arXiv subjects

Mike Y. M. Lau

Publications and source records attributed to Mike Y. M. Lau.

At least 19 recordsLinked to original sources

arXiv2026

Continuous mass ablation of planets engulfed in stellar envelopes

Most stars host short-period planets that are expected to be engulfed during post-main-sequence expansion. The dissolution of engulfed planets has been proposed as a possible mechanism for producing stars enriched in lithium and refractory elements. We perform three-dimensional hydrodynamical simulations of a Jupiter-like planet engulfed within a stellar envelope using the Seven-League Hydro code. Unlike previous studies that represent the planet as a point mass or rigid sphere, we adopt a wind-tunnel setup that resolves the planet's gaseous structure. We find that a continuous mass-ablation process operates during planetary engulfment, contrary to the common assumption that destruction occurs at a specific depth due to ram pressure, tidal forces, or thermal evaporation. The ablation rate scales nearly linearly with the wind momentum flux and is largely insensitive to the Mach number, consistent with an analytical model based on Kelvin-Helmholtz instability developing at the planetary surface. We define efficiency coefficients for drag and ablation, finding pressure-drag coefficients of 0.44-0.56 and smaller ablation efficiencies of 0.054-0.11. Applying these coefficients to a numerically integrated inspiral through a stellar profile, we find that continuous ablation could lead to complete dissolution of the planet within the convective envelope, producing observable lithium enrichment at the stellar surface. Our results provide prescriptions for drag and mass loss that enable large parameter-space studies of planetary engulfment and suggest that chemical enrichment may occur over a broader range of stellar parameters than previously thought.

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arXiv2026

Momentum-conserving self-gravity in the Phantom. Parallel dual tree traversal for the fast multipole method

Tree codes that approximate groups of distant particles with multipole expansions accelerate gravitational force calculations. While momentum-conserving fast multipole methods exist, parallelisation is non-trivial inside a modern SPH framework. We aim for a parallel momentum and angular momentum-conserving Cartesian multipole method for the computation of the gravitational force in smoothed particle hydrodynamics (SPH) with adaptive gravitational force softening in {\sc Phantom}. We modified Dehnen's original dual tree traversal algorithm to fit the parallel architecture of {\sc Phantom} by replicating the node-node interaction on the ancestors of each leaf node in the tree. While this notionally duplicates work, it greatly simplifies the parallelisation of the algorithm. We then recover the $\mathcal{O}(N)$ scaling of the algorithm by use of caches. We also adapt the tree opening criterion for adaptive softening lengths, such that all interactions within the softening kernel are handled pairwise (as in SPH) rather than with multipole expansions, also allowing the gravity calculation to be performed alongside the SPH force evaluation. We demonstrate that the new code conserves linear and angular momentum to machine precision while giving similar force accuracy and better computational performance to the previous (non-symmetric) self-gravity solver in Phantom. Our new fast multipole method is now the default for computing self-gravity in the public code.

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arXiv2026

Three-dimensional simulations of accretion disks in pre-CE systems -- II. Accretion efficiency and angular momentum transport

Rapid mass transfer preceding a common envelope event is a critical yet poorly understood stage of binary stellar evolution, setting the initial conditions for the common envelope inspiral and for the formation of compact binary stars. We present three-dimensional smoothed particle hydrodynamics simulations of accretion disk formation during this phase using the Phantom code. We model the final $21~yr$ of Roche lobe overflow from a $7~M_{\odot}$ red giant onto a $1.4~M_{\odot}$ neutron star companion, with the mass transfer rate prescribed by a 1D MESA model. Over this interval, the mass transfer rate increases from $1.3\times10^{-4}$ to $1.0\times10^{-1}~M_\odot~yr^{-1}$. An accretion disk forms around the neutron star, reading a mass of $5.0\times10^{-3} ~ M_{\odot}$, a radius of $40~R_{\odot}$ and an aspect ratio H/R$\sim 0.1$ by the end of the simulation. A direct comparison with the grid-based simulation of Juarez-Garcia et al. (2025) shows that the two codes produce disk masses that agree to within 6%. The accretion rate onto the neutron star reaches $5.2\times10^{-3}~M_\odot~yr^{-1}$, corresponding to 14% of the mass injection rate and greatly exceeding the Eddington limit. We demonstrate that this accretion rate is consistent with being driven by turbulent angular momentum transport, with an effective viscosity parameter $α_{\rm eff}~=~0.03-0.06$. Ejecta leaving the binary system carries specific angular momentum approximately 90% that of the $L_{2}$ Lagrange point, equivalent to $\sim10$ times the binary's specific orbital angular momentum. This indicates that $L_{2}$ mass loss efficiently reduces the binary orbital separation.

