arXiv subjects

Jeff Riley

Publications and source records attributed to Jeff Riley.

12 recordsLinked to original sources

arXiv2026

Modern tidal interaction models for rapid binary population synthesis: II. Binary black hole formation, mergers, and spins

We present predictions for the merger rates and effective spin ($\chi_{\rm eff}$) distribution of binary black holes (BBHs) from isolated binary evolution, using a new self-consistent tidal dissipation implementation in the rapid binary population synthesis code COMPAS. Most of the first-born black holes (BHs) in our simulated merging BBHs are formed with zero spins, with the exception of BBHs formed from chemically homogeneous evolution. The spins of the second-born BHs with the new model depend significantly on the efficiency of tidal dissipation and mass transfer history, and crucially, are not always consistent with pre-supernova synchronization. High-$\chi_{\rm eff}$ binaries preferentially merge at high redshift due to smaller binary separations at BBH formation and shorter coalescence times, thus rendering them largely inaccessible to current gravitational wave (GW) detectors. We expect the intrinsic spin distribution of merging BBHs formed from isolated evolution to be strongly biased toward low $\chi_{\rm eff}$ with current detectors, with a third of systems having $\chi_{\rm eff} < 0.05$ and only $\sim 3\%$ with $\chi_{\rm eff}>0.5$. However, $\chi_{\rm eff}$ will increase as GW detectors become sensitive to higher redshift sources, with up to $\sim 15\%$ of systems having $\chi_{\rm eff}>0.5$.

astro-ph.HE↗

arXiv2026

Electromagnetic, gravitational wave, and static gravitational transmission through throat spacetimes: a constraint-wave asymmetry

We compute the transmission properties of electromagnetic (EM), gravitational wave (GW), and static gravitational perturbations through geometric throats in spherically symmetric spacetimes. On the ultrastatic Ellis-Bronnikov background, decomposition of the four-dimensional Maxwell equations into vector spherical harmonics yields an effective Schr\"odinger problem with centrifugal barrier $V_\ell^{(\mathrm{EM})}=\ell(\ell+1)/(\sigma^2+r_0^2)$ peaked at the throat. For the lowest physical EM mode ($\ell=1$), frequencies below the barrier-top frequency $\omega_{\max}=\sqrt{2}/r_0$ are strongly suppressed by sub-barrier tunnelling. Gravitational wave perturbations ($\ell\ge 2$) see a qualitatively similar barrier and are likewise strongly suppressed below their respective barrier-top frequencies. By contrast, the static gravitational monopole ($\ell=0$), governed by the linearised Einstein equations on the same background, satisfies the source-free conservation law $(a^2\Phi')'=0$ with no potential barrier, yielding the exact solution $\Phi\propto\arctan(\sigma/r_0)$. We extend these results to a one-parameter family of throat geometries with varying profile shapes, and to a reflected-Schwarzschild (Damour-Solodukhin-type) wormhole, demonstrating that the qualitative asymmetry --- strong sub-barrier suppression for all propagating radiation ($\ell\ge 1$) versus polynomial attenuation for the static monopole ($\ell=0$) --- is universal for static, spherically symmetric throats. Numerov integration, WKB estimates, and exact analytical solutions are compared throughout. The results establish a structural constraint-wave asymmetry arising from the multipole decomposition of the field equations, independent of the matter content sourcing the geometry, on a fixed background.

gr-qc↗

arXiv2026

Modern tidal interaction models for rapid binary population synthesis: I. Methods

