On the angular momentum evolution of merged white dwarfs

Abstract

We study the angular momentum evolution of binaries containing two white dwarfs which merge and become cool helium-rich supergiants. Our object is to compare predicted rotation velocities with observations of highly evolved stars believed to have formed from such a merger, including RCrB and extreme helium stars. The principal study involves a binary containing a 0.6 solar mass CO white dwarf, and a 0.3 solar mass He white dwarf. The initial condition for the angular momentum distribution is defined where the secondary fills its Roche Lobe. We assume conservation of angular momentum to compute the angular momentum distribution in a collisionless disk and subsequently in the giant envelope. At the end of shell-helium burning, the giant contracts to form a white dwarf. We derive the surface rotation velocity during this contraction. The calculation is repeated for a range of initial mass ratios, and also for the case of mergers between two helium white dwarfs; the latter will contract to the helium main-sequence rather than the white dwarf sequence. Assuming complete conservation of angular momentum, we predict acceptable angular rotation rates for cool giants and during the initial subsequent contraction. However such stars will only survive spin-up to reach the white dwarf sequence (CO+He merger) if the initial mass ratio is close to unity. He+He merger products must lose angular momentum in order to reach the helium main sequence. Minimum observed rotation velocities in extreme helium stars are lower than our predictions by at least one half, indicating that CO+He mergers must lose at least one half of their angular momentum.