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arXiv2026

Dust Formation in Common Envelope Binary Interactions -- III. Lightcurves

Luminous red novae are transient events thought to arise from common envelope binary interactions. In this paper, we perform post-processing light-curve calculations for two, 3D hydrodynamic simulations of common envelope events. These simulations model interactions between 1.7 $M_\odot$ and 3.7 $M_\odot$ asymptotic giant branch stars and a 0.6 $M_\odot$ compact companion, including dust nucleation. In both our simulations, which are carried out for 44 years under adiabatic conditions, we observe a bright, hot peak lasting $3-5$ years, primarily due to the expansion of the photosphere before and during inspiral. Additional peaks can be seen appearing at different times and for different viewing angles, due to the asymmetry of the interaction. Dust forms about $1-3$ years after the beginning of the simulated interaction and shortly afterwards we witness a sharp decline in the bolometric luminosity, followed by a partial recovery and a plateau with an effective temperature of $\sim$400 K. The dust photosphere reaches a size of $\sim$250 au by the end of the simulations, but we predict that between 100 and 200 years, the dust will become optically thin at visible wavelengths, revealing an inner, warmer photosphere. The lightcurves obtained have two, well-quantified, but large uncertainties: insufficient surface resolution primarily affecting the first 1-2 years of the simulated lightcurves and the adiabatic assumption that affects primarily the later years. We finally contextualise our simulations within a group of observed luminous red nova transients, drawing particular attention to the outburst of OGLE-2002-BLG-360 and AT 2025abao, which are the closest match to our simulation.

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arXiv2026

Thermal-timescale accretion does not always yield critical rotation in mass gainers

Binary evolution plays a central role in producing rapidly rotating stars. Previous studies have shown that mass gainers in binaries can reach critical rotation after accreting only modest amounts of material, particularly during thermal-timescale Case B mass transfer, where tidal spin-down is ineffective due to wide orbits. However, such rapid accretion often drives the mass gainer out of thermal equilibrium, and its subsequent spin evolution during thermal relaxation has not been analysed in depth. In this study, we construct a suite of accreting detailed single-star models with different accretion prescriptions, which inflate and spin up to critical rotation during the accretion. After the accretion has ended, the models relax thermally and deflate. We find that the ratio of surface to critical angular velocity decreases to subcritical values during thermal contraction, with the magnitude of this decrease correlating with the degree of thermal disequilibrium at the end of accretion. This reduction in fractional critical rotation is even stronger when internal angular momentum transport is inefficient. Detailed binary models show the same trend, indicating that the results from our toy single-star models also apply to real binary evolution. Our results highlight that binary mass transfer does not always produce critically rotating stars, but instead may yield a wide range of spin rates depending on the mass transfer and accretion history. Our findings offer new insights into the rotational properties of mass gainers in binaries, stellar merger products, and newly formed massive stars following accretion.

