Common-envelope phases are decisive for the evolution of many binary systems. Cases with asymptotic giant branch (AGB) primary stars are of particular interest because they are thought to be progenitors of various astrophysical transients. In three-dimensional hydrodynamic simulations with the moving-mesh code AREPO, we study the common-envelope evolution of a 1.0 M⊙ early-AGB star with companions of different masses. Although the stellar envelope of an AGB star is less tightly bound than that of a red giant, we find that the release of orbital energy of the core binary is insufficient to eject more than about twenty percent of the envelope mass. Ionization energy that is released in the expanding envelope, however, can lead to complete envelope ejection. Because recombination proceeds largely at high optical depths in our simulations, it is likely that this effect indeed plays a significant role in the considered systems. The efficiency of mass loss and the final orbital separation of the core binary system depend on the mass ratio between the companion and the primary star. Our results suggest a linear relation between the ratio of final to initial orbital separation and this parameter.
Asymmetric shapes and evidence for binary central stars suggest a common-envelope origin for many bipolar planetary nebulae. The bipolar components of the nebulae are observed to expand faster than the rest, and the more slowly expanding material has been associated with the bulk of the envelope ejected during the common-envelope phase of a stellar binary system. Common-envelope evolution in general remains one of the biggest uncertainties in binary star evolution, and the origin of the fast outflow has not been explained satisfactorily. We perform three-dimensional magnetohydrodynamic simulations of common-envelope interaction with the moving-mesh code AREPO. Starting from the plunge-in of the companion into the envelope of an asymptotic-giant-branch star and covering hundreds of orbits of the binary star system, we are able to follow the evolution to complete envelope ejection. We find that magnetic fields are strongly amplified in two consecutive episodes: first, when the companion spirals in the envelope and, second, when it forms a contact binary with the core of the former giant star. In the second episode, a magnetically driven, high-velocity outflow of gas is launched self-consistently in our simulations. The outflow is bipolar, and the gas is additionally collimated by the ejected common envelope. The resulting structure reproduces typical morphologies and velocities observed in young planetary nebulae. We propose that the magnetic driving mechanism is a universal consequence of common-envelope interaction that is responsible for a substantial fraction of observed planetary nebulae. Such a mechanism likely also exists in the common-envelope phase of other binary stars that lead to the formation of Type Ia supernovae, X-ray binaries, and gravitational-wave merger events.
Modelling the evolution of progenitors of gravitational-wave merger events in binary stars faces two major uncertainties: the commonenvelope phase and supernova kicks. These two processes are critical for the final orbital configuration of double compact-object systems with neutron stars and black holes. Predictive one-dimensional models of common-envelope interaction are lacking and multidimensional simulations are challenged by the vast range of relevant spatial and temporal scales. Here, we present three-dimensional, magnetohydrodynamic simulations of the common-envelope interaction of an initially 10 M red supergiant primary star with a blackhole and a neutron-star companion. Employing the moving-mesh code arepo and replacing the core of the primary star and the companion with point masses, we show that the high-mass regime is accessible to full ab initio simulations. About half of the common envelope is dynamically ejected at the end of our simulations and the ejecta mass fraction keeps growing. Almost complete envelope ejection seems possible if all ionised gas left over at the end of our simulation eventually recombines and the released energy continues to help unbind the envelope. We find that the dynamical plunge-in of both companions terminates at orbital separations that are too wide for gravitational waves to merge the systems in a Hubble time. However, the orbital separations at the end of our simulations are still decreasing such that the true final value at the end of the common-envelope phase remains uncertain. We discuss the further evolution of the system based on analytical estimates. A subsequent mass-transfer episode from the remaining 3 M core of the supergiant to the compact companion does not shrink the orbit sufficiently either. A neutron-star-neutron-star and neutronstar-black-hole merger is still expected for a fraction of the systems if the supernova kick aligns favourably with the orbital motion. For double neutron star (neutron-star-black-hole) systems we estimate mergers in about 9% (1%) of cases while about 77% (94%) of binaries are disrupted; that is, supernova kicks actually enable gravitational-wave mergers in the binary systems studied here. Assuming orbits smaller by one-third after the common-envelope phase enhances the merger rates by about a factor of two. However, the large post-common-envelope orbital separations found in our simulations mean that a reduction in predicted gravitational-wave merger events appears possible.
We study the problem of random search in finite networks with a tree topology, where it is expected that the distribution of the first-passage time F (t) decays exponentially. We show that the slope α of the exponential tail is independent of the initial conditions of entering the tree in general, and scales exponentially or as a power law with the extent of the tree L, depending on the tendency p to jump toward the target node. It is unfeasible to uniquely determine L and p from measuring α or the mean first-passage time (MFPT) of an ordinary diffusion along the tree. To unravel the structure, we consider lazy random walkers that take steps with probability m when jumping on the nodes and return with probability q from the leaves. By deriving an exact analytical expression for the MFPT of the intermittent random walk, we verify that the structural information of the tree can be uniquely extracted by measuring the MFPT for two randomly chosen types of tracer particles with distinct experimental parameters m and q. We also address the applicability of our approach in the presence of disorder in the structure of the tree or statistical uncertainty in the experimental parameters.
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