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Ilya Mandel

Publications and source records attributed to Ilya Mandel.

At least 73 records · Page 4Linked to original sources
arXiv2022

Empirical Stability Boundary for Hierarchical Triples

The three-body problem is famously chaotic, with no closed-form analytical solutions. However, hierarchical systems of three or more bodies can be stable over indefinite timescales. A system is considered hierarchical if the bodies can be divided into separate two-body orbits with distinct time- and length-scales, such that one orbit is only mildly affected by the gravitation of the other bodies. Previous work has mapped the stability of such systems at varying resolutions over a limited range of parameters, and attempts have been made to derive analytic and semi-analytic stability boundary fits to explain the observed phenomena. Certain regimes are understood relatively well. However, there are large regions of the parameter space which remain un-mapped, and for which the stability boundary is poorly understood. We present a comprehensive numerical study of the stability boundary of hierarchical triples over a range of initial parameters. Specifically, we consider the mass ratio of the inner binary to the outer third body ($q_{\rm out}$), mutual inclination ($i$), initial mean anomaly and eccentricity of both the inner and outer binaries ($e_{\rm in}$ and $e_{\rm out}$ respectively). We fit the dependence of the stability boundary on $q_{\rm out}$ as a threshold on the ratio of the inner binary's semi-major axis to the outer binary's pericentre separation $a_{\rm in}/R_{\rm p, out} \leq 10^{-0.6 + 0.04q_{\rm out}}q_{\rm out}^{0.32+0.1q_{\rm out}}$ for coplanar prograde systems. We develop an additional factor to account for mutual inclination. The resulting fit predicts the stability of $10^4$ orbits randomly initialised close to the stability boundary with $87.7\%$ accuracy.

astro-ph.SR↗
arXiv2022

A two-stage formalism for common-envelope phases of massive stars

We propose a new simple formalism to predict the orbital separations after common-envelope phases with massive star donors. We focus on the fact that massive red supergiants tend to have a sizeable radiative layer between the dense helium core and the convective envelope. Our formalism treats the common-envelope phase in two stages: dynamical in-spiral through the outer convective envelope and thermal timescale mass transfer from the radiative intershell. With fiducial choices of parameters, the new formalism typically predicts much wider separations compared to the classical energy formalism. Moreover, our formalism predicts that final separations strongly depend on the donor evolutionary stage and companion mass. Our formalism provides a physically-motivated alternative option for population synthesis studies to treat common-envelope evolution. This treatment will impact on predictions for massive-star binaries, including gravitational-wave sources, X-ray binaries and stripped-envelope supernovae.

astro-ph.SR↗
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.

astro-ph.SR↗
arXiv2022

Rates of Compact Object Coalescences

Gravitational-wave detections are enabling measurements of the rate of coalescences of binaries composed of two compact objects -- neutron stars and/or black holes. The coalescence rate of binaries containing neutron stars is further constrained by electromagnetic observations, including Galactic radio binary pulsars and short gamma-ray bursts. Meanwhile, increasingly sophisticated models of compact objects merging through a variety of evolutionary channels produce a range of theoretically predicted rates. Rapid improvements in instrument sensitivity, along with plans for new and improved surveys, make this an opportune time to summarise the existing observational and theoretical knowledge of compact-binary coalescence rates.

astro-ph.HE↗
arXiv2022

Extreme eccentricities of triple systems: Analytic results

Triple stars and compact objects are ubiquitously observed in nature. Their long-term evolution is complex; in particular, the von-Zeipel-Lidov-Kozai (ZLK) mechanism can potentially lead to highly eccentric encounters of the inner binary. Such encounters can lead to a plethora of interacting binary phenomena, as well as stellar and compact-object mergers. Here we find explicit analytical formulae for the maximal eccentricity, $e_{\rm max}$, of the inner binary undergoing ZLK oscillations, where both the test particle limit (parametrised by the inner-to-outer angular momentum ratio $η$) and the double-averaging approximation (parametrised by the period ratio, $ε_{\rm SA}$) are relaxed, for circular outer orbits. We recover known results in both limiting cases (either $η$ or $ε_{\rm SA} \to 0$) and verify the validity of our model using numerical simulations. We test our results with two accurate numerical N-body codes, $\texttt{Rebound}$ for Newtonian dynamics and $\texttt{Tsunami}$ for general-relativistic (GR) dynamics, and find excellent correspondence. We discuss the implications of our results for stellar triples and both stellar and supermassive triple black hole mergers.

