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Formation of heavy double neutron stars II: the role of heavy first-born neutron stars and low metallicity

This paper demonstrates that heavy double neutron stars, such as GW190425, are rare outcomes (approximately 0.5% of the population) formed primarily through standard channels rather than distinct mechanisms, representing merely the high-mass tail of the standard double neutron star population with no significant dependence on metallicity.

Original authors: Ashwathi Nair, Simon Stevenson

Published 2026-08-19

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CC BY 4.0

Original authors: Ashwathi Nair, Simon Stevenson

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep in the quiet hum of the universe, two dead stars can spiral toward each other, locked in a final, fatal embrace. These are double neutron stars, the collapsed cores of massive stars that have burned out their fuel and exploded. When they collide, they send ripples through space-time called gravitational waves, a phenomenon that has revolutionized how we listen to the cosmos. For years, astronomers have cataloged these collisions, finding that most of them involve pairs with a combined weight of about two and a half times that of our Sun. But one event, detected in 2019, broke the pattern. This collision, named GW190425, involved a pair of neutron stars so heavy that their combined mass was roughly 3.4 times that of the Sun. This discovery was a puzzle because no such heavy pair has ever been seen in our own galaxy, the Milky Way, where astronomers have carefully tracked dozens of these systems using radio telescopes. The existence of this heavyweight couple challenges our understanding of how these stellar pairs are born and evolve, forcing scientists to ask if there is a hidden mechanism that creates them, or if our current theories are missing a crucial piece of the story.

To solve this mystery, a team of researchers turned to the most powerful tools available for simulating the life of stars: detailed computer models that track the physics of stellar evolution from start to finish. They focused on the specific moment when a binary system, already containing one neutron star, begins its final dance. In these systems, a second star, stripped of its outer layers to reveal a hot, dense helium core, begins to transfer material onto its neutron star companion. The researchers wanted to know if this process could naturally produce the heavy pairs seen in the gravitational wave data, or if it required exotic, unlikely scenarios. They built a vast grid of simulations, testing how the outcome changes when the first neutron star is heavier or lighter, and when the system forms in environments with different chemical compositions, known as metallicity. By running these models, they could watch the fate of thousands of potential pairs and see which ones survived to become the heavy mergers detected by observatories.

The results of this extensive simulation revealed that the formation of such heavy pairs is a rare event, occurring in only about half a percent of all double neutron star systems. Contrary to some theories that suggested these heavy systems must come from a special, fast-merging channel or require a very specific type of massive star, the researchers found that they arise from the same standard process that creates the lighter pairs we see in our galaxy. In this standard pathway, the helium star expands and dumps material onto the neutron star in a stable flow, eventually collapsing into a second neutron star. The simulations showed that even with the most massive first-born neutron stars or the most massive helium stars, the "fast-merger" channel, where a second unstable collapse happens quickly, does not contribute to the formation of these heavy pairs at the metallicity of our Sun. Instead, the heavy systems are simply the high-mass tail of the normal population, the statistical outliers of a common process.

The study also looked at whether the chemical environment of the star's birth, specifically the amount of heavy elements, played a role. In the universe, stars born in regions with fewer heavy elements lose less mass to stellar winds, potentially leaving behind heavier cores. The researchers tested this by simulating systems in environments with ten times less and one hundred times less heavy elements than our Sun. While they found that lower metallicity does allow for slightly more massive second-born neutron stars, the overall fraction of heavy double neutron stars did not change significantly. The difference was so small that it suggests the formation of these heavy systems is not strongly dependent on the chemical makeup of the galaxy. This finding contradicts some previous ideas that heavy pairs are a distinct subpopulation formed only in the early, metal-poor universe.

Ultimately, the paper concludes that the heavy double neutron star GW190425 does not require a new, exotic explanation. It is likely just a rare, naturally occurring variation of the standard formation process. The fact that we have not seen such heavy pairs in our own galaxy is likely due to the sheer rarity of the event and the difficulty of detecting them with radio telescopes, rather than a fundamental difference in how they are made. The researchers suggest that as gravitational wave detectors become more sensitive, we may find that these heavy pairs are more common in the distant universe than in our local neighborhood, simply because they are harder to spot when they are young and spinning fast. For now, the mystery of GW190425 is resolved not by finding a new law of physics, but by recognizing that nature occasionally produces a heavyweight champion from the same recipe that makes the rest of the team.

Problem Statement
The discovery of the gravitational wave source GW190425, a double neutron star (DNS) system with a total mass of $3.4^{+0.3}_{-0.1} M_\odot$ , presents a significant challenge to current formation theories. No such heavy DNS systems ( $M_{tot} \geq 3 M_\odot$ ) have been observed in the Milky Way disk, where the population is dominated by systems in the $2.3\text{--}2.9 M_\odot$ range. While several formation scenarios have been proposed to explain GW190425—ranging from "fast-merger" channels involving unstable mass transfer to pathways requiring massive first-born neutron stars (NS1) or massive helium (He) star progenitors—these hypotheses have often been tested using independent physical assumptions. There is a lack of a self-consistent framework that simultaneously calibrates to the observed Galactic DNS population and tests the viability of these alternate channels for forming heavy DNSs across varying metallicities.

