Magnetic Braking and the Evolution of the HW Vir-like Binary Stars

Abstract

HW Vir-like close binaries are thought to be the progenitors of cataclysmic variables. However, how this type of peculiar stars evolve into cataclysmic variables is an unsolved problem in astrophysics. Here we report the variations of the orbital period of HW Vir, the prototype of this group of systems. A long-term period decrease is discovered to be superimposed on the well-known cyclic change reported by previous investigators. It is suggested that the continuous decrease can be plausibly interpreted by angular momentum loss via magnetic braking of the completely convective star. This is driving the shrink of the orbit and will result in mass transfer between both components. We found that the binary will be undergoing mass transfer long before the sdB primary star evolves into a white dwarf. This conclusion suggests that HW Vir-like binaries do not directly evolve into cataclysmic variables through angular momentum loss as astronomers previously thought. On the other hand, if the observed period decrease is only part of a long-period cyclic variation, it may reveal the presence of another substellar companion in a wide orbit. This makes HW Vir an important system for future study of the formation and evolution of substellar objects.