arXiv subjects

Yangyuxin Zou

Publications and source records attributed to Yangyuxin Zou.

5 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.

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arXiv2024

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.

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arXiv2022

Jets from main sequence and white dwarf companions during common envelope evolution

It has long been speculated that jet feedback from accretion onto the companion during a common envelope (CE) event could affect the orbital evolution and envelope unbinding process, but this conjecture has heretofore remained largely untested. We present global 3D hydrodynamical simulations of CE evolution (CEE) that include a jet subgrid model and compare them with an otherwise identical model without a jet. Our binary consists of a $2M_\odot$ red giant branch primary and a $1M_\odot$ or $0.5M_\odot$ main sequence or white dwarf secondary companion modeled as a point particle. We run the simulations for 10 orbits (40 days). Our jet model adds mass at a constant rate $\dot{M}_\mathrm{j}$ of order the Eddington rate, with maximum velocity $v_\mathrm{j}$ of order the escape speed, to two spherical sectors with the jet axis perpendicular to the orbital plane, and supplies kinetic energy at the rate $\sim\dot{M}_\mathrm{j} v_\mathrm{j}^2/40$. We explore the influence of the jet on orbital evolution, envelope morphology and envelope unbinding, and assess the dependence of the results on jet mass-loss rate, launch speed, companion mass, opening angle, and whether or not subgrid accretion is turned on. In line with our theoretical estimates, we find that in all cases the jet becomes choked around the time of first periastron passage. We also find that jets lead to increases in unbound mass of up to $\sim10\%$, as compared to simulations which do not include a jet.

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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.

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arXiv2019

How Drag Force Evolves in Global Common Envelope Simulations

We compute the forces, torque and rate of work on the companion-core binary due to drag in global simulations of common envelope (CE) evolution for three different companion masses. Our simulations help to delineate regimes when conventional analytic drag force approximations are applicable. During and just prior to the first periastron passage of the in-spiral phase, the drag force is reasonably approximated by conventional analytic theory and peaks at values proportional to the companion mass. Good agreement between global and local 3D "wind tunnel" simulations, including similar net drag force and flow pattern, is obtained for comparable regions of parameter space. However, subsequent to the first periastron passage, the drag force is up to an order of magnitude smaller than theoretical predictions, quasi-steady, and depends only weakly on companion mass. The discrepancy is exacerbated for larger companion mass and when the inter-particle separation reduces to the Bondi-Hoyle-Lyttleton accretion radius, creating a turbulent thermalized region. Greater flow symmetry during this phase leads to near balance of opposing gravitational forces in front of and behind the companion, hence a small net drag. The reduced drag force at late times helps explain why companion-core separations necessary for envelope ejection are not reached by the end of limited duration CE simulations.

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