Lucky Strikes: On the Origins of GW190814 Through Isolated Binary Evolution

Abstract: The asymmetric nature of GW190814, particularly its mass ratio ($q \approx 1/10$), has made its astrophysical origin elusive. We explore isolated binary evolution as a potential explanation for GW190814's formation. Using the binary population synthesis code COSMIC, and the backpop sampling technique to map the observed parameters of GW190814 to the initial conditions of Zero Age Main Sequence binary stars while simultaneously inferring the astrophysical prescriptions for common envelope evolution, stable mass transfer and natal kick kinematics that are needed for its formation and eventual merger. We find that the initial conditions for the binary stellar population that forms GW190814 do not stand out significantly from massive star populations observed in the Local Group. Our backpop simulations recover a dominant formation pathway where the first Roche overflow phase includes a common envelope evolution and the second Roche overflow phase remains stable. Our findings suggest that natal kicks imparted during compact object formation play the strongest role in forming GW190814-like systems. Specifically, our models require a low magnitude first natal kick (independent of direction) that prevents the binary from unbinding and a large second natal kick with its direction in the plane of the orbit and toward the binary's center of mass. The second natal kick strength and direction crucially increases the orbital eccentricity, leading to shorter delay times, and thus enabling mergers within a Hubble time. We estimate the chance probability for GW190814-like events that experience such a lucky kick and find that it occurs in $\sim20\%$ of systems if natal kicks are randomly oriented. We discuss the astrophysical implications for the formation of asymmetric GW190814-like systems under the context of binary stellar evolution.

• The paper demonstrates that GW190814-like binaries form via isolated evolution with precise natal kick geometries enabling rapid merger.
• It employs the COSMIC code and BackPop statistical framework to explore a 17-dimensional parameter space and reconstruct progenitor conditions.
• Results indicate that common envelope evolution and sub-solar metallicities are crucial for successful mergers, highlighting the interplay of binary interactions.

Formation of GW190814 via Isolated Binary Evolution: Quantitative Reconstruction and Astrophysical Implications

Introduction

The detection of GW190814 presents a major challenge for models of compact binary coalescence, exhibiting an extreme mass ratio ($q \approx 0.1$) between a $23 \, M_\odot$ black hole and a $2.6 \, M_\odot$ compact object that cannot be readily classified [Abbott2020]. Previous population synthesis calculations and dynamical scenarios have struggled to reproduce such asymmetric mergers at non-negligible rates. This study utilizes the COSMIC synthesis code and the BackPop statistical framework to investigate whether isolated binary stellar evolution can account for GW190814. The paper rigorously quantifies the initial zero-age main sequence (ZAMS) conditions, binary evolution hyperparameters, and supernova natal kick properties necessary for the formation and merger of GW190814-like systems.

BackPop Statistical Inference and Evolutionary Modeling

The BackPop approach performs backward inference from GW observables to the underlying 17-dimensional model space of ZAMS masses, orbital parameters, metallicity, common-envelope efficiency, stable mass transfer limits, and kick kinematics for each compact object formation. Nautilus nested sampling is employed to efficiently navigate this high dimensional space, leveraging neural network acceleration to target rare BBH mergers consistent with GW190814 and mitigate sampling plateaus from unbound or long-delay binaries.

The COSMIC code deterministically evolves ZAMS binaries according to sampled physical hyperparameters. The study adopts wide uniform priors within observationally and physically plausible domains for all model variables to avoid prior-driven biases and ensure a physically interpretable statistical posterior.

Progenitor Parameter Space: Initial Conditions and Binary Interaction Hyperparameters

Posterior inference reveals that GW190814-compatible progenitors have ZAMS primary masses of $75$-$85 \, M_\odot$ and secondaries of $20$-$23 \, M_\odot$, with mass ratios preferentially $q_{ZAMS} \simeq 0.29$. The metallicity is sub-solar, peaking near $0.1 \, Z_\odot$. Initial orbital separations favor configurations allowing Roche-lobe overflow during the Hertzsprung Gap, with a slight inclination to wider orbits.

Figure 1: Posterior distribution for ZAMS initial conditions and hyperparameters yielding GW190814-like mergers, with a preference for finely tuned $\alpha \lambda_\mathrm{CE}$ values and mass transfer limits.

The interaction phase is characterized by a transition from short, stable mass transfer to common envelope (CE) evolution during the primary's core helium burning. Key hyperparameter correlations include $\alpha \lambda_\mathrm{CE,1} \sim 2$: wider initial orbits require greater CE ejection efficiency for binary survival. The secondary RLOF episode is always stable, exhibiting broad accretion efficiency posteriors due to Eddington-limited mass transfer.