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arXiv2025

1D stellar mergers: entropy sorting and PyMMAMS

Stellar multiple systems are the norm, not the exception, with many systems undergoing interaction phases during their lifetimes. A subset of these interactions can lead to stellar mergers, where the two components of a stellar binary system come close enough to coalesce into a single star. Accurately modeling stellar mergers requires computationally expensive 3D methods, which are not suited for exploring large parameter spaces as required e.g., by population synthesis studies. In this work, we compare two 1D prescriptions based on the concept of entropy sorting to their 3D counterparts. We employ a basic entropy sorting method ('ES'), which builds the merger remnant by sorting the progenitor stars' shells by increasing entropy, and a Python version of the 'Make Me A Massive Star' code ('PM'), which additionally applies a shock-heating prescription calibrated on SPH simulations of stellar head-on collisions. Comparing to a set of 39 more recent SPH head-on collisions different from the ones used for PM calibration, we find that PM reproduces the outcome of these mergers a lot better than ES in terms of thermal and composition structure post-merger. Both 1D methods produce remnants that are rejuvenated more strongly than expected for massive stars, indicating that increased amounts of hydrogen are being mixed into the core. In an effort to further improve PM, we introduce a scaling factor for the shock-heating. We compare 1D models with both down- and up-scaled heating to a 3D MHD $9 + 8\,\mathrm{M_\odot}$ merger of main-sequence stars. Decreasing the shock-heating improves the agreement in terms of the entropy profile, but has only a minor impact on the subsequent stellar evolution of the remnant. We find that 1D methods are able to approximate 3D stellar merger simulations well, and that shock-heating has to be considered to properly predict the post-merger structures.

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arXiv2025

Rapid stellar and binary population synthesis with COMPAS: methods paper II

The COMPAS public rapid binary population synthesis code has undergone a number of key improvements since the original COMPAS methods paper (Team COMPAS: Riley et al., 2022) was published. These include more sophisticated and robust treatments of binary interactions: mass transfer physics, common-envelope events, tides and gravitational-wave radiation reaction; and updated prescriptions for stellar evolution, winds and supernovae. The code structure and outputs have also been updated, with a focus on improving resolution without sacrificing computational speed. This paper describes the substantive changes in the code between the previous methods paper and COMPAS v03.22.01.

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arXiv2025

Common envelopes in massive stars III. The obstructive role of radiation transport in envelope ejection

We present 3D radiation hydrodynamics simulations of common-envelope (CE) evolution involving a 12 solar mass red supergiant donor and a 3 solar mass companion. Existing 3D simulations are predominantly adiabatic, focusing strongly on low-mass donors on the red giant and asymptotic giant branches. However, the adiabatic assumption breaks down once the perturbed CE material becomes optically thin or when entering a longer-timescale evolutionary phase after the dynamical plunge-in. This is especially important for high-mass red supergiant donors, which have short thermal timescales, adding significant uncertainty to our understanding of how massive binary stars evolve into gravitational-wave sources, X-ray binaries, stripped-envelope supernovae, and more. We compare our radiation hydrodynamics simulations with an adiabatic simulation from Paper I that is otherwise identical, finding that radiative diffusion strongly inhibits CE ejection. The fraction of ejected mass is roughly half that of the adiabatic case without accounting for recombination energy release. Almost no material is ejected during the dynamical plunge-in, and longer-timescale ejection during the slow spiral-in is suppressed. However, the orbital separation reached at the end of the dynamical plunge-in does not differ significantly. The large amount of remaining bound mass tentatively supports the emerging view that the dynamical plunge-in is followed by a non-adiabatic phase, during which a substantial fraction of the envelope is ejected and the binary orbit may continue to evolve.