astro-ph.EP↗
arXiv2022

Building better spin models for merging binary black holes: Evidence for non-spinning and rapidly spinning nearly aligned sub-populations

Recent work paints a conflicting portrait of the distribution of black hole spins in merging binaries measured with gravitational waves. Some analyses find that a significant fraction of merging binaries contain at least one black hole with a spin tilt $>90^\circ$ with respect to the orbital angular momentum vector, which has been interpreted as a signature for dynamical assembly. Other analyses find the data are consistent with a bimodal population in which some binaries contain black holes with negligible spin while the rest contain black holes with spin vectors preferentially aligned with the orbital angular momentum vector. In this work, we scrutinize models for the distribution of black hole spins to pinpoint possible failure modes in which the model yields a faulty conclusion. We reanalyze data from the second LIGO--Virgo gravitational-wave transient catalog (GWTC-2) using a revised spin model, which allows for a sub-population of black holes with negligible spins. In agreement with recent results by Roulet et al., we show that the GWTC-2 detections are consistent with two distinct sub-populations. We estimate that $29-75\%$ (90\% credible interval) of merging binaries contain black holes with negligible spin $χ\approx 0$. The remaining binaries are part of a second sub-population in which the spin vectors are preferentially (but not exactly) aligned to the orbital angular momentum. The black holes in this second sub-population are characterized by spins of $χ\sim0.45$. We suggest that the inferred spin distribution is consistent with the hypothesis that all merging binaries form via the field formation scenario.

gr-qc↗
arXiv2022

Stellar response after stripping as a model for common-envelope outcomes

Binary neutron stars have been observed as millisecond pulsars, gravitational-wave sources, and as the progenitors of short gamma-ray bursts and kilonovae. Massive stellar binaries that evolve into merging double neutron stars are believed to experience a common-envelope episode. During this episode, the envelope of a giant star engulfs the whole binary. The energy transferred from the orbit to the envelope by drag forces or from other energy sources can eject the envelope from the binary system, leading to a stripped short-period binary. In this paper, we use one-dimensional single stellar evolution to explore the final stages of the common-envelope phase in progenitors of neutron star binaries. We consider an instantaneously stripped donor star as a proxy for the common-envelope phase and study the star's subsequent radial evolution. We determine a range of stripping boundaries which allow the star to avoid significant rapid re-expansion and which thus represent plausible boundaries for the termination of the common-envelope episode. We find that these boundaries lie above the maximum compression point, a commonly used location of the core/envelope boundary. We conclude that stars may retain fractions of a solar mass of hydrogen-rich material even after the common-envelope episode. If we consider orbital energy as the only energy source available, all of our models would overfill their Roche lobe after ejecting the envelope, whose binding energy includes gravitational, thermal, radiation, and recombination energy terms.

astro-ph.HE↗
arXiv2022

Merging stellar-mass binary black holes

The LIGO and Virgo detectors have directly observed gravitational waves from mergers of pairs of stellar-mass black holes, along with a smaller number of mergers involving neutron stars. These observations raise the hope that compact object mergers could be used as a probe of stellar and binary evolution, and perhaps of stellar dynamics. This colloquium-style article summarises the existing observations, describes theoretical predictions for formation channels of merging stellar-mass black-hole binaries along with their rates and observable properties, and presents some prospects for gravitational-wave astronomy.

astro-ph.HE↗
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.

astro-ph.SR↗
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.

astro-ph.IM↗
arXiv2021

Impact of Massive Binary Star and Cosmic Evolution on Gravitational Wave Observations II: Double Compact Object Rates and Properties

