The Lanthanide Fraction Distribution in Metal-poor Stars: A Test of Neutron Star Mergers as the Dominant r-process Site

• Published in: The Astrophysical journal Vol. 882; no. 1; pp. 40 - 56
• Main Authors: Ji, Alexander P., Drout, Maria R., Hansen, Terese T.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01-09-2019; IOP Publishing
• Subjects: Astrophysics; Ejecta; Iron; Metallicity; Metals; Neutron stars; Neutrons; Nuclear astrophysics; process; R-process; Star mergers; Stars & galaxies; Stellar abundances; Transient sources
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ab3291

Multimessenger observations of the neutron star merger GW170817 and its kilonova proved that neutron star mergers can synthesize large quantities of r-process elements. If neutron star mergers in fact dominate all r-process element production, then the distribution of kilonova ejecta compositions should match the distribution of r-process abundance patterns observed in stars. The lanthanide fraction (XLa) is a measurable quantity in both kilonovae and metal-poor stars, but it has not previously been explicitly calculated for stars. Here we compute the lanthanide fraction distribution of metal-poor stars ([Fe/H] < − 2.5) to enable comparison to current and future kilonovae. The full distribution peaks at log XLa ∼ −1.8, but r-process-enhanced stars ([Eu/Fe] > 0.7) have distinctly higher lanthanide fractions: . We review observations of GW170817 and find general consensus that the total , somewhat lower than the typical metal-poor star and inconsistent with the most highly r-enhanced stars. For neutron star mergers to remain viable as the dominant r-process site, future kilonova observations should be preferentially lanthanide-rich (including a population of ∼10% with ). These high-XLa kilonovae may be fainter and more rapidly evolving than GW170817, posing a challenge for discovery and follow-up observations. Both optical and (mid-)infrared observations will be required to robustly constrain kilonova lanthanide fractions. If such high-XLa kilonovae are not found in the next few years, that likely implies that the stars with the highest r-process enhancements have a different origin for their r-process elements.