Actinide Production in Neutron-Star Mergers

Erika M. Holmbeck

25 October 2018

The r-process

Holmbeck with PRISM (T. Sprouse and M. Mumpower)

R-II stars

CS 22892-052

Sneden+ (2003)

The actinide boost


Holmbeck+ (2018)

Are NSMs the source of the actinide boost?

Do varying levels of neutron richness account for the actinide boost?


T. Sprouse and M. Mumpower

Low-entropy dynamical (tidal) ejecta of a NSM
(Korobkin+ 2012; Rosswog+ 2013)

Vary the initial electron fraction: $Y_e=0.005 - 0.250$

$Y_e$ = 0: all neutrons

$Y_e$ = 1: all protons

Actinide and lanthanide production

Holmbeck with PRISM (T. Sprouse and M. Mumpower)

Ages and cosmochronometry

232-Th and 238-U are radioactive

Allows radioactive decay dating

$$ t = 46.67~\text{Gyr} \left[\log\epsilon\left(\text{Th/Eu}\right)_0 - \log\epsilon\left(\text{Th/Eu}\right)_{\text{obs}}\right] $$
$$ t = 14.84~\text{Gyr} \left[\log\epsilon\left(\text{U/Eu}\right)_0 - \log\epsilon\left(\text{U/Eu}\right)_{\text{obs}}\right] $$
$$ t = 21.80~\text{Gyr} \left[\log\epsilon\left(\text{U/Th}\right)_0 - \log\epsilon\left(\text{U/Th}\right)_{\text{obs}}\right] $$

The age of actinide-boost star J0954+5246


Holmbeck+ (arXiv:1807.06662)

Actinides are currently not observed at such high levels

Need a method to dilute the actinides

Distribution of $Y_e$

Tidal ejecta centered at $Y_e=0.16$
(Bovard+ 2017)

Wind centered at $Y_e=0.22$
(Lippuner+ 2017)

$m_{\rm wind}/m_{\rm dyn}= 3$
(Rosswog+ 2017; Tanaka+ 2017)

Distribution of $Y_e$


Holmbeck+ (arXiv:1807.06662)



Holmbeck+ (arXiv:1807.06662)


We are conducting a survey to find more r-II stars

During this survey, we found the most actinide-enhanced r-II star to date

Low-$Y_e$, low-entropy NSM conditions (e.g., in the tidal ejecta) over-produce the actinides

NSMs could be a source of the actinide boost if we add another r-process from a higher-$Y_e$ environment (e.g., the accretion disk wind)