Progenitors of gravitational wave mergers: Binary evolution with the stellar grid-based code ComBinE (1801.05433v3)

Abstract: The first gravitational wave detections of mergers between black holes and neutron stars represent a remarkable new regime of high-energy transient astrophysics. The signals observed with LIGO-Virgo detectors come from mergers of extreme physical objects which are the end products of stellar evolution in close binary systems. To better understand their origin and merger rates, we have performed binary population syntheses at different metallicities using the new grid-based binary population synthesis code ComBinE. Starting from newborn pairs of stars, we follow their evolution including mass loss, mass transfer and accretion, common envelopes and supernova explosions. We apply the binding energies of common envelopes based on dense grids of detailed stellar structure models, make use of improved investigations of the subsequent Case BB Roche-lobe overflow and scale supernova kicks according to the stripping of the exploding stars. We demonstrate that all the double black hole mergers, GW150914, LVT151012, GW151226, GW170104, GW170608 and GW170814, as well as the double neutron star merger GW170817, are accounted for in our models in the appropriate metallicity regime. Our binary interaction parameters are calibrated to match the accurately determined properties of Galactic double neutron star systems, and we discuss their masses and types of supernova origin. Using our default values for the input physics parameters, we find a double neutron star merger rate of about 3.0 Myr^-1 for Milky-Way equivalent galaxies. Our upper limit to the merger-rate density of double neutron stars is R=400 yr^-1 Gpc^-3 in the local Universe (z=0).

• The paper introduces a grid-based binary evolution model that accurately simulates double compact object formation and key interaction phases.
• The paper demonstrates that predicted merger rates for NS-NS, BH-NS, and BH-BH systems align with LIGO-Virgo observations across diverse metallicity environments.
• The paper highlights the need to refine supernova kick treatments and mass transfer efficiency parameters to improve gravitational wave event predictions.

An Analysis of Binary Evolution and Gravitational Wave Merger Rates

The paper by Kruckow et al. undertakes a comprehensive exploration of the origins and merger rates of gravitational wave (GW) sources, focusing on double compact object (DCO) binaries comprising neutron stars (NSs) and black holes (BHs). These systems play a pivotal role in modern astrophysics as they culminate in high-energy astrophysical phenomena and contribute to the chemical enrichment of the universe. The paper utilizes a novel grid-based binary population synthesis approach to simulate binary star evolution, carefully considering critical interaction phases that affect the lifecycle of such systems.

Key Methodologies and Models

The simulations in the paper track the evolutionary history of binary systems from the zero-age main sequence (ZAMS) through potential phases of mass transfer, common envelope (CE) evolution, supernova (SN) events, and ultimately the formation of DCOs. The synthesis models incorporate a range of metallicity regimes, acknowledging the significant influence of metallicity on stellar wind strength and evolutionary paths. The simulations utilize dense grids of detailed stellar models, which afford greater accuracy in evaluating mass transfer and CE dynamics compared to traditional fitting formulae.

A noteworthy aspect of the methodology is the scaling of supernova kick velocities based on the degree of envelope stripping, thereby introducing a nuanced treatment of post-SN binary dynamics. Additionally, the implementation of mass transfer efficiency and the binding energy parameters during CE evolution is intricately tied to empirical observations of Galactic double NS systems, thus grounding the simulations in observable data.

Numerical Outcomes and Implications

The simulation results demonstrate that all known GW events from LIGO-Virgo, such as GW150914, GW151226, and GW170817, fit within the model's framework across the considered metallicity spectrum. In Milky Way-equivalent galaxies, the paper estimates a double NS merger rate of $\unit{3.0}{\mega^{-1}}$, which is consistent with empirical double NS populations. The paper further identifies BH-NS binaries as potential significant sources of GW events and addresses the challenge of simulating such systems given their rarity and the constraints imposed by mass transfer physics.

Analysis of Merger Rate Densities

At $z=0$, the paper estimates a binary black hole (BBH) merger-rate density of $\unit{0.6}{^{-1}\usk\giga{\rm pc}^{-3}}$ for high-metallicity environments and $\unit{16.8}{^{-1}\usk\giga{\rm pc}^{-3}}$ for low-metallicity regimes. These values align well with empirical LIGO-Virgo constraints, particularly when accounting for potential observational biases towards lower metallicity progenitors for massive BBH mergers. The correlation between local universe merger rates and the observed frequencies at varying redshifts is well analyzed, highlighting the robustness of the modeled evolutionary pathways across cosmic time scales.

Conclusions and Future Prospects

The implications of this research extend beyond merely matching observed GWs events; they offer insights into the efficiency of stellar processes like mass transfer and CE ejection, pivotal for accurately predicting the evolution of massive binaries. A crucial element remains the need to refine models of the mass-transfer efficiency, as current constraints are largely based on NS mass distributions in double NS systems, largely assuming low-efficiency scenarios.

The findings rightfully encourage future research to refine SN kick models further and examine the effects of metallicity more granularly on binary evolution. The advent of future LIGO-Virgo observations will likely provide additional constraints, enabling researchers to recalibrate models to better predict DCO mergers across different cosmic environments. Lastly, elucidating the mechanisms behind rare NS-BH systems remains a key frontier in improving the comprehensive understanding of binary star evolution.