Actinide-Rich or Actinide-Poor, Same r -Process Progenitor
Erika M. Holmbeck
23 May 2019
JINA-CEE Frontiers in Nuclear Astrophysics
Good afternoon everyone, and thanks for being here. I am very excited to present some recently submitted work about actinides in r-process enhanced stars and the r-process.
The r -Process Pattern
Hotokezaka+ (2018)
Yesterday, we heard about the r-process. This is the Solar isotopic abundance pattern with some important features highlighted.
We have the three peaks that correspond to the closed neutron shells and the rare-earth that Nicole talked about yesterday.
One part of this plot that is relevant to my work is....[NEXT]
The r -Process Pattern
Hotokezaka+ (2018)
The actinides. Th and U are special since they are only made by the r-process.
Actinides in r -II Stars
Thorium in J2038-0023

Placco, Holmbeck+ (2017)
Uranium in J0954+5246

Holmbeck+ (2018)
In some stars, we observe the entire range of r-process elements.
And sometimes, we can measure the actinides, Th and U.
Here is an example of a Th measurement and a U measurement in two different r-II stars.
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)
For stars that show the entire r-process pattern, there is a variation from star to star in the actinide-to-lanthanide ratio.
For example, in Reticiulum II, the "r-process dwarf galaxy," the one star where a thorium abundance could be measured shows a low actinide-to-lanthanide abundance.
Most r-proces enhanced stars have a relative thorium abundance that approximately agrees with the Solar value, and some are enhanced in thorium, these actinide-boost stars.
These differences in the relative actinide abundances could be a hint that certain r-process conditions, or point to different r-proces sites entirely, are responsible for producing the actinides.
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)
...this plot, which shows the final actinide and Eu abundance as a function of decreasing Ye.
We thought this variation could explain the actinide-boost.
It turns out that **if you assume all the r-process ejecta has very low Ye**, the actinide production is too great to explain even actinide-boost stars,
and this production doesn't agree with stellar abundances derived from observations.
So instead of picking one Ye, we chose a combination.
Actinide Boost Stars
Abundances of stars enhanced with Th and U can be reproduced by a combination of Y_{e}

Instead of picking one very low Ye, we chose a combination that was motivated by literature and observations of the GW170817 kilonova.
This combination of Ye---in red here---does much better matching to actinide-boost abundances than any single Ye---in blue.
Can we make a more general argument? [NEXT]
Going backwards
What would the abundances themselves suggest for this ejecta distribution?

Can we instead go backwards and describe a Ye combination from the stellar abundances themselves...
And therefore build a distribution that characterizes the r-process event needed in order to reproduce those abundances.
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

So we built a model to do this that uses a Monte Carlo method.
First, we ran about one hundred r-process nucleosynthesis simulations using PRISM
starting at different Ye's, each of which produces a distinct final abundance pattern.
Then, we give the model constraints using three elements.
[PLAY]
The model randomly picks a number of initial Ye's, then adds together the final abundance patterns.
If the summed abundance pattern agrees with the input abundances, then we keep that randomly-chosen set of Ye's and add them to this distribution on the left.
This is the orange line in this movie. We run this hundred of thousands of times until this distribution converges.
In the end, this distribution shows how the ejecta of an r-process event could have been distributed in Ye
in order to produce the input stellar abundances on the right.
ADM
The low-Y_{e} component
No discrete difference between actinide-rich and actinide-poor

Holmbeck+ (2019b)
We can quantify the contribution from the very-low Ye component.
This plot is a little complicated. Basically, we ran a series of these simulations where we varied the required actinide contribution systematically,
and this axis shows what percentage of the total ejected mass is allowed to be in that low-Ye tail.
The gray dots show what the low-Ye contribution would be for individual stars after running this model.
Notice a few things. First---if you were to project this into a histogram---the actinide-boost stars are just at the tail of an otherwise normal/Gaussian distribution.
Secondly, the low-Ye contribution that we get from the ADM results varies smoothly from actinide-poor to actinide-boost.
So what does this imply?
Nuclear and Astrophysical Variations
Holmbeck+ (2019b)
Actinide-boost stars do not necessarily call for a separate r -process progenitor
Is this source an NSM?
This could mean that stars showing the extra enhancement of actinides could be made by the same r-process site as the actinide-deficient case.
For example, this is accommodated in a NSM model by different amounts of dynamical ejecta where very robust actinide production may occur.
[NEXT] Well, we have observations of one NSM event, so we can compare our distribution to observations to test if they agree with a NSM.
GW170817 lightcurve
Lanthanide-poor blue ejecta + Lanthanide-rich red ejecta

Cowperthwaite+ (2017)
In addition to gravitational waves, GW170817 released electromagnetic radiation.
To explain the lightcurve from this event, many studies have argued that there must have been at least two components of the NSM ejecta:
a lanthanide-poor blue component, and a lanthanide-rich red component.
So we can do the same thing with our distribution to test if it matches with this observation.
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
What we derived from observations of stars is similar to results derived from a NSM observation.
This could suggest that the r-process enhanced stars like Ret II were indeed enriched by a NSM event.
It does not need to be a NSM merger. For example, a collapsar could yield similar results.
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)