Constraining the r-Process with
Actinide Production Studies


Erika M. Holmbeck


20 November 2019

eholmbeck.github.io/talks.html

The Origin of the Elements


processes

from data in Sneden+ (2008)

The r-process

The (Solar) r-Process Pattern


r-process

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

processes

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

see talk by O. Korobkin

Neutron Star Mergers: GW170817

SSS2017a


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


lit

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

Magellan Telescopes, Las Campanas Observatory, Chile

see talks by T. Beers and C. Sakari

High-Resolution Spectroscopy

RPA

Hansen, Holmbeck+ (2018)

Doubled Number of Known r-II Stars


RPA

from RPA data (2017-2019)

New Separation at [Eu/Fe] = 0.7?


RPA

Holmbeck+ (in prep.)

Actinides in r-II Stars


Thorium in J2038-0023 and Uranium in J0954+5246

U

J095442         

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


Th

Holmbeck+ (2018)

The Actinide Boost


J095442

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

Ye

Holmbeck+ (2019a)

see talk by M. Eichler

Actinide Production and Ye


Only a narrow Ye range reproduces observations of Th and U

Ye

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


ADM


Empirical ejecta mass distributions


Distributions differ in very low-Ye region

Holmbeck+ (2019b)

Astrophysical Variations

Ye

Holmbeck+ (2019b)

Nuclear Physics Variations

Ye

Holmbeck+ (2019b)

The low-Ye component

No discrete difference between actinide-rich and actinide-poor

Ye

Holmbeck+ (2019b)

see talks by H. Schatz and F. Montes
Ye

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)

J095442

GW170817


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)