Actinide-Boost Stars may not
Suggest a Separate r-Process Site


Erika M. Holmbeck


28 March 2019

r-process enhanced stars


RPA_logo

from data in McWilliam+ (1995), Sneden+ (2003)

RPA_logo

Uranium in RAVE J203843.2-002333


U

Placco, Holmbeck+ (2017)

Uranium in J0954+5246


J095442

Holmbeck+ (2018)

The actinide boost and J0954+5246


J095442

Holmbeck+ (2018)

Ages and cosmochronometry


Th

Holmbeck+ (2018)

Ages and cosmochronometry


232-Th and 238-U are radioactive

Allows radioactive decay dating


$$ t = 46.67~\text{Gyr} \left[\log\epsilon\left(\text{Th/Eu}\right)_0 - \log\epsilon\left(\text{Th/Eu}\right)_{\text{obs}}\right] $$
$$ t = 14.84~\text{Gyr} \left[\log\epsilon\left(\text{U/Eu}\right)_0 - \log\epsilon\left(\text{U/Eu}\right)_{\text{obs}}\right] $$
$$ t = 21.80~\text{Gyr} \left[\log\epsilon\left(\text{U/Th}\right)_0 - \log\epsilon\left(\text{U/Th}\right)_{\text{obs}}\right] $$

Actinides and the r-process

Can varying levels of neutron richness
in a NSM account for the actinide boost?

Ye


M. Mumpower and T. Sprouse

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

Vary the initial electron fraction: $Y_e=0.005 - 0.250$


$Y_e = \left[1+(n/p)\right]^{-1}$

Actinide and lanthanide production


Holmbeck with PRISM (T. Sprouse and M. Mumpower)

Actinide and lanthanide production


Ye

Holmbeck+ (2019)

Ages and cosmochronometry


$$ t = 46.67~\text{Gyr} \left[\log\epsilon\left(\text{Th/Eu}\right)_0 - \log\epsilon\left(\text{Th/Eu}\right)_{\text{obs}}\right] $$
$$ t = 14.84~\text{Gyr} \left[\log\epsilon\left(\text{U/Eu}\right)_0 - \log\epsilon\left(\text{U/Eu}\right)_{\text{obs}}\right] $$
$$ t = 21.80~\text{Gyr} \left[\log\epsilon\left(\text{U/Th}\right)_0 - \log\epsilon\left(\text{U/Th}\right)_{\text{obs}}\right] $$

The age of J0954+5246


Ye

Holmbeck+ (2019)

Nuclear physics variations


PRs


from data in Holmbeck+ (2019)

The age of J0954+5246


PRs


Holmbeck+ (2019)

Actinides are currently not observed
at such high levels


Need a method to dilute the actinides to reduce the Th/Eu production ratio

Actinide-Dilution model

Distribution of $Y_e$


Tidal ejecta
($Y_e\approx 0.16$, Bovard+ 2017)

Disk wind
($Y_e\approx 0.22$; Lippuner+ 2017)

$m_{\rm wind}/m_{\rm dyn}= 3$
from estimates of GW170817
(Rosswog+ 2017; Tanaka+ 2017)

Actinide-Dilution model


Ye

Holmbeck+ (2019)

Ages


Ye


Holmbeck+ (2019)

Ages


Ye


Holmbeck+ (2019)

AD Model


Can we go backwards?


Reticulum II


Ji+ (2016)

Groups of r-process enhanced stars

Holmbeck+ (in prep. 2019)

Assume one event

that produces the entire r-process pattern


Not necessarily an NSM

Actinide-Dilution with Matching model



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

To explain entire pattern from Zr to U

ADM abundance pattern for Ret II


Ye

Holmbeck+ (in prep. 2019)

Empirically built ejecta mass distributions

Ye

Holmbeck+ (in prep. 2019)

NSM ejecta (literature)

Ye

Fernandez+ (2015)

The low-Ye component


Ye

Holmbeck+ (in prep. 2019)

Low-Ye component under variations


Ye

Holmbeck+ (in prep. 2019)

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

Is this source an NSM?


Ye

Ye

Results derived from r-II stars
agree* with NSM observation

Summary and Outlook


The actinides are over-produced in very cold, neutron-rich (tidal) ejecta

NSMs could still be an actinide-boost source if most of the ejecta mass does not contribute actinides

The same r-process source can in principle account for observed actinide variations

How do the empirically built ejecta distributions compare to NSM simulations?

Entropy, dynamical timescale, nuclear physics variations...

Special Thanks


Rebecca Surman (ND), Nicole Vassh (ND), Matthew Mumpower (LANL), Trevor M. Sprouse (ND)
Gail C. McLaughlin (NC State), Anna Frebel (MIT)

Timothy C. Beers (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)