Formation of Thorne–Żytkow objects in close binaries

Abstract

Thorne–Żytkow objects (TŻOs), originally proposed by Thorne and Żytkow, may form as a result of unstable mass transfer in a massive X-ray binary after a neutron star (NS) is engulfed in the envelope of its companion star. Using a rapid binary evolution program and the Monte Carlo method, we simulated the formation of TŻOs in close binary stars. The Galactic birth rate of TŻOs is about $1.5\times 10^{-4}~\hbox {yr}^{-1}$. Their progenitors may be composed of a NS and a main-sequence star, a star in the Hertzsprung gap or a core-helium burning, or a naked helium star. The birth rates of TŻOs via the above different progenitors are $1.7\times 10^{-5}$, $1.2\times 10^{-4}$, $0.7\times 10^{-5}$, $0.6\times 10^{-5}~\hbox {yr}^{-1}$, respectively. These progenitors may be massive X-ray binaries. We found that the observational properties of three massive X-ray binaries (SMC X-1, Cen X-3 and LMC X-4) in which the companions of NSs may fill their Roche robes were consistent with those of their progenitors.