Making the most of the rapidly increasing population of gravitational-wave detections of black hole (BH) and neutron star (NS) mergers requires comparing observations with population synthesis predictions. In this work we investigate the combined impact from the key uncertainties in population synthesis modelling of the isolated binary evolution channel: the physical processes in massive binary-star evolution and the star formation history as a function of metallicity, $Z$, and redshift $z, \mathcal{S}(Z,z)$. Considering these uncertainties we create 560 different publicly available model realizations and calculate the rate and distribution characteristics of detectable BHBH, BHNS, and NSNS mergers. We find that our stellar evolution and $\mathcal{S}(Z,z)$ variations can impact the predicted intrinsic and detectable merger rates by factors $10^2$-$10^4$. We find that BHBH rates are dominantly impacted by $\mathcal{S}(Z,z)$ variations, NSNS rates by stellar evolution variations and BHNS rates by both. We then consider the combined impact from all uncertainties considered in this work on the detectable mass distribution shapes (chirp mass, individual masses and mass ratio). We find that the BHNS mass distributions are predominantly impacted by massive binary-star evolution changes. For BHBH and NSNS we find that both uncertainties are important. We also find that the shape of the delay time and birth metallicity distributions are typically dominated by the choice of $\mathcal{S}(Z,z)$ for BHBH, BHNS and NSNS. We identify several examples of robust features in the mass distributions predicted by all 560 models, such that we expect more than 95% of BHBH detections to contain a BH $\gtrsim 8\,\rm{M}_{\odot}$ and have mass ratios $\lesssim 4$. Our work demonstrates that it is essential to consider a wide range of allowed models to study double compact object merger rates and properties.

astro-ph.HE↗
arXiv2021

The Next Generation Global Gravitational Wave Observatory: The Science Book

The next generation of ground-based gravitational-wave detectors will observe coalescences of black holes and neutron stars throughout the cosmos, thousands of them with exceptional fidelity. The Science Book is the result of a 3-year effort to study the science capabilities of networks of next generation detectors. Such networks would make it possible to address unsolved problems in numerous areas of physics and astronomy, from Cosmology to Beyond the Standard Model of particle physics, and how they could provide insights into workings of strongly gravitating systems, astrophysics of compact objects and the nature of dense matter. It is inevitable that observatories of such depth and finesse will make new discoveries inaccessible to other windows of observation. In addition to laying out the rich science potential of the next generation of detectors, this report provides specific science targets in five different areas in physics and astronomy and the sensitivity requirements to accomplish those science goals. This report is the second in a six part series of reports by the GWIC 3G Subcommittee: i) Expanding the Reach of Gravitational Wave Observatories to the Edge of the Universe, ii) The Next Generation Global Gravitational Wave Observatory: The Science Book (this report), iii) 3G R&D: R&D for the Next Generation of Ground-based Gravitational Wave Detectors, iv) Gravitational Wave Data Analysis: Computing Challenges in the 3G Era, v) Future Ground-based Gravitational-wave Observatories: Synergies with Other Scientific Communities, and vi) An Exploration of Possible Governance Models for the Future Global Gravitational-Wave Observatory Network.

gr-qc↗
arXiv2021

GW200115: a non-spinning black hole -- neutron star merger

GW200115 was the second merger of a black hole and a neutron star confidently detected through gravitational waves. Inference on the signal allows for a large black hole spin misaligned with the orbital angular momentum, but shows little support for aligned spin values. We show that this is a natural consequence of measuring the parameters of a black hole -- neutron star binary with non-spinning components while assuming the priors used in the LIGO-Virgo-KAGRA analysis. We suggest that, a priori, a non-spinning binary is more consistent with current astrophysical understanding.

astro-ph.HE↗
arXiv2021

Constraints on Weak Supernova Kicks from Observed Pulsar Velocities

Observations of binary pulsars and pulsars in globular clusters suggest that at least some pulsars must receive weak natal kicks at birth. If all pulsars received strong natal kicks above \unit[50]{\kms}, those born in globular clusters would predominantly escape, while wide binaries would be disrupted. On the other hand, observations of transverse velocities of isolated radio pulsars indicate that only $5\pm2\%$ have velocities below \unit[50]{\kms}. We explore this apparent tension with rapid binary population synthesis modelling. We propose a model in which supernovae with characteristically low natal kicks (e.g., electron-capture supernovae) only occur if the progenitor star has been stripped via binary interaction with a companion. We show that this model naturally reproduces the observed pulsar speed distribution and without reducing the predicted merging double neutron star yield. We estimate that the zero-age main sequence mass range for non-interacting progenitors of electron-capture supernovae should be no wider than ${\approx}0.2 M_\odot$.

astro-ph.HE↗
arXiv2021

Conditions for accretion disc formation and observability of wind-accreting X-ray binaries

