IRRADIATION-DRIVEN MASS-TRANSFER IN LOW-MASS X-RAY BINARIES

IN a low-mass X-ray binary (LMXB), mass accreted onto a neutron star from a larger companion generates a flux of X-rays which irradiate the companion. Previous studies of irradiated stars have been restricted to the effects on their outermost layers 1-5, or did not address conditions found in interacting binaries6. Here I show that, under the radiative flux typical of a LMXB, the companion will expand towards a new state of thermal equilibrium, and that this expansion provides a new mechanism to drive mass transfer onto the neutron star. The evolution of LMXBs can be drastically altered in this way. In particular, this mechanism may introduce a new evolutionary phase during which the orbital period increases, leading to larger orbital periods during and at the end of mass transfer, and (if the companion is a subgiant) shortening the duration of the LMXB phase. These modifications may help to resolve a number of observational puzzles concerning the numbers and properties of LMXBs and related objects.