A rare and prolific r-process event
observed in the ultra-faint dwarf galaxy Reticulum IIelements made in the rapid (r-) neutron-capture process

Anna Frebel

Anna Frebel

A long time ago...
First stars(100 M)

2nd and later generationsof stars (<1 M)Big Bang

Today

Introduction

Dark matter-only N-body simulation

Red:
first star minihalos

Firstgalaxies

Yellow:
first galaxies

Today’s galaxies
Larson & Bromm 2001

Cosmic time (not to scale)chemical evolution

Zentrum fuer Astronomie und Astrophysik, TU Berlin

Chemical evolution &cosmic recyclingStars are made from ~75% H and ~25% He, but:
⇒ Early stars contain little of all elements⇒ Younger stars contain larger amounts

ExamplesSun: contains 1.4 % heavy elements (by mass)
Oldest stars: 10-4 to 10-7 % heavy elements

We look for stars
with the least amountsof elements heavier
than H and He!

chemical evolution
Stars are made from ~75% H and ~25% He, but:Sun

2nd gen star

Abundances are derived from
integrated absorption line strengths[Fe/H] = log(NFe/NH)* − log(NFe/NH)

Galactic chemical evolution⇒ Early stars contain little of all elements⇒ Younger stars contain larger amounts“Look-back time”

Zentrum fuer Astronomie und Astrophysik, TU Berlin

Chemical evolution &cosmic recycling

chemical evolution

Zentrum fuer Astronomie und Astrophysik, TU Berlin

Cosmic recycling

Periodic Table

With time, more and more of
all elements were made!

We look for stars
withthatthe
areleast
metal-poor
amounts
comparedof elements
to theheavier
Sun’sthancomposition!
H and He!

chemical evolution

Zentrum fuer Astronomie und Astrophysik, TU Berlin

Cosmic recycling

H: X

Astronomers’
Periodic Table

He: Y

Metals Z

With time, more and more of
all elements were made!

We look for stars
that are metal-poorcompared to the Sun’scomposition!

The Story of Reticulum II

NuclearAstrophysics

StellarArchaeology

Dwarf Galaxy
Archaeology

Cosmic origin ofthe chemicalelements

Clues to theastrophysicalsite of r-process
nucleosynthesis

Ancient, clean
chemicalenrichment
signaturesnucleosynthesis in stars(Depends on stellar mass, stellar evolutionary status, others...)

56Fe

Iron
new heavy atom createdZ(# ofprotons)

N (# of neutrons)

Fusion: H to iron

during stellar evolution

Neutron-capture processes

slow n-capture: advanced stellar evolution
rapid n-capture: site undetermined(!)

rapid neutron-capturenucleosynthesis(neutron star merger scenario)
rapid n-capture
within ß-decay

Pb
Ir, Os, Pt

Te, XeSnneutron-capture r-process elemental patternrelative abundance

Anna Frebel

Nuclear astrophysics

r-process pattern

3rd peak
1stpeak2ndpeak

Anna Frebel

Nuclear astrophysics

Universal r-process patternobserved in metal-poor stars

Universal r-process pattern• r-processr-processabundance
abundance
patternsthe
are thepatternsame in the Sun
sametheand oldinmetalSun
poor andstarsmetalpoor stars
r-process stars• r-process stars
are all extremelyall
extremely
metal-poor:[Fe/H]~−3.0
metalpoor(= 1/1000th~of-3)
solar([Fe/H]Fe value)

Definition: [Fe/H] = log10(NFe/NH)star − log10(NFe/NH)Sun

Sneden et al. 2008

Rare r-process stars
in the Milky Way
How common are r-process metalpoor stars in the Milky Way?

3 to 5% of metal-poor stars w/[Fe/H]<−2.5 (Barklem et al. 05)
Only ~30 stars known so far w/[Eu/Fe] > 1.0; i.e. clear rprocess pattern above Ba
More stars known with lower
levels of 0.3 < [Eu/Fe] < 1.0;
unclear what lowest level is=> Origin of these stars is unknown✷ ✷ ✷✷ ✷✷ ✷✷Dwarf✷✷
galaxies

HaloBulge
Metal-poor
halo stars

DiskClues
tother-processsite
stellar archaeology•

Using metal-poorstars
to probe
the early
universe
StellarArchaeology:
usingmetal-poorstars
toLow-massstarschemicalwith M < 1 M : Lifetimes> 10 billionof
years=> they
are still universearound!
probe
thecontent
theearly

Through chemical abundance studies

Low-massstars found in ourGalaxy today!

Metals12-13 billion years

FirstEarlyStarsstars

Star forminggas cloud(e.g. early dwarfgalaxy)[Fe/H] ≤ −3(= 1/1000th of solar Fe)

Observe chemical
6.5m Magellan
abundances oftelescopethese -> chemical
content of old gas=> only ~1 progenitor star produced that ironclouds!

=> only ~1 nucleosynthesis event made heavier elements

Galactic metal-poor stars are a great tool for near-field cosmology
because they are the local equivalent of the high-redshift Universe!

