Triple star evolution and the formation of short-period, low mass X-ray binaries

To explain the presence of a relatively massive black hole in 2A0620−00 we propose a new evolutionary scenario for the formation of short-period, low-mass X-ray binaries. The progenitor in this scenario is a triple star in which a massive close binary is accompanied at large distance by a late dwarf. After the evolution of the close binary into an ordinary X-ray binary, the compact object is engulfed by its expanding massive companion, and spirals in to settle at its centre. The resulting Thorne–$$\rm{\dot Zytkow}$$ supergiant gradually expands until it attains the size of the late-dwarf orbit. Then a second spiral-in phase ensues, leading to the formation of a low-mass close binary. Depending on the initial parameters of the massive binary, this close low-mass binary may contain either a back hole or a neutron star.