Constraining the r-Process with
Actinide Production Studies

Erika M. Holmbeck

20 November 2019

The Origin of the Elements


from data in Sneden+ (2008)

The r-process

The (Solar) r-Process Pattern


Hotokezaka+ (2018)

What is the site of the r-Process?

Core-collapse supernovae?

NASA / JPL-Caltech

Exotic supernovae?

NASA / SkyWork Digital
see talk by D. Siegel

Neutron Star Mergers


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

see talk by O. Korobkin

Neutron Star Mergers: GW170817


Drout+ (2017)

see talk by S. Vitale

How (else) can we study the r-process observationally?

Chemically-enhanced 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 1767 (of target 2500) stars

Published 225 (325 in prep.)

Identified over 25 new r-II stars (of ~600 analyzed)


Magellan Telescopes, Las Campanas Observatory, Chile

see talks by T. Beers and C. Sakari

High-Resolution Spectroscopy


Hansen, Holmbeck+ (2018)

Doubled Number of Known r-II Stars


from RPA data (2017-2019)

New Separation at [Eu/Fe] = 0.7?


Holmbeck+ (in prep.)

Actinides in r-II Stars

Thorium in J2038-0023 and Uranium in J0954+5246



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

Thorium in Metal-Poor Stars

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


Holmbeck+ (2018)

The Actinide Boost


Holmbeck+ (2018)

What is the source of this actinide-boost?

Actinide Production and Ye

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

Ye = [1+(n/p)]-1
1 = all protons; 0 = all neutrons

Actinide Production and Ye

Only a narrow Ye range reproduces observations of Th and U


Holmbeck+ (2019a)

see talk by M. Eichler

Actinide Production and Ye

Only a narrow Ye range reproduces observations of Th and U


Holmbeck+ (2019a)

see talk by M. Eichler

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?

Reticulum II

Ji+ (2016)

A. Ji; Dark Energy Survey/Fermilab

Enhanced stars are kinematically related

Roederer+ (2018)

Galactic halo stars may be relicts of
disrupted satellite galaxies

Groups of kinematically related stars were
polluted by the same r-process source(s)

Actinide Variation among Groups

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

Holmbeck+ (2019b)

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)

see talks by H. Schatz and F. Montes

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)