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arXiv2025

Magnetically driven outflows in 3D common-envelope evolution of massive stars

Recent three-dimensional magnetohydrodynamical simulations of the common-envelope interaction revealed the self-consistent formation of bipolar magnetically driven outflows launched from a toroidal structure resembling a circumbinary disk. So far, the dynamical impact of bipolar outflows on the common-envelope phase remains uncertain and we aim to quantify its importance. We illustrate the impact on common-envelope evolution by comparing two simulations -- one with magnetic fields and one without -- using the three-dimensional moving-mesh hydrodynamics code AREPO. We focus on the specific case of a $10 M_\odot$ red supergiant star with a $5 M_\odot$ black hole companion. By the end of the magnetohydrodynamic simulations (after $\sim 1220$ orbits of the core binary system), about $6.4 \%$ of the envelope mass is ejected via the bipolar outflow, contributing to angular momentum extraction from the disk structure and core binary. The resulting enhanced torques reduce the final orbital separation by about $24 \%$ compared to the hydrodynamical scenario, while the overall envelope ejection remains dominated by recombination-driven equatorial winds. We analyze field amplification and outflow launching mechanisms, confirming consistency with earlier studies: magnetic fields are amplified by shear flows, and outflows are launched by a magneto-centrifugal process, supported by local shocks and magnetic pressure gradients. These outflows originate from $\sim 1.1$ times the orbital separation. We conclude that the magnetically driven outflows and their role in the dynamical interaction are a universal aspect, and we further propose an adaptation of the $α_\mathrm{CE}$-formalism by adjusting the final orbital energy with a factor of $1+ M_\mathrm{out}/μ$, where $M_\mathrm{out}$ is the mass ejected through the outflows and $μ$ the reduced mass of the core binary. (abridged)

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arXiv2025

Stellar mergers and common-envelope evolution

Stellar mergers and common-envelope evolution are fast (dynamical-timescale) interactions in binary stars that drastically alter their evolution. They are key to understanding a plethora of astrophysical phenomena. Stellar mergers are thought to produce blue straggler stars, blue supergiants, and stars with peculiar rotation and surface chemical abundances. Common-envelope evolution is proposed as a key stage in the formation of gravitational wave sources, X-ray binaries, type Ia supernovae, cataclysmic variables, and other systems. A significant fraction (tens of percent) of binary stars undergo such a phase during their evolution. In this chapter, we first discuss processes leading to a stellar merger or common-envelope phase. We then explain these complex interactions, starting from underlying physical principles like entropy sorting in stellar mergers and the energy formalism in common envelopes. This is followed by a more complete picture revealed by three-dimensional (magneto)hydrodynamical simulations. The outcomes of these interactions are discussed comprehensively and special emphasis is given to the role of magnetic fields. Both stellar mergers and common-envelope evolution remain far from fully understood, and we conclude by highlighting open questions in their study.

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arXiv2025

Hot Jupiter engulfment by a red giant in 3D hydrodynamics

Hot Jupiters are gas giant planets with orbital periods of a few days and are found in 0.1-1% of Sun-like stars. They are expected to be engulfed during their host star's radial expansion on the red giant branch, which may account for observed rapidly rotating and chemically enriched giant stars. We performed 3D hydrodynamical simulations of hot Jupiter engulfment by a 1 solar mass, 4 solar radii early red giant. Our 'global' simulations simultaneously resolve the stellar envelope and planetary structure, modelling the hot Jupiter as a polytropic gas sphere. The hot Jupiter spirals in due to ram-pressure drag. A substantial fraction of its mass is continuously ablated in this process, although the mass-loss rate is resolution dependent. We estimate that this could enhance the surface lithium abundance by up to 0.1 dex. The hot Jupiter is disrupted by a combination of ram pressure and tidal forces near the base of the convective envelope, with the deepest material penetrating to the radiative zone. The star experiences modest spin-up (~ 1 km/s), and engulfing a more massive companion may be required to produce a rapidly rotating giant. Drag heating near the surface and hydrogen recombination in the small amount of unbound ejecta recorded in the simulation could power an optical transient, although this needs to be confirmed by a calculation that has adequate resolution at the stellar surface.

