Rethinking Thorne–Żytkow Object Formation: Assembly via Common Envelope in Field Binaries

Abstract Thorne–Żytkow objects (TŻOs), hypothetical merger products in which a neutron star is embedded in a stellar core, are traditionally considered steady-state configurations. Their assembly, especially through dynamical channels, is not well understood. The predominant focus in the literature has been on the observational signatures related to the evolution and long-term fate of TŻOs, with their initial formation often treated as a given. However, the foundational calculations supporting the existence of TŻOs assume nonrotating spherically symmetric initial conditions that we find to be inconsistent with a binary merger scenario. In this work, we explore the implications of postmerger dynamics in TŻO formation scenarios with field binary progenitors, specifically the role that angular momentum transport during the common envelope phase plays in constraining the possible merger products, using the tools of stellar evolution and three-dimensional hydrodynamics. We also propose an alternative steady-state outcome for these mergers: the thin-envelope TŻO, an equilibrium solution consisting of a low-mass spherical envelope supported by the accretion disk luminosity of a central stellar-mass black hole. These configurations may be of interest to upcoming time-domain surveys as potential X-ray sources that may be preceded by a series of bright transient events.