In this work, we present an updated prescription of contemporary tidal dissipation theory adapted for rapid binary population synthesis. Our simplified expressions encode the dependence of tidal dissipation on stellar structure, stratification, and tidal forcing frequency, while remaining computationally efficient. We implement these prescriptions in the rapid population synthesis code COMPAS, and demonstrate the self-consistent coupling of tides with stellar evolution and binary properties such as orbital periods, spins, and eccentricities for several representative binary systems. When compared with commonly used tidal prescriptions, our equilibrium tidal dissipation efficiencies can be stronger by 1-2 orders of magnitude for low mass main sequence and giant type stars, and dynamical tides can be stronger by 1-7 orders of magnitude due to the explicit dependence on internal stellar structure and the presence of inertial wave dissipation. Despite our simplistic approach, our models agree with detailed stellar simulations to within an order of magnitude across tidal dissipation mechanisms.

astro-ph.SR↗

arXiv2025

Implications of modern mass-loss rates for massive stars

Massive stars lose a significant fraction of their mass through stellar winds at various stages of their lives, including on the main sequence, during the red supergiant phase, and as helium-rich stripped stars. In stellar population synthesis, uncertainty in the mass-loss rates in these evolutionary stages limits our understanding of the formation of black holes and merging compact binaries. In the last decade, the theoretical predictions, simulation, and direct observation of wind mass-loss rates in massive stars have improved significantly, typically leading to a reduction in the predicted mass-loss rates of massive stars. In this paper we explore the astrophysical implications of an updated treatment of winds in the COMPAS population synthesis code. There is a large amount of variation in predicted mass-loss rates for massive red supergiants; some of the prescriptions we implement predict that massive red supergiants are able to lose their hydrogen envelopes through winds alone (providing a possible solution to the so-called missing red supergiant problem), while others predict much lower mass-loss rates that would not strip the hydrogen envelope. We discuss the formation of the most massive stellar-mass black holes in the Galaxy, including the high-mass X-ray binary Cygnus X-1 and the newly discovered Gaia BH3. We find that formation rates of merging binary black holes are sensitive to the mass-loss rate prescriptions, while the formation rates of merging binary neutron stars and neutron-star black hole binaries are more robust to this uncertainty.

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2024

A Trifecta of Modelling Tools: A Bayesian Binary Black Hole Model Selection combining Population Synthesis and Galaxy Formation Models

Gravitational waves (GWs) have revealed surprising properties of binary black hole (BBH) populations, but there is still mystery surrounding how these compact objects evolve. We apply Bayesian inference and an efficient method to calculate the BBH merger rates in the Shark host galaxies, to determine the combination of COMPAS parameters that outputs a population most like the GW sources from the LVK transient catalogue. For our COMPAS models, we calculate the likelihood with and without the dependence on the predicted number of BBH merger events. We find strong correlations between hyper-parameters governing the specific angular momentum (AM) of mass lost during mass transfer, the mass-loss rates of Wolf-Rayet stars via winds and the chemically homogeneous evolution (CHE) formation channel. We conclude that analysing the marginalised and unmarginalised likelihood is a good indicator of whether the population parameters distribution and number of observed events reflect the LVK data. In doing so, we see that the majority of the models preferred in terms of the population-level parameters of the BBHs greatly overpredict the number of events we should have observed to date. Looking at the smaller number of models which perform well with both likelihoods, we find that those with no CHE, AM loss occurring closer to the donor during the first mass-transfer event, and/or higher rates of mass-loss from Wolf-Rayet winds are generally preferred by current data. We find these conclusions to be robust to our choice of selection criteria.

astro-ph.HE↗

arXiv2023

Surrogate forward models for population inference on compact binary mergers

Rapidly growing catalogs of compact binary mergers from advanced gravitational-wave detectors allow us to explore the astrophysics of massive stellar binaries. Merger observations can constrain the uncertain parameters that describe the underlying processes in the evolution of stars and binary systems in population models. In this paper, we demonstrate that binary black hole populations - namely, detection rates, chirp masses, and redshifts - can be used to measure cosmological parameters describing the redshift-dependent star formation rate and metallicity distribution. We present a method that uses artificial neural networks to emulate binary population synthesis computer models, and construct a fast, flexible, parallelisable surrogate model that we use for inference.

