arXiv subjects

Baowei Liu

Publications and source records attributed to Baowei Liu.

14 recordsLinked to original sources

arXiv2026

Effect of Neutron Star Jets on Common Envelope Evolution

The common envelope (CE) phase is a key stage in binary star evolution that is still not very well understood. Once engulfed by the giant star, the binary companion may accrete envelope material. For neutron star (NS) companions, such accretion may in principle occur at mass rates several orders of magnitude above the Eddington limit and may result in outflows dominated by powerful bi-polar jets with mass-loss rates similar to the accretion rates. Such jets would impact the morphology of the system and the rate of envelope unbinding, which affect the duration and outcome of the CE event. Employing 3D global hydrodynamic simulations, we study the role of such NS jets in a CE event involving a red giant branch star. The jets eventually drill through and break out of the envelope, producing prominent low-density bi-polar lobes. The jets cause about twice as much envelope mass to be unbound ($\sim20\%$ of the envelope) as compared to simulations of the same duration without NS jets ($\sim10\%$). However, the rate of mass unbinding due to the jets decreases towards the ends of the simulations as the jets break out and energetically decouple from the envelope. Moreover, jet activity leads to slightly reduced drag on the binary, decreasing the rate of orbital energy transfer to the envelope. Hence, while such powerful jets can play an important role, negative feedback effects tend to prevent them from dominating envelope unbinding and dictating CE outcomes.

astro-ph.SR↗

arXiv2025

Understanding the Drag Torque in Common Envelope Evolution

Common envelope (CE) evolution is largely governed by the drag torque applied on the in-spiralling stellar components by the envelope. Previous work has shown that idealized models of the torque based on a single body moving in rectilinear motion through an unperturbed atmosphere can be highly inaccurate. Progress requires new models for the torque that account for binarity. Toward this end we perform a new 3D global hydrodynamic CE simulation with the mass of the companion point particle set equal to the mass of the asymptotic giant branch star core particle to maximize symmetry and facilitate interpretation. First, we find that a region around the particles of a scale comparable to their separation contributes essentially all of the torque. Second, the density pattern of the torque-dominating gas and, to an extent, this gas itself, is roughly in corotation with the binary. Third, approximating the spatial distribution of the torquing gas as a uniform-density prolate spheroid whose major axis resides in the orbital plane and lags the line joining the binary components by a constant phase angle reproduces the torque evolution remarkably well, analogous to studies of binary supermassive black holes. Fourth, we compare the torque measured in the simulation with the predictions of a model that assumes two weak point-mass perturbers undergoing circular motion in a uniform background without gas self-gravity, and find remarkable agreement with our results if the background density is taken to be equal to a fixed fraction (~0.44) of the density at the spheroid surface. Overall, this work makes progress toward developing simple time-dependent models of the CE phase, for example by informing the development of drag force prescriptions for 1D spherically symmetric CE simulations, which could be used to explore the parameter space of luminous red novae or in binary population synthesis studies.

astro-ph.SR↗

arXiv2023

How negative feedback and the ambient environment limit the influence of recombination in common envelope evolution

We perform 3D hydrodynamical simulations to study recombination and ionization during the common envelope (CE) phase of binary evolution, and develop techniques to track the ionic transitions in time and space. We simulate the interaction of a $2\,M_\odot$ red giant branch primary and a $1\,M_\odot$ companion modeled as a particle. We compare a run employing a tabulated equation of state (EOS) that accounts for ionization and recombination, with a run employing an ideal gas EOS. During the first half of the simulations, $\sim15$ per cent more mass is unbound in the tabulated EOS run due to the release of recombination energy, but by simulation end the difference has become negligible. We explain this as being a consequence of (i) the tabulated EOS run experiences a shallower inspiral and hence smaller orbital energy release at late times because recombination energy release expands the envelope and reduces drag, and (ii) collision and mixing between expanding envelope gas, ejecta and circumstellar ambient gas assists in unbinding the envelope, but does so less efficiently in the tabulated EOS run where some of the energy transferred to bound envelope gas is used for ionization. The rate of mass unbinding is approximately constant in the last half of the simulations and the orbital separation steadily decreases at late times. A simple linear extrapolation predicts a CE phase duration of $\sim2\,\mathrm{yr}$, after which the envelope would be unbound.

