Mergers Fall Short: Non-merger Channels Required for Galactic Heavy Element Production

Abstract

Since the discovery of the binary neutron star merger GW170817 and its associated kilonova, neutron star mergers have been established as a key production channel for r-process elements in the Universe. However, various lines of evidence, including observations of r-process abundances inferred from stellar spectra of Milky Way disk stars, suggest that additional channels are needed to fully account for r-process element enrichment in the Milky Way. Neutron star-black hole mergers and fast-merging binary neutron star systems are among the leading alternative candidates. In this paper, we combine gravitational-wave observations from LIGO-Virgo-KAGRA with data from short gamma-ray bursts, Galactic pulsars, and Galactic Eu/Fe versus Fe/H abundance observations to assess the contribution of these mergers to r-process enrichment in the Galactic disk. Our analysis employs a unified, likelihood-based inference framework that consistently propagates uncertainties in merger rates, delay-time distributions, mass- and spin-dependent ejecta yields, and stellar abundance measurements. We find that neither neutron star-black hole mergers nor fast- merging binary neutron star populations can serve as the dominant additional channel without generating strong tension with existing observations and theoretical expectations. These results constrain the viable sources of Galactic r-process enrichment and underscore the necessity of non- merger production channels.

Figures (7)

• Figure 1: Comoving BNS merger-rate density as a function of redshift for the delayed and fast-merging BNS channels, required if they were solely to explain the [Eu/Fe] versus [Fe/H] abundance trends observed from Milky Way's disk stars. Solid curves are posterior medians; shaded regions show central 90% credible intervals. Here, we assume the fast-merging systems are detectable by current gravitational-wave searches and have no delay relative to SFR. The figure shows that the fast-merging BNSs have to dominate the BNS rates across the Milky Way history in order to explain the observed abundance trends.
• Figure 2: Same as Fig. \ref{['fig:ecc_case1_summary']} but under the assumption that the gravitational-wave searches are not sensitive to fast-merging BNS events. The figure shows that a gravitational-wave undetectable fast-merging BNS population can explain the observed $r$-process abundance trends only if their merger rate significantly exceeds that of the delayed population across the Milky Way history.
• Figure 3: Posterior distributions for the BNS (blue), NSBH (orange), and third-channel (green) $r$-process contributions. We show the fractional contribution from each channel integrated over cosmic history, inferred simultaneously to best fit the observed abundance data from Milky Way disk stars.
• Figure 4: Current GW searches can detect fast-merging BNSs. Diagonal panels show marginalized posteriors (medians and 68% credible intervals); off-diagonal panels show joint posteriors (filled credible regions). Parameters are the total BNS rate $R_{\rm BNS}$ (${\rm Gpc}^{-3}\, {\rm yr}^{-1}$), ejecta mass $m_{\rm ej}$ (${\rm M}_\odot$), delayed-channel delay-time parameters $\alpha$ and $t_{\rm min}$, and the fast-merging fraction $f_{\rm fast}$. The subscript '0' in the parameter labels indicate that the quantities are evaluated in the local universe ($z=0$). The large panel at upper right displays the posterior-predicted credible regions (orange) in $[\mathrm{Eu}/\mathrm{Fe}]$ versus $[\mathrm{Fe}/\mathrm{H}]$, with Milky Way disk measurements from Battistini shown as blue points.
• Figure 5: Current GW searches miss fast-merging BNSs. Diagonal panels show marginalized posteriors (medians and 68% credible intervals); off-diagonal panels show joint posteriors (filled credible regions). Parameters are the delayed-channel BNS merger rate $R_{\rm BNS}$ (${\rm Gpc}^{-3}\, {\rm yr}^{-1}$) (here referring to the delayed component only), the common ejecta mass $m_{\rm ej}$ (${\rm M}_\odot$), the delayed-channel delay-time parameters $\alpha$ and $t_{\rm min}$ (${\rm Gyr}$), and the scaling factor $X_{\rm fast}$ that sets the fast-merging rate as $R_{\rm fast} = X_{\rm fast}\,R_{\rm BNS}$. The subscript '0' in the parameter labels indicate that the quantities are evaluated in the local universe ($z=0$). The large panel at upper right displays the posterior-predicted credible regions (orange) in $[\mathrm{Eu}/\mathrm{Fe}]$ versus $[\mathrm{Fe}/\mathrm{H}]$, with Milky Way disk measurements from Battistini shown as blue points.