The fate of merging white dwarfs

• Applicant: Professor Marcus Brüggen, Ph.D., since 8/2012
• Subject Area: Astrophysics and Astronomy
• Term: from 2007 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 54698620

White dwarfs (WDs) are the end points of the evolution of the vast majority of the stars in the universe. Our Galaxy harbours of order 1010 WDs and about 2.5 x 108 close binary WD systems. A good fraction of this population merges under the influence of gravitational wave emission. Such coalescences are relevant for a number of astrophysical questions, where the arguably most important one is whether such a merger can trigger a type Ia supernovae or will undergo an accretion-induced collapse.In the first funded period of this project we have focussed on accurate simulations of the onset of mass transfer in the initial stages of a WD merger. Contrary to what was the general belief in the community, the mass transfer does not last for only about two orbital periods, but instead our new simulations with accurate initial conditions show many dozens of orbits during which numerically resolvable transfer persists. This dynamics is clearly imprinted on the gravitational wave signal. Moreover, we have shown in a hybrid approach using Smoothed Particle Hydrodynamics and Adaptive Mesh Refinement simulations that this mass transfer is not necessarily “uneventful". On the contrary, in cases where the stream of transferred matter directly impacts on the accreting WD, Kelvin-Helmholtz instabilities can trigger thermonuclear surface explosions which could possibly initiate a subsequent type Ia supernova. We propose to focus now on the merging process itself with emphasis on high-mass binaries and on systems that are doomed to undergo an accretion-induced collapse. Our simulations will serve as input for a collaborating group, which will use them to draw a dividing line between accretion-induced collapse and type Ia candidates. As a service to the astrophysics community we will make our remnant structures available via a dedicated website for subsequent stellar evolution studies.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Professor Dr. Stephan Rosswog, until 5/2012