What do gravitational-wave observations tell us about Luminous Red Novae?

TL;DR

This paper investigates how gravitational-wave observations can inform us about the connection between Luminous Red Novae and the formation of merging compact binaries, providing constraints on the fraction of LRNe leading to such mergers.

Contribution

It introduces a method to constrain the fraction of LRNe that produce merging compact binaries using GW and LRN rates, linking stellar phenomena to GW observations.

Findings

Only about 0.1% of LRNe lead to GW-detected binary mergers.

Most LRNe likely result in stellar mergers or other outcomes.

The study constrains the LRN-to-merger fraction using GWTC-4 data.

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 compact binaries 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 compact binaries in LVK tell us about the fraction of LRNe that lead to the formation of compact binaries that merge in Hubble time. Using the observed volumetric rates of LRNe from the Zwicky Transient Facility (ZTF) and of compact binary mergers from LVK observations, we derive limits on the fraction of LRNe that produce compact binaries 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 compact binaries 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 $\sim 10^{-3}$ (median) of the LRNe correspond to the GW-observed binary neutron star (BNS) and neutron star-black hole (NSBH) 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.