Nature: "A massive white-dwarf merger product before final collapse"

[tovisonnenberg]

Hey guys! I read this fascinating paper about the discovery of a white dwarf merger remnant: https://www.nature.com/articles/s41586-019-1216-1
To quote the abstract: "For white dwarfs, the mass of the merger product may exceed the Chandrasekhar limit, leading either to a thermonuclear explosion as a type Ia supernova or to a collapse forming a neutron star. The latter case is expected to result in a hydrogen- and helium-free circumstellar nebula and a hot, luminous, rapidly rotating and highly magnetized central star with a lifetime of about 10,000 years." My question is: why are two outcomes to the merger possible and what determines the outcome? As I understood it, degeneracy pressure is independent of temperature; white dwarfs are unable to regulate temperature in the manner of normal stars, so they undergo runaway fusion reactions.

 

[stefan r]

If we are playing with billiards we can hit a cue ball at slightly different angles. The impacts can knock the target ball into different pockets. That is two different outcomes.

Merging stars are going to spiral down rather than direct impact in almost all cases. However, the axis of rotation can be adjusted 360 degree for each star. If both are rotating prograde the merger will occur much slower than if they are both rotating retrograde. That is enough to at least consider the possibility of outcome diversity. We could also consider the magnetic field, the mass of the white dwarfs, the ratio of masses, the internal composition, and the temperature of each.

The type 1a supernova is a runaway reaction. That can happen because the star cools down and then mass is added. If your material is hot enough for carbon burning it would ignite as the pressure rises.