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Abstract

Thorne-\.{Z}ytkow objects (T\.{Z}Os), 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 T\.{Z}Os, with their initial formation often treated as a given.
However, the foundational calculations supporting the existence of T\.{Z}Os
assume non-rotating spherically-symmetric initial conditions that we find to be
inconsistent with a binary merger scenario. In this work, we explore the
implications of post-merger dynamics in T\.{Z}O formation scenarios with field
binary progenitors, specifically the role that angular momentum transport
during the common envelope phase plays in constraining 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 T\.{Z}O, 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.