwarfs who have a close binary companion star. Either the white dwarf pulls material from a “normal” star, slowly growing its mass, or two white dwarfs can merge forming an new white dwarf that exceeds the Chandrasekhar-Mass which undergoes rapid collapse.

Recently a new subclass of type Ia supernovae has been identified called type Iax. The Iaxs are much less luminous than normal type Ia supernovae, suggesting that the white dwarf does not fully explode leaving behind some intact fragment of the former star, which is then ejected from the binary star system. In 2017 an usual white dwarf called GD492/LP40-365 has been identified which fits the description of a surviving remnant from a type Iax supernova. GD492 moves through the Milky Way so fast that it is essentially being flung out of the Galaxy. Furthermore it has a truly remarkable surface composition – instead of the usual hydrogen or helium atmosphere typical of white dwarfs, these elements are entirely absent, and instead a unique atmosphere dominated by oxygen, neon, and magnesium is seen, with traces of heavier elements all the way up to nickel. The set of elements and their abundances are signatures of nuclear burning of carbon, oxygen, and silicon. Stars like GD492 therefore provide a novel way to study a nearby objects and connecting them to supernovae seen distant Galaxies.