We explore the effect of anisotropic wind driving on the properties of accretion onto black holes in close binaries. We specifically focus on line-driven winds, which are common in high-mass X-ray binaries. In close binary systems, the tidal force from the companion star can modify the wind structure in two different ways. One is the reduction of wind terminal velocity due to the weaker effective surface gravity. The other is the reduction in mass flux due to gravity darkening. We incorporate these effects into the so-called CAK theory in a simple way and investigate the wind flow around the accretor on the orbital scale. We find that a focused accretion stream is naturally formed when the Roche lobe filling factor is $\gtrsim0.8$-0.9, analogous to that of wind Roche lobe overflow, but only when the velocity reduction is taken into account. The formation of a stream is necessary to bring in sufficient angular momentum to form an accretion disc around the black hole. Gravity darkening effects reduce the amount of accreted angular momentum, but not enough to prevent the formation of a disc. Based on these results, we expect there to be a discrete step in the observability of high-mass X-ray binaries depending on whether the donor Roche lobe filling factor is below or above $\sim$0.8-0.9.

astro-ph.HE↗
arXiv2021

Impact of Massive Binary Star and Cosmic Evolution on Gravitational Wave Observations I: Black Hole-Neutron Star Mergers

Mergers of black hole-neutron star (BHNS) binaries have now been observed by GW detectors with the recent announcement of GW200105 and GW200115. Such observations not only provide confirmation that these systems exist, but will also give unique insights into the death of massive stars, the evolution of binary systems and their possible association with gamma-ray bursts, $r$-process enrichment and kilonovae. Here we perform binary population synthesis of isolated BHNS systems in order to present their merger rate and characteristics for ground-based GW observatories. We present the results for 420 different model permutations that explore key uncertainties in our assumptions about massive binary star evolution (e.g. mass transfer, common-envelope evolution, supernovae), and the metallicity-specific star formation rate density, and characterize their relative impacts on our predictions. We find intrinsic local BHNS merger rates spanning $\mathcal{R}_{\rm{m}}^0 \approx 4$-$830\,\rm{Gpc}^{-3}\,\rm{yr}^{-1}$ for our full range of assumptions. This encompasses the rate inferred from recent BHNS GW detections, and would yield detection rates of $\mathcal{R}_{\rm{det}} \approx 1$-$180\, \rm{yr}^{-1}$ for a GW network consisting of LIGO, Virgo and KAGRA at design sensitivity. We find that the binary evolution and metallicity-specific star formation rate density each impact the predicted merger rates by order $\mathcal{O}(10)$. We also present predictions for the GW detected BHNS merger properties and find that all 420 model variations predict that $\lesssim 5\%$ of the BHNS mergers have BH masses $\gtrsim 18\,M_{\odot}$, total masses $ \gtrsim 20\,M_{\odot}$, chirp masses $\gtrsim 5.5\,M_{\odot}$, mass ratios $ \gtrsim 12$ or $\lesssim 2$. Moreover, we find that massive NSs $\gtrsim 2\,M_{\odot}$ are expected to be commonly detected in BHNS mergers in almost all our model variations.

astro-ph.HE↗
arXiv2021

The impact of the COVID-19 pandemic on academic productivity

'Publish or perish' is an expression describing the pressure on academics to consistently publish research to ensure a successful career in academia. With a global pandemic that has changed the world, how has it changed academic productivity? Here we show that academics are posting just as many publications on the arXiv pre-print server as if there were no pandemic: 168,630 were posted in 2020, a +12.6% change from 2019 and $+1.4σ$ deviation above the predicted 162,577 $\pm$ 4,393. However, some immediate impacts are visible in individual research fields. Conference cancellations have led to sharp drops in pre-prints, but laboratory closures have had mixed effects. Only some experimental fields show mild declines in outputs, with most being consistent on previous years or even increasing above model expectations. The most significant change is a 50% increase ($+8σ$) in quantitative biology research, all related to the COVID-19 pandemic. Some of these publications are by biologists using arXiv for the first time, and some are written by researchers from other fields (e.g., physicists, mathematicians). While quantitative biology pre-prints have returned to pre-pandemic levels, 20% of the research in this field is now focussed on the COVID-19 pandemic, demonstrating a strong shift in research focus.

cs.DL↗