Novel modelling of ultracompact X-ray binary evolution - stable mass transfer from white dwarfs to neutron stars

Tight binaries of helium white dwarfs (He WDs) orbiting millisecond pulsars (MSPs) will eventually 'merge' due to gravitational damping of the orbit. The outcome has been predicted to be the production of long-lived ultracompact X-ray binaries (UCXBs), in which the WD transfers material to the accreting neutron star (NS). Here we present complete numerical computations, for the first time, of such stable mass transfer from a He WD to a NS. We have calculated a number of complete binary stellar evolution tracks, starting from pre-low-mass X-ray binary systems, and evolved these to detached MSP+ WD systems and further on to UCXBs. The minimum orbital period is found to be as short as 5.6 min. We followed the subsequent widening of the systems until the donor stars become planets with a mass of similar to 0.005M(circle dot) after roughly a Hubble time. Our models are able to explain the properties of observed UCXBs with high helium abundances and we can identify these sources on the ascending or descending branch in a diagram displaying mass-transfer rate versus orbital period.