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

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.

A neutron star merger, or a neutron star tidally disrupted by a black hole outside its capture region, can leave a hot accretion disk around the remnant. Rapid accretion can power a relativistic jet and a short gamma-ray burst. Beaming means that an observer must lie sufficiently near the jet axis, and not every merger produces an observable burst. A clean binary black hole merger has no substantial baryonic fuel and is not ordinarily expected to power this mechanism. Disruption in a neutron-star–black-hole pair depends on binary mass ratio, black-hole spin and the neutron star's stellar radius.

The changing mass quadrupole moment produces an inspiral gravitational wave signal whose frequency and amplitude increase toward merger. The chirp mass controls the leading frequency evolution, while higher-order phasing and the merger carry information about binary mass ratio and spins. Neutron stars also introduce tidal effects. These signals motivate a search with an interferometer such as LIGO; the supplied ranges quantify the paper's proposed detection prospects, without assuming any particular later observing-run performance.

Neutron-rich material ejected dynamically or in remnant-disk outflows can undergo the r-process: successive neutron captures, followed by beta decays, make heavy nuclei. The yield depends on the amount ejected, its entropy, expansion and electron fraction; not all outflow is equally neutron rich. Radioactive products can power a kilonova. A neutron-star–black-hole event contributes if matter escapes capture. An isolated binary black hole merger has no analogous reservoir of neutron-rich baryonic material, so it contributes essentially no such nucleosynthesis.