astro-ph.SR↗

arXiv2020

Bipolar Planetary Nebulae from Outflow Collimation by Common Envelope Evolution

The morphology of bipolar planetary nebulae (PNe) can be attributed to interactions between a fast wind from the central engine and dense toroidal shaped ejecta left over from common envelope (CE) evolution. Here we use the 3-D hydrodynamic AMR code AstroBEAR to study the possibility that bipolar PN outflows can emerge collimated even from an uncollimated spherical wind in the aftermath of a CE event. The output of a single CE simulation via the SPH code PHANTOM serves as the initial conditions. Four cases of winds, all with high enough momenta to account for observed high momenta preplanetary nebula outflows, are injected spherically from the region of the CE binary remnant into the ejecta. We compare cases with two different momenta and cases with no radiative cooling versus application of optically thin emission via a cooling curve to the outflow. Our simulations show that in all cases highly collimated bipolar outflows result from deflection of the spherical wind via the interaction with the CE ejecta. Significant asymmetries between the top and bottom lobes are seen in all cases. The asymmetry is strongest for the lower momentum case with radiative cooling. While real post CE winds may be aspherical, our models show that collimation via "inertial confinement" will be strong enough to create jet-like outflows even beginning with maximally uncollimated drivers. Our simulations reveal detailed shock structures in the shock focused inertial confinement (SFIC) model and develop a lens-shaped inner shock that is a new feature of SFIC driven bipolar lobes.

astro-ph.SR↗

arXiv2019

Models of the Mass-Ejection Histories of pre Planetary Nebulae, IV. Magnetized Winds and the Origins of Jets, Bullets, and FLIERs

The influences and consequences of toroidal magnetic fields in shaping the visible lobes of pre planetary nebulae ("prePNe") are explored in this, the last of a series of papers of parameter studies of prePN evolution. To probe these influences we start with the steady, diverging, and field-free wind model of our previous papers and add weak to moderate toroidal fields to the winds in order to generate arrays of outcomes after 500 y, after which the structures grow almost homologously. As expected, toroidal fields in the stellar winds invariably form very thin and dense axial features whose structure is best described as a thin cold jet with an ultra-dense and neutral leading knot, or "bullet", at its tip. The speed of the leading knot depends only on the density contrast (the ratio of injected to ambient gas densities at the nozzle) and wind injection speed, but not on the field strength or opening angle. The lobes formed by the ram pressure of the winds take a variety of forms and sizes that depend primarily on the geometric structure of the injected gas and the density contrast. About 20% of the HST images of prePNe show the unique signatures of shaping by toroidal fields. Pairs of low-ionization knots seen along the major axis of fully ionized PNe, often called "FLIERs" are easily explained as the very dense, cold , and neutral remnants of magnetically formed knots.

astro-ph.SR↗

arXiv2019

Hydrodynamic Simulations of Disrupted Planetary Accretion Discs Inside the Core of an AGB Star

Volume complete sky surveys provide evidence for a binary origin for the formation of isolated white dwarfs with magnetic fields in excess of a MegaGauss. Interestingly, not a single high-field magnetic white dwarf has been found in a detached system suggesting that if the progenitors are indeed binaries, the companion must be removed or merge during formation. An origin scenario consistent with observations involves the engulfment, inspiral, and subsequent tidal disruption of a low-mass companion in the interior of a giant star during a common envelope phase. Material from the shredded companion forms a cold accretion disc embedded in the hot ambient around the proto-white dwarf. Entrainment of hot material may evaporate the disc before it can sufficiently amplify the magnetic field, which typically requires at least a few orbits of the disc.Using three-dimensional hydrodynamic simulations of accretion discs with masses between 1 and 10 times the mass of Jupiter inside the core of an Asymptotic Giant Branch star, we find that the discs survive for at least 10 orbits (and likely for 100 orbits), sufficient for strong magnetic fields to develop.

