Study Models How Metallicity Affects Double Neutron Star Merger Timescales Across Cosmic History
A new study accepted in The Astrophysical Journal finds that double neutron star (DNS) mergers predominantly occur between 80 and 250 million years after star formation, with metallicity playing a key role in shaping this delay time distribution. Using the POSYDON binary evolution code, researchers show that the helium star radius sets a physical lower limit on DNS orbital size at birth, constraining how quickly pairs can spiral together and merge. The findings have direct implications for understanding the cosmic origins of heavy elements via r-process nucleosynthesis and the host environments of short gamma-ray bursts and kilonovae.
Researchers Abhishek Chattaraj and collaborators have modeled how the delay time distribution (DTD) of double neutron star mergers depends on stellar metallicity, using the detailed binary population synthesis code POSYDON. Their central physical argument is that the radius of a helium star during its main-sequence phase sets a hard lower limit on the orbital separation of a newly formed DNS system, meaning mergers cannot occur arbitrarily quickly after star formation. Across a wide range of metallicities and regardless of common envelope efficiency or natal kick assumptions, the study finds that fewer than 15% of DNS systems merge within 80 million years of formation, while the peak merger rate falls between 80 and 250 million years. Approximately 20% or more of systems merge on timescales exceeding 1 billion years, which helps explain why short gamma-ray bursts are observed in old, metal-poor galaxies. The DTD can exhibit a complex double-peaked structure driven by a metallicity-dependent split between dominant formation channels. The early-merging minority (~15%) may account for observed r-process enrichment in dwarf galaxies and other environments with short star formation histories. The authors also find that producing very short merger delay times via specially oriented natal kicks would require kick magnitudes inconsistent with pulsar observations.
What's missing
The study relies on population synthesis modeling with POSYDON and acknowledges that results depend on input assumptions such as common envelope efficiency, natal kick distributions, and mass transfer prescriptions; uncertainties in these parameters are not fully propagated into quoted fractions.
What different sources said
- arXiv astro-phCenter
Double Neutron Star Delay Times Across Cosmic Metallicities: The Role of Helium Star Progenitors
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