Actinide-Rich or Actinide-Poor,
Same r-Process Progenitor

Erika M. Holmbeck

23 May 2019

JINA-CEE Frontiers in Nuclear Astrophysics

The r-Process Pattern

Hotokezaka+ (2018)

The r-Process Pattern

Hotokezaka+ (2018)

Actinides in r-II Stars

Thorium in J2038-0023

Placco, Holmbeck+ (2017)

Uranium in J0954+5246

Holmbeck+ (2018)

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 Y_e

Th and U are produced by the r-process

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

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

In an astrophysical r-process event, the initial electron fraction---or Ye---is one parameter that sensitively affects the final r-process abundances that are produced. Just as a reminder, a lower Ye means more neutron-rich, and higher Ye is more proton-rich and essentially sets the initial amount of neutrons that are available to participate in neutron-capture. [MOVIE] For example, with all else being equal, in certain situations, starting at a somewhat low Ye may produce just the second r-process peak---elements like Barium and Tellurium. Decreasing the initial Ye produces the lanthanides, but not necessarily the third peak. Further decreasing the initial Ye can eventually produce actinides in the final abundance pattern. Then, as the initial material gets more and more neutron-rich, the actinides oscillate. We see this explicitly in [NEXT]

Actinide Production and Y_e

Th and U overproduced at very low Y_e

Holmbeck+ (2019a)

Actinide Boost Stars

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

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 Y_e (0.005-0.450)

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

ADM

Empirical ejecta mass distributions

Distributions differ in very low-Y_e region

Holmbeck+ (2019b)

Astrophysical Variations

Holmbeck+ (2019b)

Nuclear Physics Variations

Holmbeck+ (2019b)

The low-Y_e 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

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 like Ret II

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)

Actinide-Rich or Actinide-Poor,Same r-Process Progenitor

Erika M. Holmbeck

23 May 2019 JINA-CEE Frontiers in Nuclear Astrophysics

The r-Process Pattern

Hotokezaka+ (2018)

The r-Process Pattern

Hotokezaka+ (2018)

Actinides in r-II Stars

Placco, Holmbeck+ (2017)

Holmbeck+ (2018)

Actinide Variation

Holmbeck+ (2019b)

Actinide Production and Ye

Actinide Production and Ye

Holmbeck+ (2019a)

Actinide Boost Stars

Going backwards

Actinide-Dilution with Matching Model

ADM

Empirical ejecta mass distributions

Holmbeck+ (2019b)

Astrophysical Variations

Holmbeck+ (2019b)

Nuclear Physics Variations

Holmbeck+ (2019b)

The low-Ye component

Holmbeck+ (2019b)

Nuclear and Astrophysical Variations

Holmbeck+ (2019b)

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

Is this source an NSM?

GW170817 lightcurve

Cowperthwaite+ (2017)

Two ejecta components

Stellar Abundances

Holmbeck+ (2019b)

GW170817

Kasen+ (2017)

Results derived from r-enhanced stars areconsistent with the GW170817 kilonova

Further evidence supporting that an NSM producedthe material in r-enhanced stars like Ret II

Special Thanks

Actinide-Rich or Actinide-Poor,
Same r-Process Progenitor

23 May 2019

JINA-CEE Frontiers in Nuclear Astrophysics

Actinide Production and Y_e

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 like Ret II