Past exam of the astrophysics course of the University of Oxford / 2015 / 1 / a / Solution

A close binary star can produce two compact stellar objects if both initial stellar masses are sufficiently large. The initially more massive star evolves first, transfers material or loses its envelope, and leaves a neutron star or black hole after core collapse. The system must remain bound despite mass loss and any natal kick. Subsequent evolution of the companion can produce a common envelope: orbital energy transferred to the envelope can expel it and greatly reduce the separation. A second collapse must again leave a bound pair. A sufficiently close surviving binary star then loses orbital energy and angular momentum through gravitational waves and may merge. Stable transfer, winds and kicks make this a possible channel rather than an inevitable outcome; dynamical encounters in dense stellar systems provide another channel.