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arXiv2024

Dust formation in common envelope binary interactions -- II: 3D simulations with self-consistent dust formation

We performed numerical simulations of the common envelope (CE) interaction between thermally-pulsing asymptotic giant branch (AGB) stars of 1.7~\Msun and 3.7~\Msun, respectively, and a 0.6~\Msun compact companion. We use tabulated equations of state to take into account recombination energy. For the first time, formation and growth of dust is calculated explicitly, using a carbon dust nucleation network with a C/O abundance ratio of 2.5 (by number). The first dust grains appear within $\sim$1--3~yrs after the onset of the CE, forming an optically thick shell at $\sim$10--20~au, growing in thickness and radius to values of $\sim$400--500~au over $\sim$40~yrs, with temperatures around 400~K. Most dust is formed in unbound material, having little effect on mass ejection or orbital evolution. By the end of the simulations, the total dust yield is $\sim8.4\times10^{-3}$~\Msun and $\sim2.2\times10^{-2}$~\Msun for the CE with a 1.7~\Msun and a 3.7~\Msun AGB star, respectively, corresponding to a nucleation efficiency close to 100\%, if no dust destruction mechanism is considered. Despite comparable dust yields to single AGB stars, \textit{in CE ejections the dust forms a thousand times faster, over tens of years as opposed to tens of thousands of years}. This rapid dust formation may account for the shift in the infrared of the spectral energy distribution of some optical transients known as luminous red novae. Simulated dusty CEs support the idea that extreme carbon stars and "water fountains" may be objects observed after a CE event.

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arXiv2024

Dust formation during the interaction of binary stars by common envelope

We performed numerical simulations of the common envelope (CE) interaction between two intermediate-mass asymptotic giant branch (AGB) stars and their low-mass companions. For the first time, formation and growth of dust in the envelope is calculated explicitly. We find that the first dust grains appear as early as $\sim$1-3 yrs after the onset of the CE, and are smaller than grains formed later. As the simulations progress, a high-opacity dusty shell forms, resulting in the CE photosphere being up to an order of magnitude larger than it would be without the inclusion of dust. At the end of the simulations, the total dust yield is $0.0082~M_{\odot}$ ($0.022~M_{\odot}$) for a CE with a $1.7~M_{\odot}$ ($3.7~M_{\odot}$) AGB star. Dust formation does not substantially lead to more mass unbinding or substantially alter the orbital evolution.

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arXiv2024

Expansion of accreting main-sequence stars during rapid mass transfer

Accreting main-sequence stars expand significantly when the mass accretion timescale is much shorter than their thermal timescales. This occurs during mass transfer from an evolved giant star onto a main-sequence companion in a binary system, and is an important phase in the formation of compact binaries including X-ray binaries, cataclysmic variables, and gravitational-wave sources. In this study, we compute 1D stellar models of main-sequence accretors with different initial masses and accretion rates. The calculations are used to derive semi-analytical approximations to the maximum expansion radius. We assume that mass transfer remains fully conservative as long as the inflated accretor fits within its Roche lobe, leading stars to behave like hamsters, stuffing excess material behind their expanding cheeks. We suggest a physically motivated prescription for the mass growth of such "hamstars", which can be used to determine mass-transfer efficiency in rapid binary population synthesis models. With this prescription, we estimate that progenitors of high-mass X-ray binaries and gravitational-wave sources may have experienced highly non-conservative mass transfer. In contrast, for low-mass accretors, the accretion timescale can exceed the thermal timescale by a larger factor without causing significant radial expansion.

