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

Erika M. Holmbeck

16 October 2019 --- DNP 2019

Anna Frebel, Gail C. McLaughlin, Rebecca Surman,
Trevor M. Sprouse, Matthew Mumpower
2019, ApJ, 881, 5

The r-Process Pattern

Hotokezaka+ (2018)

The r-Process Pattern

Hotokezaka+ (2018)

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)

Actinide Variation

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

Holmbeck+ (2019b)

Actinide Production and Y_e

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

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

To investigate this, we studied the affect of initial electron fraction on the final r-process abundances. 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. For those a but rusty on this notation, 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] 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

Only a narrow Y_e range reproduces observations of Th and U

Holmbeck+ (2019a)

Actinide Production and Y_e

Only a narrow Y_e range reproduces observations of Th and U

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)

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, the low-Ye contribution that we get from the ADM results varies smoothly from actinide-poor to actinide-boost. Secondly, the actinide-boost stars are just at the tail of an otherwise normal/Gaussian distribution. To explain the majority of the range of actinide abundances, the low-Ye ejecta need only be between 10 and 30% of the total ejected mass, which isn't too significance of a difference. So what does this imply?

Nuclear and Astrophysical Variations

Holmbeck+ (2019b)

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

Special Thanks

Rebecca Surman (ND), 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)

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 some numbers. Now compare that to the same numbers from an independent study of GW170817. We see rough consistency, which is interesting 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

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

Erika M. Holmbeck

16 October 2019 --- DNP 2019

Anna Frebel, Gail C. McLaughlin, Rebecca Surman, Trevor M. Sprouse, Matthew Mumpower2019, ApJ, 881, 5

The r-Process Pattern

Hotokezaka+ (2018)

The r-Process Pattern

Hotokezaka+ (2018)

Actinides in r-II Stars

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

Thorium in Metal-Poor Stars

Holmbeck+ (2018)

Actinide Variation

Holmbeck+ (2019b)

Actinide Production and Ye

Actinide Production and Ye

Holmbeck+ (2019a)

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

Special Thanks

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

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

Anna Frebel, Gail C. McLaughlin, Rebecca Surman,
Trevor M. Sprouse, Matthew Mumpower
2019, ApJ, 881, 5

Actinide Production and Y_e

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