Can Neutron-star Mergers Explain the r-process Enrichment in Globular Clusters?

Abstract

Star-to-star dispersion of r-process elements has been observed in a significant number of old, metal-poor globular clusters. We investigate early-time neutron-star mergers as the mechanism for this enrichment. Through both numerical modeling and analytical arguments, we show that neutron-star mergers cannot be induced through dynamical interactions early in the history of the cluster, even when the most liberal assumptions about neutron-star segregation are assumed. Therefore, if neutron-star mergers are the primary mechanism for r-process dispersion in globular clusters, they likely result from the evolution of isolated, primordial binaries in the clusters. Through population modeling of double neutron-star progenitors, we find that most enrichment candidates are fast-merging systems that undergo a phase of mass transfer involving a naked He-star donor. Only models where a significant number of double neutron stars proceed through this phase give rise to enrichment fractions that are comparable to the observed number of enriched globular clusters. Under various assumptions for the initial properties of globular clusters, we find if the secondary phase of mass transfer from a naked He-star donor proceeds stably (unstably), a neutron-star merger with the potential for enrichment will occur in ∼ 2 − 12% (∼ 4 − 25%) of globular clusters. The strong anti-correlation between the pre-supernova orbital separation and post-supernova systemic velocity due to mass loss in the supernova leads to efficient ejection of most enrichment candidates from their host clusters. Thus, most enrichment events occur shortly after the double neutron stars are born. This requires star-forming gas that can absorb the r-process ejecta to be present in the globular cluster 30-50 Myr after the initial burst of star formation. If scenarios for redistributing gas in globular clusters cannot act on these timescales, the number of neutron-star merger enrichment candidates drops severely, and it is likely that another mechanism, such as r-process enrichment from collapsars, is at play.

Kilonovae

“…2019) and Globular Clusters (Zevin et al. 2019). Depending on the efficiency of compositional mixing between the merger ejecta and the ISM of the Galaxy, realistic delay time distributions for NS–NS/NS–BH mergers within a consistent picture of structure formation via hierarchical growth (Kelley et al.…”

Internal r-process abundance spread of M15 and a single stellar population model

“…Then, the SG stars have higher [Eu/H] than the FG stars. The observed Eu abundance spread can be reproduced in this scenario (Bekki & Tsujimoto 2017;Zevin et al 2019). An important feature is that the r-process enrichment affects only the SG stars.…”

“…The physical process that caused the abundance spread is not well understood. An interesting possibility is r-process enrichment between multiple star formation epochs (Bekki & Tsujimoto 2017;Zevin et al 2019). Since a GC is thought to host multiple star formation epochs, r-process event after the first-generation (FG) star formation can enrich only the next generation stars.…”

Heavy elements form short and long gamma-ray bursts

“…In contrast to NS mergers, collapsars trace star formation history without significant delay (∼few Myr due to their lifetime) and occur directly in the star-forming region that gave birth to the massive progenitor star. Prompt enrichment without spatial dislocation is favorable for r-process enrichment of halo stars (van de Voort et al 2019) 3 , as well as for enrichment in ultra-faint dwarfs and globular clusters (Bonetti et al 2019;Zevin et al 2019). Furthermore, prompt enrichment by a dominant contribution of collapsars also reproduces the late-time trends of r-process elements relative to iron and alpha-elements in Galactic disk stars.…”