The (detailed) astronomer’s
periodic tableBig Bang nucleosynthesis
Spallation

α-rich freezeout, νp-proc., weak s-proc.
r-process

Evolved giant stars
Odd-Z elements
α-elements
Iron group elements

Isotope distribution of Sun

The (detailed) astronomer’s
periodic table
Big Bang nucleosynthesis
Spallation

α-rich freezeout, νp-proc., weak s-proc.
s-process

Evolved giant stars
Odd-Z elements
α-elements
Iron group elements

Isotope distribution of Sun

90

Th

Thorium
232.038

92

U

Uranium
238.029

The (detailed) astronomer’s
periodic table
Big Bang nucleosynthesis
Spallation
Evolved giant stars
Odd-Z elements
α-elements
Iron group elements

α-rich freezeout, νp-proc., weak s-proc.
s-processWeak r-proc., light n-cap. primary proc.
r-processLong-livedr-process
radioactive(also r-process)Isotope distribution of solar nebula(~8 billion yrs of chemical evolution)

The Big Question★ What is the (dominant) astrophysical site of ther-process?

➡ Core-collapse supernovae➡ Neutron star mergers➡ Others (e.g., jet-driven supernovae)★ What is the rate and yield of the event?
★ Is the dominant site changing over cosmic time?

Core-collapse supernova(death of a massive star with M > 8 M)Supernovae are common; produce light elements w/ Z<30 in their cores
Responsible for these light elements when observed in metal-poor stars
Theoretical element yield:

Woosley & Janka 2005

~10-6 Msun of total r-process material=> ~10-7.5 Msun of Eu (per event)

Pros✓Metal-poor stars only have one/few progenitors✓Provides the fast enrichment needed; small & steady r-process yields
Con Theoretical difficulties for r-process nucleosynthesis to produce
elements heavier than Ba (e.g. Arcones et al.)

Neutron star binary merger(two compact supernova remnants)

Pros Easily produces elements heavier than Ba
Cons Rare One binary per ~1000- 2000 supernovae
Long(er) enrichment timescale => Inspiral time >100 MyrνYield: ~10-3 -10-2 Msun of r-process material (across all n-cap elements)=> ~10-4.5 Msun of Eu (per event)
Additional (indirect) evidence for local r-process nucleosynthesis
1) Short gamma-ray bursts: Afterglow from decay of radioactive r-process elements detected (Tanvir et al. 13)
2) Radioactive deep sea measurements suggest local neutron star mergers (Wallner et al. 15, Hotokezaka et al.15)

Neutron star binary merger(two compact supernova remnants)

Pros Easily produces elements heavier than Ba
Cons Rare One binary per ~1000- 2000 supernovae

GRB?
ɣLong(er) enrichment timescale => Inspiral time >100 MyrνwindνwindDynamic ejecta:
1) tidal ejecta
before merger2) collisionalejecta (on contact)ν ν ν ν νν ν ν
disk ν νHMNSν diskνν ν ννννwindνwindedge onɣ

Yield: ~10-3 -10-2 Msun of r-process material (across all n-cap elements)=> ~10-4.5 Msun of Eu (per event)
Additional (indirect) evidence for local r-process nucleosynthesis
1) Short gamma-ray bursts: Afterglow from decay of radioactive r-process elements detected (Tanvir et al. 13)
2) Radioactive deep sea measurements suggest local neutron star mergers (Wallner et al. 15, Hotokezaka et al.15)

Neutron star binary merger(two compact supernova remnants)

Pros Easily produces elements heavier than Ba
Cons Rare One binary per ~1000- 2000 supernovaeGRB?
ɣLong(er) enrichment timescale => Inspiral time >100 Myr

Delay time toolong to occur
Dynamicmetal-poorejecta:before1) tidal ejecta
starsform??
beforemergerνwindνwindν ν ν ν νν ν νdisk ν νHMNSν diskνν ν ννννwindνwind2) collisionalejecta (on contact)edge onɣ

Yield: ~10-3 -10-2 Msun of r-process material (across all n-cap elements)=> ~10-4.5 Msun of Eu (per event)
Additional (indirect) evidence for local r-process nucleosynthesis
1) Short gamma-ray bursts: Afterglow from decay of radioactive r-process elements detected (Tanvir et al. 13)
2) Radioactive deep sea measurements suggest local neutron star mergers (Wallner et al. 15, Hotokezaka et al.15)

Milky Way
Satellite galaxies

New Milky Way Satellites
Dwarf galaxies are useful tools to study star formation and chemical
evolution, early galaxy formation and the build-up of the Milky Way

Anna Frebel

Ultra-faint dwarf galaxy
properties (UFDs)

Dwarf galaxy archaeology

Low luminosity (300 - 3,000 Lsun)
Dark matter-dominated (M/L > 100)
Metal-poor (mean [Fe/H] ~ −2)Stars are old (mean age 13.3 +/− 1 Gyr)Few bursts of star formation
Ultra-faint dwarfs