astro-ph.SR↗

arXiv2019

Hydrodynamic and Magnetohydrodynamic Simulations of Wire Turbulence

We report on simulations of laboratory experiments in which magnetized supersonic flows are driven through a wire mesh. The goal of the study was to investigate the ability of such a configuration to generate supersonic, MHD turbulence. We first report on the morphological structures that develop in both magnetized and non-magnetized cases. We then analyze the flow using a variety of statistical measures, including power spectra and probability distribution functions of the density. Using these results we estimate the sonic mach number in the flows downstream of the wire mesh. We find the initially hypersonic (M=20) planar shock through the wire mesh does lead to downstream turbulent conditions. However, in both magnetized and non-magnetized cases, the resultant turbulence was marginally supersonic to transonic (M~1), and highly anisotropic in structure.

physics.flu-dyn↗

arXiv2019

Energy Budget and Core-Envelope Motion in Common Envelope Evolution

We analyze a 3D hydrodynamic simulation of common envelope evolution to understand how energy is transferred between various forms and whether theory and simulation are mutually consistent given the setup. Virtually all of the envelope unbinding in the simulation occurs before the end of the rapid plunge-in phase, here defined to coincide with the first periastron passage. In contrast, the total envelope energy is nearly constant during this time because positive energy transferred to the gas from the core particles is counterbalanced by the negative binding energy from the closer proximity of the inner layers to the plunged-in secondary. During the subsequent slow spiral-in phase, energy continues to transfer to the envelope from the red giant core and secondary core particles. We also propose that relative motion between the centre of mass of the envelope and the centre of mass of the particles could account for the offsets of planetary nebula central stars from the nebula's geometric centre.

astro-ph.SR↗

arXiv2018

Accretion in Common Envelope Evolution

Common envelope evolution (CEE) occurs in some binary systems involving asymptotic giant branch (AGB) or red giant branch (RGB) stars, and understanding this process is crucial for understanding the origins of various transient phenomena. CEE has been shown to be highly asymmetrical and global 3D simulations are needed to help understand the dynamics. We perform and analyze hydrodynamic CEE simulations with the adaptive mesh refinement (AMR) code AstroBEAR, and focus on the role of accretion onto the companion star. We bracket the range of accretion rates by comparing a model that removes mass and pressure using a subgrid accretion prescription with one that does not. Provided a pressure-release valve, such as a bipolar jet, is available, super-Eddington accretion could be common. Finally, we summarize new results pertaining to the energy budget, and discuss the overall implications relating to the feasibility of unbinding the envelope in CEE simulations.

astro-ph.SR↗

arXiv2018

Accretion in common envelope evolution

Common envelope evolution (CEE) is presently a poorly understood, yet critical, process in binary stellar evolution. Characterizing the full 3D dynamics of CEE is difficult in part because simulating CEE is so computationally demanding. Numerical studies have yet to conclusively determine how the envelope ejects and a tight binary results, if only the binary potential energy is used to propel the envelope. Additional power sources might be necessary and accretion onto the inspiraling companion is one such source. Accretion is likely common in post-asymptotic giant branch (AGB) binary interactions but how it operates and how its consequences depend on binary separation remain open questions. Here we use high resolution global 3D hydrodynamic simulations of CEE with the adaptive mesh refinement (AMR) code AstroBEAR, to bracket the range of CEE companion accretion rates by comparing runs that remove mass and pressure via a subgrid accretion model with those that do not. The results show that if a pressure release valve is available, super-Eddington accretion may be common. Jets are a plausible release valve in these environments, and they could also help unbind and shape the envelopes.

astro-ph.SR↗

arXiv2018

Models of the Mass-Ejection Histories of pre Planetary Nebulae. II. The Formation of the Butterfly and its Proboscis in M2-9

