Constraining the r-Process with
Actinide Production Studies

Erika M. Holmbeck

2 December 2019

eholmbeck.github.io/talks.html

The Origin of the Elements

from data in Sneden+ (2008)

Neutron capture

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

Neutron Star Mergers

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

Neutron Star Mergers: GW170817

Drout+ (2017)

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

Galactic Archaeology

Stellar Genealogy

Chemically-enhanced stars

Solar [Fe/H] = 0

Metal-poor [Fe/H] < -3

r-II Stars

McWilliam+ (1995), Sneden+ (2003)

Universality of the r-Process?

r-Process Enhanced Stars are Rare

from data in Abohalima & Frebel (2018)

_{est. 2017}

Snapshot high-resolution data obtained for 1767 (of target 2500) stars

Goal: Identify 75-100 new r-II stars

Identified over 25 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)

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?

Nuclear or Astrophysical?

Fission cycling?

Neutron Star Merger Ejecta

Metzger & Fernandez (2014)

Neutron Star Merger Ejecta

Metzger & Fernandez (2014)

T. Sprouse and M. Mumpower

Low-entropy dynamical (tidal) ejecta of a NSM with Y_e=0.035
(Korobkin+ 2012; Rosswog+ 2013)

Vary the initial electron fraction: Y_e=0.005 - 0.250

Y_e = [1+(n/p)]^-1

Y_e = 0: all neutrons
Y_e = 1: all protons

To investigate this, I use PRISM: an r-process network solver written by Trevor Sprouse and Matt Mumpower. Prism allows for complete flexibility of the nuclear and astrophysical input. For the actinide boost problem, I choose that low-entropy, NSM tidal ejecta trajectory, which has a nominal very low Ye of 0.035. From here on out, I'll be talking about this Ye a lot, which is the electron fraction. The Ye essentially measures the fraction of nucleons that are protons and sets the initial composition of the r-process material, basically determining how neutron-rich the material starts out. All you need to remember is that a lower Ye means more neutrons, and a higher Ye is more protons. The suggested Ye for this trajectory is very low, but I also ran 50 separate, full r-process calculations, changing the initial Ye between 0 and 0.25.

Actinide Production and Y_e

The electron fraction (Y_e) is a key parameter determining the extent of an r-process event

Fission Cycling

Actinides over-produced in fission cycling conditions

<- proton-rich

neutron-rich ->

...this plot, which shows the final actinide and Eu abundance as a function of decreasing Ye. As we saw in the previous slide, as the Ye decreases, the material is neutron rich enough to produce the lanthanide, then neutron rich enough to get beyond the third peak and produce the actinides. Instead of runaway increase, there are enough neutrons at this very low Ye to induce fission in the actinide, so their abundances decrease, and those fission products deposit somewhere around the 2nd peak. Then, as more neutrons are available, those fission products capture neutrons too, increasing the lanthanides, and eventually making way to the actinides. These are fission cycles, and we explored if fission cycling could explain the actinide variations we observe in stars, where this peak might reproduce the actinide-boost abundances.

Fission Cycling

Actinides over-produced in fission cycling conditions

<- proton-rich

neutron-rich ->

Nuclear Variations

Baseline: FRDM2012 mass model with Möller beta-decay rates and 50/50 fission fragment distribution

Holmbeck+ (2019a)

Fission Fragment Distribution?

Fission products of ²³⁸U compared to Kodama & Takahashi (1975) description (K&T)

from data in Nagy+ (1978)

Fission Fragment Distribution?

Using the K&T case over-produces Eu

Holmbeck+ (2019a)

Beta-Decay Rates?

β-decay rates from Marketin+ (2016) are over 10x faster at N>126

Holmbeck+ (2019a)

Beta-Decay Rates?

Using the Marketin produces the wrong U/Th ratio

Holmbeck+ (2019a)

Nuclear Mass Model?

The Duflo & Zuker (1995) mass model (DZ) has weaker shell closures

Nuclear Mass Model?

The DZ case still over-produces the actinides

Holmbeck+ (2019a)

In fission cycling conditions, no single choice of nuclear data reproduces the actinide-boost

from data in Holmbeck+ (2019)

Astrophysical Mixture?

