The Stellar Actinide Boost and
its r-Process Implications

Erika M. Holmbeck

1 October 2019

Elements in the Universe


from data in Sneden+ (2008)

The Nuclear Physicist's Periodic Table


National Nuclear Data Center, Brookhaven National Laboratory

Neutron capture


The r-process

The r-Process Pattern


Hotokezaka+ (2018)

What is the site of the r-Process?

Core-collapse supernovae?

NASA / JPL-Caltech

MHD-jet supernovae?

NASA / SkyWork Digital

Neutron Star Mergers


Daniel Price (U/Exeter) and Stephan Rosswog (Int. U/Bremen)

Neutron Star Mergers: GW170817


Drout+ (2017)

How can we study the r-process observationally?

Chemically-enhanced r-II stars

r-II Stars

CS 22892-052

McWilliam+ (1995), Sneden+ (2003)

r-Process Enhanced Stars are Rare


from data in Abohalima & Frebel (2018)

RPA_logoest. 2017

Snapshot high-resolution data obtained for ~1500 (of target 2500) stars

Expect 3-5% to be extremely r-process enhanced (r-II)

Identified over 30 new r-II stars (of ~600 analyzed)


Magellan Telescopes, Las Campanas Observatory, Chile

High-Resolution Spectroscopy


Hansen, Holmbeck+ (2018)

Doubled Number of Known r-II Stars


from RPA data (2017-2019)

Actinides in r-II Stars

Thorium in J2038-0023 and Uranium in J0954+5246



Placco, Holmbeck+ (2017), Holmbeck+ (2018)

The Actinide Boost


Holmbeck+ (2018)

What is the source of this actinide-boost?

Actinide Variation

The actinide-to-lanthanide ratio (Th/Eu) is not the same in all r-process enhanced stars

Actinide variations could be a hint to key r-process characteristics

Holmbeck+ (2019b)

Actinide Production and Ye

Th and U are produced by the r-process

The electron fraction, Ye, is a key parameter determining the extent of an r-process event

Ye = [1+(n/p)]-1

Actinide Production and Ye

Th and U overproduced at very low Ye


Holmbeck+ (2019a)

Actinide Boost Stars

Abundances of stars enhanced with Th and U can be reproduced by a combination of Ye

Going backwards

What would the abundances themselves suggest for this ejecta distribution?

Actinide-Dilution with Matching Model

Builds empirical mass ejecta distributions as a function of Ye (0.005-0.450)

To explain entire pattern using Zr, Dy, and Th only


Empirical ejecta mass distributions

Distributions differ in very low-Ye region


Holmbeck+ (2019b)

Astrophysical Variations


Holmbeck+ (2019b)

Nuclear Physics Variations


Holmbeck+ (2019b)

The low-Ye component

No discrete difference between actinide-rich and actinide-poor


Holmbeck+ (2019b)

Nuclear and Astrophysical Variations


Holmbeck+ (2019b)

Actinide-boost stars do not necessarily call
for a separate r-process progenitor

Is this source an NSM?

GW170817 lightcurve

Lanthanide-poor blue ejecta + Lanthanide-rich red ejecta

Cowperthwaite+ (2017)

Two ejecta components

Stellar Abundances

Xlan = 10-3.8

Xlan = 10-0.8

mred / mblue = 1.7

Holmbeck+ (2019b)



Xlan = 10-4

Xlan = 10-1.5

mred / mblue = 1.6

Kasen+ (2017)

Results derived from r-enhanced stars are
consistent with the GW170817 kilonova

Further evidence supporting that an NSM produced
the material in r-enhanced stars?

Special Thanks

Rebecca Surman (ND), Gail C. McLaughlin (NC State), Anna Frebel (MIT)
Trevor M. Sprouse (ND), Matthew Mumpower (LANL)

Timothy C. Beers (ND), Nicole Vassh (ND), Terese T. Hansen (TAMU), Chris Sneden (UT-Austin)
Vinicius M. Placco (ND), Ian U. Roederer (UMich.), Charli M. Sakari (UW), Rana Ezzeddine (MIT)
Grant Mathews (ND), Ani Aprahamian (ND), Toshihiko Kawano (LANL)