What Do Gravitational-wave Observations Tell Us About Luminous Red Novae?

Abstract Luminous red novae (LRNe) have been argued to be related to the ejection of common envelopes (CEs) in binary star systems. Ejection of CEs leads to tightened stellar orbits capable of forming CBs that merge in Hubble time. As these mergers are seen by gravitational-wave (GW) detectors such as LIGO, Virgo, and KAGRA (LVK), we ask what the merger rates of CBs in LVK tell us about the fraction of LRNe that lead to the formation of CBs that merge in Hubble time. Using the observed volumetric rates of LRNe from the Zwicky Transient Facility and of compact binary mergers from LVK observations, we derive limits on the fraction of LRNe that produce CBs that merge in Hubble time. Assuming the LRNe rate closely follows the star formation rate at any redshift, we use the delay time distribution models for CBs to compute the compact binary merger rate. A comparison of this merger rate with the latest volumetric rates of compact binary mergers from the fourth GW Transient Catalog (GWTC-4) at the present epoch of LVK allows us to constrain the above fraction. We find that only a fraction as small as ∼10 −3 (median) of the LRNe correspond to the GW-observed binary neutron star and neutron star–black hole mergers. This potentially implies that the majority of the LRNe population will not lead to mergers of compact objects, but other end products, such as stellar mergers.