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

TL;DR

This work investigates how gravitational-wave observations constrain the fraction of Luminous Red Novae (LRNe) that produce compact binaries capable of merging within the Hubble time. By linking the LRNe rate, assumed to follow the star-formation rate, with the delay-time distribution of compact-binary mergers and comparing the projected rate to LVK results from GWTC-4, the authors derive a fraction $f^{\rm LVK}_{\rm LRNe}$ for BNS/NSBH formation. They find a median fraction of ${\cal O}(10^{-3})$, implying most LRNe do not yield CBC mergers and are more consistent with stellar mergers; however, the brightest LRNe could account for a significant portion of LVK-detected CBCs. This highlights the potential of gravitational-wave data to illuminate CE physics and the end states of LRNe, with future surveys expected to sharpen these constraints further.$

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.

Figures (3)

• Figure 1: Schematic illustration of close-binary evolutionary pathways that produce LRNe and GW progenitors. An initially detached binary undergoes Roche-lobe overflow and a contact phase that can trigger a CE episode; CE ejection produces an LRN and a dusty circumstellar medium, whereas complete merger yields a single remnant. Systems that survive the CE are driven to tighter orbits, may produce stripped-envelope supernovae, and can form compact-object binaries (BNS or NSBH) that later coalesce as GW sources.
• Figure 2: Star formation rate along with the merger rate. Solid lines represents 2014ARAA..52..415M, whereas the dashed lines represents the work of wanderman2015rate. The SFR is given in the color black, the blue, green and red lines represents the CBC rate with the power law, lognormal and exponential delay distributions, respectively.
• Figure 3: Left: Posteriors of the ratio of the sum of BNS and NSBH mergers observed by the LVK collaboration with the CBC rates calculated from LRNe rates (see Eq. \ref{['eq:frac']}) for the different delay distributions. The LRNe are assumed to follow the SFR estimated by 2014ARAA..52..415M. Right: Fraction of LRNe brighter than the absolute magnitude $M_V=-11$ resulting in BNS/NSBH mergers. The trend shows the fraction of events brighter than the given absolute magnitude, with the shaded region showing the $90\%$ confidence interval.