Type Ia supernova progenitors and white dwarfs

5 problems

• Chandrasekhar-mass or sub-Chandrasekhar-mass white dwarfs in normal Type Ia supernovae

  Contested

  The exploding white dwarf may be close to the maximum mass a white dwarf can have (about 1.4 Suns), or lighter, with a helium layer on its surface detonating first and setting off the core. Each picture predicts different elements in the debris.

• Origin of strong magnetic fields in white dwarfs

  Contested

  Some white dwarfs carry magnetic fields millions of times stronger than the Sun's, and they become more common as white dwarfs age. It is unknown whether the fields are inherited from the parent star, produced in mergers, or generated when the white dwarf's core freezes.

• Why are surviving companion stars absent near Type Ia remnants?

  Open

  If a white dwarf explodes while pulling gas from a normal star, that companion should survive and be seen near the center of the debris. Deep searches of young remnants have found no such star.

• Does neon-22 distillation fully explain the Gaia white-dwarf Q branch delay?

  Partially resolved

  Gaia found a group of heavy white dwarfs that stop cooling for billions of years, piling up in a narrow strip (the Q branch) of the color-brightness diagram. One proposed cause is that crystals rich in neon float upward and release energy, but whether this works in detail is not settled.

• What fraction of Type Ia supernovae come from two white dwarfs?

  Contested

  A white dwarf may explode after taking matter from an ordinary companion star, or after merging or interacting with a second white dwarf. Astronomers still do not know which route makes most of the observed explosions.