Classical dSphs

Ideal targets for Dwarf Galaxy Archaeology
Use entire galaxy as fossil record of the early universe.
Bonus: get environmental information because we know where stars were bornAnna Frebel

r-process galaxy Reticulum II

Meet Reticulum IIReticulum IIAll
Allstarsstars

Reticulum
ReticulumIIIIstars
Stars
Energy(Dark EnergyDarkSurvey,2015)Survey

Magellan observations
Simon et al. 2015: radial velocity members confirm Ret II to be a galaxy
Brightest members (V=17-19) observable with high-resolution spectroscopy=> Ji et al. (2015) spent 2-3 hours on each of 9 brightest targets (~23h)

CMD of Ret

Color-magnitude-diagram of Ret II(red = confirmed members)

Clay 6.5m Magellan telescope(on left) at Las Campanas Observatory, Chile

(C,Mg,Al,Si, abundances
Ca, Sc, Ti,Mn,Co,Ni)
RetNa,II starshavesimilarto Cr,
UFD andhalostars

Reticulum II stars have same abundances as typical metal-poor halo stars

Core-collapse supernovae
areareprimary
light element
sourceCore-collapse
supernovae
primary
metal source
Ji et al 2016, Nature, 531, 610

Anna Frebel

r-process galaxy Reticulum II

Light elementabundances
Light elements

gray dots:
metalpoor
halostars

Anna Frebel

r-process galaxy Reticulum II

neutron-capture element
abundances(Ba andEu)
Stars in first nine UFDs
have low
The otherelementten UFDs haveneutron-capture
abundances
uniquely low neutron-capture element abundances!

gray dots:
metalpoor
halostars

UFD
UFD
star
star

r-processrprocessstarstar

Ji et al 2016, Nature, 531, 610

Anna Frebel

r-process galaxy Reticulum II

All seven Ret II stars display
thepatternAll
Ret r-process
II stars match r-process
pattern

Anna Frebel

r-process galaxy Reticulum II

Neutron-cap.
UFD neutron-captureelementsabundances inUFD stars(beforeaddingRet II)
BeforeReticulumIIAnna Frebel

r-process galaxy Reticulum II

Ret II stars:
II stars>100x
higher neutron-capture
> Ret
100x
highern-captureelement
element abundances
otherUFDs
UFDs
abundancesthanthanother[Eu/H]=−1[Ba/H]=−1.6

Dwarf galaxy archaeology( = using an entire dwarf galaxy to study the early universe)

How Rare?

How Prolific?

Population of 10 UFDs:➡1 of 10 r-process events

Estimate gas mass of UFD:

➡Est. stellar mass of all UFDs:~2000 SNe expected➡Consistent w/ expected NSM
rate of 1 per 1000-2000 SNe(LIGO will deliver answer in 2+ yrs)

Total gas in UFD galaxy➡Max. dilution mass: ~107 Msun
Gas swept up by a 1051erg
energy injection into typical ISM➡Min. dilution mass: ~105 Msun

Back-of-the-envelope calculation
Mix NSM yield mass of 10-4.5 Msun into 106 Msun of H gas (can NOW be estimated!)=> [Eu/H] = −1.2 is abundance of next-generation star=> Agrees with Ret II abundance results!

Ji et al 2016, Nature, 531, 610

Supernova
Supernova

Neutron star
Neutronstar merger
mergerRet II abundances consistent
Ret II abundance consistent withw/ neutron-star merger yieldneutron star merger

Rare and prolific jet-driven
Ret II abundance consistent withsupernova remains possibility
neutron star merger

Supernova
Supernova

Neutron star
Neutronstar merger
mergerJet-driven
supernova(e.g. Winteler et al. 2012)...but ordinary supernovae remain ruled out!

Anna Frebel

r-process galaxy Reticulum II

Reticulum II was enriched by arare, prolific and delayedr-process event
A typical core-collapse supernova could not beresponsible for the Ret II r-process signature!
Can’t you increase the # of supernovae to get higher yield?
No, 1000+ supernovae would disrupt the system
Need to be just one/few massive events➡
➡

Aren’t NSM taking too long to enrich the galaxy?
➡ After the few (initial) supernovae, it takes time for thesystem to reassemble again (~100 Myr)

➡ Minimum time scales for coalesence is ~100 Myr

Enrichment and starformation timeline~13 Gyrof galaxy
assemblyStars form fromenriched gasBigBang...are found inthe Milky Way today

First stars
explodeBlown out gas
reassembles

NSM
goes off

Stars form fromenriched gas...are found indwarf galaxies today~13 Gyrof galaxy
assemblydelay of 100 Myr✷ ✷ ✷✷ ✷✷ ✷ ✷✷ ✷

Anna Frebel

r-process galaxy Reticulum II

Answers tothe Big Question★ What is the (dominant) astrophysical site of ther-process?
➡No, but a rare and prolific site
➡ Core-collapse supernovae➡Consistent w/ Ret II abundances➡ Neutron star mergers➡Remain possible➡ Others (e.g., jet-driven supernovae)★ What is the rate and yield of the event?
➡ ~1 event per 2000 SN; ~10-2.5 Msun of r-process★ Is the dominant site changing over cosmic time?
➡Probably not!