Gravitational Wave Data Reveals Most Luminous Red Novae Don’t End in Neutron Star Mergers
For the first time, astronomers have used gravitational wave observations to put hard numbers on what happens when stars in a binary system tear each other apart in spectacular fashion. The answer is surprising: the vast majority of these events produce something other than the neutron star and black hole mergers that LIGO and Virgo can detect.
The study, led by Dhruv Jain and colleagues and accepted for publication in the Astrophysical Journal, tackles a long-standing question about luminous red novae (LRNe): brilliant optical transients that erupt when two stars in a binary system share a common envelope of gas. Astronomers have long suspected that these events could be the birth cry of compact binary systems that eventually spiral together and merge, producing gravitational waves. The new work uses real data to test that assumption.
The team compared the observed rate of luminous red novae measured by the Zwicky Transient Facility (ZTF) with the gravitational wave merger rate measured by the LIGO-Virgo-KAGRA (LVK) collaboration’s fourth observing run. Their central result is stark: only about 0.1% of LRNe produce binary neutron star (BNS) or neutron star-black hole (NSBH) mergers within the age of the universe.
Common Envelopes and Their Aftermath
A luminous red nova is thought to occur when the outer layers of two stars in a close binary system become shared, forming a single “common envelope” of gas that surrounds both stellar cores. Friction within this envelope causes the cores to spiral inward, ejecting the envelope material into space and producing the characteristically red optical outburst.
The conventional picture held that this spiral-in process could tighten the orbit of the remaining stellar cores so dramatically that they would eventually merge as a compact binary system, detectable by gravitational wave observatories. But the numbers have never added up: the observed rate of LRNe is far higher than the rate of compact binary mergers that gravitational wave detectors see.
Jain’s team built a rigorous framework to quantify the mismatch. Using the ZTF LRN rate and applying delay-time distribution models for how long it takes a compact binary to merge, they calculated how many LRNe should produce gravitational wave mergers if every LRN led to a compact binary. The predicted rate was orders of magnitude higher than what LVK has observed.
A Fraction of a Percent
The derived constraint of roughly one in a thousand means that the vast majority of common envelope ejections have different outcomes. The leading alternative is a direct stellar merger: the two cores inside the common envelope collide and coalesce before they can form a tight-enough orbit to later produce a detectable gravitational wave event. Some systems may also produce wide binaries that simply never merge within Hubble time.
The result provides an observational anchor for binary evolution models that have long relied on theoretical assumptions about common envelope efficiency. If most LRNe produce stellar mergers instead of compact binaries, the common envelope phase is far less efficient at tightening orbits than many models assume.
The paper is available on arXiv under the identifier 2511.19243 and has been accepted for publication in the Astrophysical Journal.