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Abstract We propose a new predictive theory for the analysis of common envelope (CE) events that incorporates the effects of relevant hydrodynamical processes into a simple analytical framework. We introduce the ejection and dynamical parametersξandβ, which define whether envelope ejection is energetically or hydrodynamically favorable, respectively, during CE inspiral. When combined, these parameters offer a detailed narrative of how inspiral begins, proceeds, and ends that is consistent with preliminary comparisons to 3D hydrodynamical models. This physically motivated framework impacts predictions for CE outcomes, especially for systems that have energy excess, and offers promise as a potential alternative for the treatment of CEs in binary population synthesis. 

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. 

Abstract We present a start-to-end simulation aimed at studying the long-term fate of high-mass X-ray binaries and whether a Thorne–Żytkow object (TŻO) might ultimately be assembled. We analyze results from a 3D hydrodynamical simulation that models the eventual fate of LMC X-4, a compact high-mass X-ray binary system, after the primary fills its Roche lobe and engulfs the neutron star companion. We discuss the outcome of this engulfment within the standard paradigm of TŻO formation. The post-merger angular momentum content of the stellar core is a key ingredient, as even a small amount of rotation can break spherical symmetry and produce a centrifugally supported accretion disk. Our findings suggest the inspiraling neutron star, upon merging with the core, can accrete efficiently via a disk at high rates (≈10⁻²M_⊙s⁻¹), subsequently collapsing into a black hole and triggering a bright transient with a luminosity and duration typical of an ultra-long gamma-ray burst. We propose that the canonical framework for TŻO formation via common envelope needs to be revised, as the significant post-merger accretion feedback will unavoidably unbind the vast majority of the surrounding envelope. 

Abstract Triple stellar systems allow us to study stellar processes that cannot be attained in binary stars. The evolutionary phases in which the stellar members undergo mass exchanges can alter the hierarchical layout of these systems. Yet, the lack of a self-consistent treatment of common-envelope (CE) in triple-star systems hinders the comprehensive understanding of their long-term fate. This paper examines the conditions predicted around binaries embedded within CEs using local 3D hydrodynamical simulations. We explore varying the initial binary separation, the flow Mach number, and the background stellar density gradients as informed by a wide array of CE conditions, including those invoked to explain the formation of the triple system hosting PSR J0337+1715. We find that the stellar density gradient governs the gaseous drag force, which determines the final configuration of the embedded binary. We observe a comparable net drag force on the center of mass but an overall reduction in the accretion rate of the binary compared to the single-object case. We find that, for most CE conditions, and in contrast to the uniform background density case, the binary orbital separation increases with time, softening the binary and preventing it from subsequently merging. We conclude that binaries spiraling within CEs become more vulnerable to disruption by tidal interactions. This can have profound implications on the final outcomes of triple-star systems.