Methodology
This study extends the work of Nair & Stevenson (2025, Paper I) by employing a detailed binary evolution framework using the 1D stellar evolution code MESA (version 23.05.01). The authors model the post-common envelope (post-CE) evolution of He star + NS binaries, treating the He star + NS system as the starting point.

Key methodological features include:

Parameter Space Expansion: Unlike Paper I, which used a fixed NS1 mass of $1.4 M_\odot$ , this work explores a broad range of first-born neutron star masses ( $1.1\text{--}1.9 M_\odot$ ) and initial He star masses ( $2.5\text{--}10 M_\odot$ ).
Metallicity Variations: Simulations are conducted at solar metallicity ( $Z = Z_\odot = 0.02$ ) and two low-metallicity environments ( $0.1 Z_\odot$ and $0.01 Z_\odot$ ) to assess the impact of stellar winds on remnant masses.
Mass Transfer and Winds: The models assume Eddington-limited accretion ( $\dot{M}_{Edd} = 3 \times 10^{-8} M_\odot \text{yr}^{-1}$ ) onto the NS. Wind mass loss for He stars is modeled using the Nugis & Lamers (2000) prescription at solar metallicity and a modified Belczynski et al. (2010a) prescription with Vink & de Koter (2005) metallicity dependence ( $\beta=0.86$ ) for low-metallicity models.
Population Synthesis: To compare with observations, the authors apply weighting schemes based on observed distributions for initial He star masses (power law), orbital periods (flat in log), and NS1 masses (double-Gaussian fit to observed recycled pulsars and a Turn-on Power Law model).
Post-SN Dynamics: The formation of the second neutron star (NS2) and the survival of the binary are determined using a modified Mandel & Müller (2020) prescription for remnant mass and natal kicks, calibrated to reproduce the observed Galactic DNS population.

Key Contributions and Results
The paper systematically tests four proposed formation channels for heavy DNSs ( $M_{tot} \geq 3 M_\odot$ ):

Standard Formation: Stable Case BB Roche-lobe overflow (RLO) with $M_{NS1} < 1.7 M_\odot$ .
Massive He-star Pathway: $M_{He} \geq 8.8 M_\odot$ with varying NS1, avoiding further mass transfer.
Massive NS1 Pathway: $M_{NS1} \geq 1.7 M_\odot$ undergoing stable Case BB/BC RLO.
Low-Metallicity Binaries: Formation at $Z < Z_\odot$ .

The results indicate:

Rarity of Heavy DNSs: Heavy DNSs are rare, accounting for only $\sim 0.5\%$ of the total DNS population at solar metallicity.
Dominance of Standard Channel: The "standard formation" channel is the dominant pathway for heavy DNSs, contributing $\sim 0.5\%$ of the population. The "massive He-star" and "massive NS1" channels contribute negligibly ( $\sim 6 \times 10^{-5}\%$ and $\sim 2 \times 10^{-4}\%$ , respectively) because massive He stars and massive NS1s are intrinsically rare in the adopted distributions.
No Fast-Merger Channel at Solar Metallicity: The study finds no contribution from the "fast-merger" channel (unstable mass transfer leading to a second CE phase) at solar metallicity.
Metallicity Dependence: The fraction of heavy DNSs shows very weak dependence on metallicity. While low-metallicity models ( $0.1 Z_\odot$ ) produce slightly more massive NS2s due to reduced wind mass loss, the overall fraction of heavy DNSs remains similar to solar metallicity ( $\sim 4.8\%$ vs. $\sim 5\%$ ). However, the mechanism differs: at low metallicity, heavy DNSs are predominantly formed through unstable mass transfer, whereas at solar metallicity, they form via stable mass transfer.
Population Consistency: The simulated DNS population, when weighted by the extended parameter space, broadly matches the observed Galactic DNS total mass distribution and orbital period-eccentricity distribution, though it marginally overproduces systems with $M_{tot} \approx 2.55 M_\odot$ .

Significance and Conclusions
The authors conclude that heavy DNSs do not constitute a separate subpopulation requiring exotic formation physics. Instead, they represent the high-mass tail of the standard DNS population formed via isolated binary evolution. The study suggests that the standard formation channel (Tauris et al. 2017) is sufficient to explain the existence of GW190425-like systems, provided the parameter space includes the full range of NS1 masses and metallicities.

The paper notes that while their models reproduce the observed Galactic DNS population, they do not fully explain the high merger rate of GW190425 relative to the lack of such systems in the Galactic field. The authors suggest that if future gravitational wave observations continue to reveal a large fraction of heavy DNSs, it may imply that a significant fraction of DNSs are born heavy but remain undetected in radio surveys due to selection effects (e.g., lack of recycling, short lifetimes, or acceleration effects). The work underscores the importance of self-consistent frameworks in distinguishing between distinct formation channels and highlights that metallicity plays a secondary role in the fraction of heavy DNSs, though it influences the stability of the mass transfer process leading to their formation.

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