M2-9, or the "Butterfly Nebula" is one of the most iconic outflow sources from an evolved star. In this paper we present a hydrodynamic model of M2-9 in which the nebula is formed and shaped by a steady, low-density ("light"), mildly collimated "spray" of gas injected at 200 km s^-1 that interacts with a far denser, intrinsically simple pre-existing AGB wind has slowly formed all of the complex features within M2-9's lobes (including the knot pairs N3/S3 and N4/S4 at their respective leading edges, and the radial gradient of Doppler shifts within 20" of the nucleus). We emphasize that the knot pairs are not ejected from the star but formed in situ. In addition, the observed radial speed of the knots is only indirectly related to the speed of the gas injected by the star. The model allows us to probe the early history of the wind geometry and lobe formation. We also formulate a new estimate of the nebular distance D = 1.3 kpc. The physical mechanism that accounts for the linear radial speed gradient in M2-9 applies generally to many other pre planetary nebulae whose hollow lobes exhibit similar gradients along their edges.

astro-ph.SR↗

arXiv2017

Models of the Hydrodynamic Histories of post-AGB Stars. I. Multiflow Shaping of OH231.8+04.2

We present a detailed hydrodynamic model that matches the present structure of the well-observed preplanetary nebula OH231.8+04.2. The purpose of the model is to present a physically justified and coherent picture of its evolutionary history from about 100 years of the start of the formation of its complex outer structures to the present. We have adopted a set of initial conditions that are heavily constrained by high-quality observations of its present structure and kinematics. The shaping of the nebula occurs while the densities of the flows are "light": less than the surrounding AGB-wind environment. The simulations show that pairs of essentially coeval clumps and sprays of the same extent and density but different outflow speeds sculpted both the pair of thin axial flow "or spine" and the bulbs. The total ejected mass and momentum in the best fit model are surprisingly large -- 3 M_sun and 2.2 x 10^41 gm cm s-1, respectively -- however, these values are reduced by up to a factor of ten in other models that fit the data almost as well. Our ultimate goal is to combine the present model results of masses, momenta, flow speeds, and flow geometries for OH231 with those of other models to be published in the future in order to find common attributes of their ejection histories.

astro-ph.SR↗

arXiv2016

Hot planetary winds near a star: dynamics, wind-wind interactions, and observational signatures

Signatures of "evaporative" winds from exo-planets on short (hot) orbits around their host star have been observed in a number of systems. In this paper we present global AMR simulations that track the launching of the winds, their expansion through the circumstellar environment, and their interaction with a stellar wind. We focus on purely hydrodynamic flows including the anisotropy of the wind launching and explore the orbital/fluid dynamics of the resulting flows in detail. In particular we find that a combination of the tidal and Coriolis forces strongly distorts the planetary "Parker" wind creating "up-orbit" and "down-orbit" streams. We characterize the flows in terms of their orbital elements which change depending on their launch position on the planet. We find that the anisotropy in the atmospheric temperature leads to significant backflow on to the planet. The planetary wind interacts strongly with the stellar wind creating instabilities that cause eventual deposition of planetary gas onto the star. We present synthetic observations of both transit and absorption line-structure for our simulations. For our initial conditions, we find that the orbiting wind material produces absorption signatures at significant distances from the planet and substantial orbit to orbit variability. Ly-α absorption shows red and blueshifted features out to 70 km/s. Finally, using semi-analytic models we constrain the effect of radiation pressure, given the approximation of uniform stellar absorption.

astro-ph.EP↗

arXiv2004

High-Tc Superconductors - based Nanocomposites with Improved Intergrain Coupling and Enhanced Bulk Pinning

Heterogeneous sonochemical synthesis was used to modify superconducting properties of granular YBa2Ca3CuO7-y and Bi2Sr2CaCu2O8+x. Sonication of liquid-powder alkane slurries produces material with enhanced intergrain coupling and improved current-carrying capabilities. Co-sonication with metals and organometallics results in highly compact nanocomposites with increased magnetic irreversibility. Ultrasonic irradiation of YBa2Ca3CuO7-y carried under partial oxygen atmosphere produces similar morphological effects and increases superconducting transition temperature due to effective surface saturation with oxygen. Detailed chemical and physical characterization of sonochemically prepared high-Tc nanocomposites is presented.

cond-mat.mtrl-sci↗