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arXiv2022

Common envelope binary interaction simulations between a thermally-pulsating AGB star and a low mass companion

At least one in five of all planetary nebulae are the product of a common envelope (CE) interaction, where the companion in-spirals into the envelope of an asymptotic giant branch (AGB) star ejecting the nebula and leaving behind a compact binary. In this work we carry out 3D smoothed particle hydrodynamics simulations of the CE interaction between a 1.7 $M_{\odot}$ AGB star and a 0.6 $M_{\odot}$ companion. We model the AGB structure using a 1D stellar model taken at the seventh thermal pulse. The interaction takes place when the giant is on the expanding phase of the seventh thermal pulse and has a radius of 250 $R_{\odot}$. The post-CE orbital separations varies between 20 and 31 $R_{\odot}$, with the inclusion of recombination energy resulting in wider separations. Based on the observed short in-spiral time-scales, we suggest that thermal pulses can trigger CEs, extending the ability of AGB stars to capture companions into CEs, that would lead to the prediction of a larger population of post-AGB, post-CE binaries. Simulations that include a tabulated equation of state unbind a great deal more gas, likely unbinding the entire envelope on short time-scales. The shape of the CE after the in-spiral is more spherical for AGB than red giant branch stars, and even more so if recombination energy is included. We expect that the planetary nebula formed from this CE will have different features from those predicted by Zou et al. 2020.

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arXiv2022

Common envelopes in massive stars II: The distinct roles of hydrogen and helium recombination

The role of recombination during a common-envelope event has been long debated. Many studies have argued that much of hydrogen recombination energy, which is radiated in relatively cool and optically-thin layers, might not thermalise in the envelope. On the other hand, helium recombination contains 30% of the total recombination energy, and occurs much deeper in the stellar envelope. We investigate the distinct roles played by hydrogen and helium recombination in a common-envelope interaction experienced by a 12 solar mass red supergiant donor. We perform adiabatic, 3D hydrodynamical simulations that (i) include hydrogen, helium, and molecular hydrogen recombination, (ii) include hydrogen and helium recombination, (iii) include only helium recombination, and (iv) do not include recombination energy. By comparing these simulations, we find that the addition of helium recombination energy alone ejects 30% more envelope mass, and leads to a 16% larger post-plunge-in separation. Under the adiabatic assumption, adding hydrogen recombination energy increases the amount of ejected mass by a further 40%, possibly unbinding the entire envelope, but does not affect the post-plunge separation. Most of the ejecta becomes unbound at relatively high (>70%) degrees of hydrogen ionisation, where the hydrogen recombination energy is likely to expand the envelope instead of being radiated away.

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arXiv2022

Common envelopes in massive stars: Towards the role of radiation pressure and recombination energy in ejecting red supergiant envelopes

We perform 3D hydrodynamical simulations of a common-envelope event involving a 12 solar mass red supergiant donor. Massive stars are expected to be qualitatively different from low-mass stars as their envelopes have significant support from radiation pressure, which increases both the final separation and amount of mass ejected through the common-envelope interaction. We perform adiabatic simulations that include radiation energy through the equation of state, which results in ejecting 60 per cent more mass (up to two thirds of the total envelope mass becoming unbound, or more) and yield a 10 per cent larger final separation compared to simulations that assume an ideal gas. When also including recombination energy, we find that at least three quarters of the envelope, and possibly the entire envelope, may be unbound. The final separation further increases by almost 20 per cent. The additional amount of ejected material is mainly due to energy injected from helium recombination. Hydrogen recombination plays a comparatively small role, as it mainly occurs in gas that has already become unbound. We conclude that the internal energy of the envelope can be a significant energy source for ejecting the common envelope, but ultimately radiation transport and convection need to be included.

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arXiv2021

Rapid stellar and binary population synthesis with COMPAS

Compact Object Mergers: Population Astrophysics and Statistics (COMPAS; https://compas.science) is a public rapid binary population synthesis code. COMPAS generates populations of isolated stellar binaries under a set of parametrized assumptions in order to allow comparisons against observational data sets, such as those coming from gravitational-wave observations of merging compact remnants. It includes a number of tools for population processing in addition to the core binary evolution components. COMPAS is publicly available via the github repository https://github.com/TeamCOMPAS/COMPAS/, and is designed to allow for flexible modifications as evolutionary models improve. This paper describes the methodology and implementation of COMPAS. It is a living document which will be updated as new features are added to COMPAS; the current document describes COMPAS v02.21.00.

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