Dominant Evolutionary Channel and Merger Requirements

The reconstructed formation pathway is summarized in a schematic that traces the full stellar evolution and binary interaction sequence necessary for GW190814-like mergers.

Figure 2: Evolutionary channel for GW190814-like binaries: initial mass transfer followed by CE evolution, low-kick BH formation, stable RLOF, high-kick secondary compact object formation, and merger within a Hubble time.

After CE, the first compact object (the BH) forms with a low natal kick to preserve binary binding. The secondary (with comparable mass) donates matter during Eddington-limited stable RLOF, minimally increasing BH mass. Formation of the second compact object requires a "lucky strike" -- a strong natal kick ($v_2 \gtrsim 150 \, \mathrm{km\,s}^{-1}$) delivered in the orbital plane, perpendicular to motion, to impart high post-SN eccentricity ($e \gtrsim 0.86$), enabling GW emission to shorten the delay time below a Hubble time.

Natal Kick Distribution and Merger Likelihood

Natal kick analysis demonstrates that the merger is contingent not only on kick magnitude but also on precise geometric orientation.

Figure 3: Marginalized posterior for second SN natal kick parameters: only a narrow range of kick magnitude and geometry produces mergers, implying low probability for the required configuration.

Approximately $20\%$ of systems experience the necessary directional configuration ($\delta \Omega \approx 2.57 \, \rm{sr}$ solid angle for the kick), given isotropic kicks. The rarity of such kicks, rather than initial binary conditions or generic evolutionary channels, dominates the statistical history of observed GW190814-like systems.

Robustness and Comparative Simulations

The study validates its channel and parameter inference by comparing simulations that fix the first BH's natal kick to zero. Both 17-parameter (free kick) and 13-parameter (no BH kick) spaces yield qualitatively similar outcomes for the chirp mass and mass ratio distributions, with only minor quantitative differences in the mass ratio extension.

Figure 4: Simulated chirp mass and mass ratio posteriors compared for unconstrained kick and zero-kick BH formation channels, both consistent with GW190814 observational posteriors.

Astrophysical Interpretation and Population Constraints

Analysis shows the preferred ZAMS mass ratios and metallicities are consistent with observed populations in the Local Group, though precise matches to $80\,M_\odot$ stars at $0.1\,Z_\odot$ are rare but observed. The necessity of CE evolution during the HG phase introduces sensitivity to the mass-loss and CE formalism; detailed stellar evolution models may modify envelope binding parameters ($\lambda_\mathrm{CE}$), potentially increasing merger or widening rates.

Stable mass transfer throughout first RLOF remains an alternative, but the channel reconstructed herein indicates orbital shrinkage through CE is crucial for bringing the binary to a separations compatible with post-SN merger. High-kick formation for $2-5\,M_\odot$ compact objects is supported by recent multi-dimensional core-collapse models, though the preferred magnitude here ($v_{k,2} \simeq 220\,\mathrm{km\,s}^{-1}$) is less than values of $300-1000\,\mathrm{km\,s}^{-1}$ predicted for some SN outcomes.

Full Posterior Landscape

For completeness, the study includes full multidimensional posterior distributions for both unconstrained and zero-kick scenarios, reaffirming the specific parameter correlations and channel dominance.

Figure 5: Comprehensive posterior over the binary population synthesis parameter space from BackPop/COSMIC modeling.

Figure 6: Full posterior for the fixed-kick BH scenario, confirming minor changes in BBH mass ratios and chirp mass distribution.

Conclusion

This study rigorously demonstrates that GW190814-like binaries can be formed in isolated binary evolution scenarios without requiring progenitor ZAMS properties that are atypical relative to local massive star populations. The decisive factor for forming and merging such asymmetric BBHs is the occurrence of a highly specific and low-probability natal kick for the second-formed compact object, which imparts merger-enabling eccentricity without disrupting the system. The statistical likelihood for GW190814-like events is therefore predominantly governed by natal kick geometry, not by restricted initial conditions or interaction prescriptions.

The formalism and statistical inference deployed here are broadly extensible to other GW events, enabling population-level constraints on binary physics such as CE efficiency, mass transfer, and natal kicks. Detailed stellar evolution and improved binary interaction models will be essential to refine the merger rate predictions and reconcile CE and mass transfer pathways with full observational datasets. As the GW event catalog expands, such multidimensional backward inference frameworks will be crucial for constraining the astrophysical drivers underlying the diversity of compact object mergers.

Essay based on "Lucky Strikes: On the Origins of GW190814 Through Isolated Binary Evolution" (2511.16648).