Nuclear inputs and their impact on the $r$-process


Matthew Mumpower

April APS Meeting

Sunday April 19$^{th}$ 2020

FIRE Collaboration
Fission In R-process Elements

The rapid neutron capture process

Believed to be responsible for the production of half the heavy elements above iron on the periodic table

Proceeds primarily via a series of rapid neutron captures and $\beta$-decays, hence $r$-process

Most improtant question: What is the astrophysical location where this can occur?

Burbidge, Burbidge, Fowler, Hoyle Rev. Mod. Phys. 29, 547 (1957) • Arnould, Goriely, Takahashi Phys. Rep. 450 4 (2007)

Understanding of the formation of the elements

One of the grand challenges of our time

The theoretical modeling of astrophysical environments

Atomic opacities for light curve observations

More generally, multi-messenger observations (gravitational waves, EM waves, etc.)

Precision experiments to constrain theory modeling

Nuclear theory for exotic nuclei

Data and observations are limited

All of these groups must work together to solve unanswered questions of $r$-process nucleosynthesis

I will focus on today the impact of one of the most critical pieces:

Nuclear theory & experiments

Abbott et al. ApJL 848 (2017) • Barnes & Kasen ApJ 775 (2013) • Kasen et al. Nature 551 (2017) • Metzger et al. MNRAS 406 (2010) • Foucart et al. PRD 87 (2015) • Lippuner et al. MNRAS 472 (2017) • Rosswog et al. MNRAS 439 (2014) • Fontes et al. MNRAS 493 (2020) • Hix & Thielemann JCAM 109 (1999) • Engel et al. PRC 60 (1999) • Côté et al. ApJ 855 (2018) • Zhu et al. ApJL 863 (2018) • Wu et al. PRC accepted (2020)

Nuclear physics as the language of the $r$-process

1st order: masses, $\beta$-decay rates, capture rates & fission

Dillmann et al. EPJA (2002) • Aprahamian et al. (2018) • See review paper: Mumpower et al. PPNP 86 (2016)

What do we know?

The chart of nuclides

Figure by Mumpower

What do we know?

All half-lives

Figure by Mumpower

What do we know?

Nuclear masses

Figure by Mumpower

What do we know?

Neutron capture rates

Figure by Mumpower

What do we know?

As of today, to varying degrees of accuracy

Figure by Mumpower

The r-process requires nuclear theory

Even in the era of FRIB

Figure by Mumpower

Variation in masses

Large variation in mass model predictions further from stability

Impacts our ability to accurately gauge the resultant composition of nucleosynthesis

Figure from Mumpower et al. PPNP 86 (2016)

Modern uncertainty bands from masses

State-of-the-art DFT calculations used to predict abundances under three different astrophysical conditions

Varied uncertain masses ($\Delta \sim$ 600 keV) with correlations around the minimum DFT solution

Result: $\sim$1 order of magnitude uncertainty in abundances predictions; in agreement with our past work

Mumpower et al. PRC 92 (2015) • Sprouse et al. accepted to PRC (2020)

Improvements to uncertainty band

Possible improvement with FRIB Currently available measuremens
Range of DFT calculations Range of 15 mass models

New measurements at radioactive beam facilities will help reduce uncertainties

However, if fission plays a dominate role (bottom panel) we will need to reduce uncertainties elsewhere

Sprouse et al. accepted to PRC (2020) • See Nicole's talk - next

Precise mass measurements

Also have the ability to diagnose $r$-process conditions

Here we predict trends along the mass chain based off a Bayesian technique - see Nicole's talk - next

Mumpower et al. ApJ 833 (2016) • Mumpower et al. J. Phys. G 44 (2017) • Orford et al. PRL 120 (2018) • Vassh et al. in prep. (2020)

Spallation Reactions

Some of the ejecta of neutron star mergers may be propelled at high velocity ($\sim$ 0.5c)

This material will eventually impact the interstellar medium

Spallation reactions on light nuclei might ensue, transmuting heavier species to slighly ligther ones

In contrast to the previous discussion on masses, these reactions occur on stable or near-stable nuclei

Figure by Kjerish - Own work, CC BY-SA 4.0 • Wang et al. ApJ (2020)

Spallation Reactions

We find that spallation can alter the shape of the major abundance peaks, but the cross sections are uncertain

This is important for determining the correct production ratios of highly populated elements e.g. Pt, Os, Ir

Experimental efforts in this direction may be able to put a cosmic speed limit on $r$-process ejecta

Spallation cross sections calculated with TALYS • Wang et al. ApJ (2020)

Importance of decays

Nuclear decays strongly impact nearly every aspect of the $r$-process

the $r$-process path, production of superheavies, composition, energy generation, and observations

More effort should be focused on the next generation potential for $\gamma$-ray observations

$\gamma$-ray emission may proceed after a multitude of nuclear processes (as with $\beta$-decay in the above figure)

Mumpower et al. PRC 94 (2016) • Spyrou et al. PRL (2016) • Mumpower et al. ApJ (2018) • Holmbeck et al. ApJ 870 (2019) • Vassh et al. J. Phys. G (2019) • Möller et al. ADNDT (2019) • Fryer et al. (2018) • Figure from Mumpower et al. in prep. (2020)

Merger $\gamma$-rays

One possible (yet very difficult) option is to attempt to observe the spectra from transients / remnants

For the $r$-process we should search for signatures of actinides, then we know we've produced everything

This involves following potentially complex decay chains...

Wu et al. ApJ (2019) • Korobkin et al. ApJ (2020)

$\gamma$-ray spectrum at 10 kyr

Distinct signatures do arise; despite line broadening

This depends sensitively on observational timescale

Can we do this with future space missions?

Wu et al. ApJ (2019) • Korobkin et al. ApJ (2020)

Observing $\gamma$-rays

Differing composition may be able to be ascertained, but the event has to be close (10 Mpc or less)

Possible candidates:

The Lunar Occultation Explorer (LOX)

Compton Spectrometer and Imager (COSI)

All-sky Medium Energy Gamma-ray Observatory (AMEGO) may all be promising

Korobkin et al. ApJ (2020)

Special thanks to

My collaborators

A. Aprahamian, J. Barnes, B. Côté, J. Clark, C. Fryer, E. Holmbeck, A. Hungerford, P. Jaffke, T. Kawano, O. Korobkin, S. Liddick, G. C. McLaughlin, J. Miller, G. W. Misch, P. Möller, R. Orford, J. Randrup, G. Savard, A. Sierk, N. Schunck, T. Sprouse, A. Spyrou, R. Surman, P. Talou, N. Vassh, M. Verriere, R. Vogt, X. Wang, Y. Zhu
& many more...

Students Postdocs FIRE LANL


To understand the $r$-process requires a coordinated, multidisciplinary effort

In particular, a deep understanding of nuclear physics will be required

Nuclear physics is the language of the $r$-process, impacting:

Production pathwaysEnergy generationMulti-messenger observations

FRIB, etc. will help to constrain nuclear models, but the heaviest elements will remain relatively inaccessible

We therefore need to keep developing and studying theoretical models of nuclear physics

Nuclear modeling is absolutely crucial if we want to prove definitively that heaviest elements, such as the actinides, were made in an event

Results / Data / Papers @