Inference of Multichannel r-process Element Enrichment in the Milky Way Using Binary Neutron Star Merger Observations

• Published in: The Astrophysical journal Vol. 985; no. 2; pp. 154 - 160
• Main Authors: Chen, Hsin-Yu, Landry, Philippe, Read, Jocelyn S., Siegel, Daniel M.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.06.2025; IOP Publishing; American Astronomical Society
• Subjects: Abundance; Binary stars; Chemical evolution; Constraints; Delay time; Enrichment; Equations of state; Galaxies; Galaxy distribution; Gamma ray bursts; Gamma rays; Gravitational wave astronomy; Gravitational waves; Milky Way; Neutron stars; Neutrons; Nuclear astrophysics; Nuclear capture; Nuclear fusion; R-process; Star & galaxy formation; Star formation; Star mergers; Stars; Stellar evolution; Uncertainty
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/add0af

Observations of GW170817 strongly suggest that binary neutron star (BNS) mergers produce rapid neutron-capture nucleosynthesis ( r -process) elements. However, it remains an open question whether these mergers can account for all the r -process element enrichment in the Milky Way’s history. Here, we constrain the contributions of the BNS channel using astrophysical neutron star observations. The rate and mass distributions are constrained by LIGO/Virgo/Kagra through the latest catalog GWTC-3, the neutron star equation of state by gravitational-wave, radio, and X-ray observations, and the delay time distribution by short gamma-ray burst (GRB) host galaxy associations. We present a Bayesian framework to consistently combine these observations with abundance information to quantify the contribution and uncertainties of single and multiple astrophysical enrichment sources, and obtain a distribution of per-event BNS r -process element yields consistent with geophysical and astrophysical abundance constraints. We then adopt a Galactic chemical evolution model assuming an instantaneous and fixed amount of Fe enrichment from core-collapse supernovae, and show that BNS-only enrichment scenarios remain inconsistent with the observed r -process abundance trend of disk stars in the Galaxy even with the uncertainties in BNS merger observations. Using stellar abundance observations instead of the short GRB constraints, we can infer a shorter BNS delay time distribution with power-law index α ≤ −2.0 and minimum delay time t min ≤ 40 Myr at 90% confidence, consistent with detailed Galactic chemical evolution models. Such delay times are in tension with those predicted by standard BNS formation models. Alternatively, we confirm that a two-channel scenario, in which the second channel tracks the star formation history without significant delay, can account for both Galactic stellar and short GRB observations. We estimate that 45%–90% of the r -process abundance in the Milky Way today would have been produced by this star formation-tracking channel, rather than BNS mergers with significant delay times.