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↗

arXiv2020

Massive Stellar Triples Leading to Sequential Binary Black-Hole Mergers in the Field

Stellar triples with massive stellar components are common, and can lead to sequential binary black-hole mergers. Here, we outline the evolution towards these sequential mergers, and explore these events in the context of gravitational-wave astronomy and the pair-instability mass gap. We find that binary black-hole mergers in the pair-instability mass gap can be of triple origin and therefore are not exclusively formed in dense dynamical environments. We discuss the sequential merger scenario in the context of the most massive gravitational-wave sources detected to date: GW170729 and GW190521. We propose that the progenitor of GW170729 is a low-metallicity field triple. We support the premise that GW190521 could not have been formed in the field. We conclude that triple stellar evolution is fundamental in the understanding of gravitational-wave sources, and likely, other energetic transientsas well.

astro-ph.HE↗

arXiv2020

Chemically Homogeneous Evolution: A rapid population synthesis approach

We explore chemically homogeneous evolution (CHE) as a formation channel for massive merging binary black holes (BBHs). We develop methods to include CHE in a rapid binary population synthesis code, Compact Object Mergers: Population Astrophysics and Statistics (COMPAS), which combines realistic models of binary evolution with cosmological models of the star-formation history of the Universe. For the first time, we simultaneously explore conventional isolated binary star evolution under the same set of assumptions. This approach allows us to constrain population properties and make simultaneous predictions about the gravitational-wave detection rates of BBH mergers for the CHE and conventional formation channels. The overall mass distribution of detectable BBHs is consistent with existing gravitational-wave observations. We find that the CHE channel may yield up to ~70% of all gravitational-wave detections of BBH mergers coming from isolated binary evolution.

astro-ph.SR↗

arXiv2020

Binary population synthesis with probabilistic remnant mass and kick prescriptions

We report on the impact of a probabilistic prescription for compact remnant masses and kicks on massive binary population synthesis. We find that this prescription populates the putative mass gap between neutron stars and black holes with low-mass black holes. However, evolutionary effects reduce the number of X-ray binary candidates with low-mass black holes, consistent with the dearth of such systems in the observed sample. We further find that this prescription is consistent with the formation of heavier binary neutron stars such as GW190425, but over-predicts the masses of Galactic double neutron stars. The revised natal kicks, particularly increased ultra-stripped supernova kicks, do not directly explain the observed Galactic double neutron star orbital period--eccentricity distribution. Finally, this prescription allows for the formation of systems similar to the recently discovered extreme mass ratio binary GW190814, but only if we allow for the survival of binaries in which the common envelope is initiated by a donor crossing the Hertzsprung gap, contrary to our standard model.

astro-ph.HE↗

arXiv2020

BARD: A structured technique for group elicitation of Bayesian networks to support analytic reasoning

In many complex, real-world situations, problem solving and decision making require effective reasoning about causation and uncertainty. However, human reasoning in these cases is prone to confusion and error. Bayesian networks (BNs) are an artificial intelligence technology that models uncertain situations, supporting probabilistic and causal reasoning and decision making. However, to date, BN methodologies and software require significant upfront training, do not provide much guidance on the model building process, and do not support collaboratively building BNs. BARD (Bayesian ARgumentation via Delphi) is both a methodology and an expert system that utilises (1) BNs as the underlying structured representations for better argument analysis, (2) a multi-user web-based software platform and Delphi-style social processes to assist with collaboration, and (3) short, high-quality e-courses on demand, a highly structured process to guide BN construction, and a variety of helpful tools to assist in building and reasoning with BNs, including an automated explanation tool to assist effective report writing. The result is an end-to-end online platform, with associated online training, for groups without prior BN expertise to understand and analyse a problem, build a model of its underlying probabilistic causal structure, validate and reason with the causal model, and use it to produce a written analytic report. Initial experimental results demonstrate that BARD aids in problem solving, reasoning and collaboration.

cs.AI↗