If nuclear physics can't explain the actinide-boost, perhaps astrophysics can

Metzger & Fernandez (2014)

Astrophysical Mixture?

Astrophysical Mixture!

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

Astrophysical Mixture!...?

What would the abundances themselves suggest for this ejecta distribution?

Actinide Differences

Patterns of r-II stars differ markedly in the actinides

Actinide Differences

Do these actinide variations point to different r-process sites/characteristics?

Holmbeck+ (2019b)

Actinide-Dilution with Matching Model

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

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

Empirical ejecta mass distributions

Distributions differ in very low-Y_e region

Holmbeck+ (2019b)

Nuclear Physics Variations

Holmbeck+ (2019b)

Astrophysical Variations

Holmbeck+ (2019b)

The low-Y_e component

No discrete difference between actinide-rich and actinide-poor

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

X_lan = 10^-3.8
X_lan = 10^-0.8
m_red / m_blue = 1.7

Holmbeck+ (2019b)

GW170817

X_lan = 10^-4
X_lan = 10^-1.5
m_red / m_blue = 1.6

Kasen+ (2017)

Inspired by this red and blue component model, we split our mass distributions into red and blue components, and calculated the lanthanide mass fractions and the mass ratio between them. And we get numbers. We compare that to the same numbers from an independent study of GW170817. It's amazing that they are so similar considering we built these distribution from spectroscopic observations of r-process enhanced stars, and the numbers from the kilonova are from an entirely different kind of observation. This is what we come up with just from stellar abundances---three elemental abundances in particular. It's still possible that our results from the r-process enhanced stars could be from a different r-process site, but these results are surprisingly similar considering our vastly different approaches.

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)

Constraining the r-Process with Actinide Production Studies

Erika M. Holmbeck

2 December 2019

eholmbeck.github.io/talks.html

The Origin of the Elements

from data in Sneden+ (2008)

Neutron capture

The r-process

The (Solar) r-Process Pattern

Hotokezaka+ (2018)

What is the site of the r-Process?

Core-collapse supernovae?

Exotic supernovae?

Neutron Star Mergers

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

Neutron Star Mergers: GW170817

Drout+ (2017)

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

Galactic Archaeology Stellar Genealogy

Chemically-enhanced stars

r-II Stars

McWilliam+ (1995), Sneden+ (2003)

Universality of the r-Process?

r-Process Enhanced Stars are Rare

from data in Abohalima & Frebel (2018)

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

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

Thorium in Metal-Poor Stars

Holmbeck+ (2018)

The Actinide Boost

Holmbeck+ (2018)

What is the source of this actinide-boost?

Nuclear or Astrophysical?

Fission cycling?

Neutron Star Merger Ejecta

Metzger & Fernandez (2014)

Neutron Star Merger Ejecta

Metzger & Fernandez (2014)

Actinide Production and Ye

Fission Cycling

Fission Cycling

Nuclear Variations

Holmbeck+ (2019a)

Fission Fragment Distribution?

from data in Nagy+ (1978)

Fission Fragment Distribution?

Holmbeck+ (2019a)

Beta-Decay Rates?

Holmbeck+ (2019a)

Beta-Decay Rates?

Holmbeck+ (2019a)

Nuclear Mass Model?

Nuclear Mass Model?

Holmbeck+ (2019a)

from data in Holmbeck+ (2019)

Astrophysical Mixture?

Metzger & Fernandez (2014)

Astrophysical Mixture?

Astrophysical Mixture!

Astrophysical Mixture!...?

Actinide Differences

Actinide Differences

Holmbeck+ (2019b)

Actinide-Dilution with Matching Model

Empirical ejecta mass distributions

Holmbeck+ (2019b)

Nuclear Physics Variations

Holmbeck+ (2019b)

Astrophysical Variations

Holmbeck+ (2019b)

The low-Ye component

Holmbeck+ (2019b)

Holmbeck+ (2019b)

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

Is this source an NSM?

Constraining the r-Process with
Actinide Production Studies

Galactic Archaeology

Stellar Genealogy

Actinide Production and Y_e

The low-Y_e component

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

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?