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Nuclear robustness of the r process in neutron-star mergers
Gabriel Martínez Pinedo
International Nuclear Physics ConferenceAdelaide, Australia, September 11-16, 2016
Nuclear Astrophysics Virtual Institute
Introduction r process in mergers Summary
Observational signatures
Elements heavier than iron produced by neutroncapture processes: s and r process.
r process requires an environment with largeneutron to seed ratios.
0 20 40 60 80 100 120 140 160 180 200 220
Mass Number
10−2
100
102
104
106
108
1010
Ab
un
dan
ce r
ela
tiv
e t
o S
ilic
on
= 1
06
He
D
H
Li
Be
B
CONeSiS
Ca
Fe
Ni
GeSr
XeBa
Pt
Pb
r sr s
Cowan & Sneden, Nature 440, 1151 (2006)
30 40 50 60 70 80 90Atomic Number
−8
−6
−4
−2
0
Rel
ativ
e lo
g ε
30 40 50 60 70 80 90−8
−6
−4
−2
0
Stars rich in heavy r-process elements (Z > 50) and poor iniron (r-II stars, [Eu/Fe] > 1.0).
Robust abundance patter for Z > 50, consistent with solarr-process abundance.
Possible Astrophysical Scenario: Neutron star mergers.
Introduction r process in mergers Summary
Core-collapse supernova: neutrino driven winds
Neutrino interactions determine Ye
νe + n� p + e−
ν̄e + p� n + e+
Neutron-rich ejecta:
〈Eν̄e 〉 − 〈Eνe 〉 > 4∆np −
[Lν̄eLνe− 1
] [〈Eν̄e 〉 − 2∆np
]
neutron-rich ejecta: weak r-process
proton-rich ejecta: νp-process
Energy difference related to symmetry energy (GMP+ 2012,Roberts+ 2012)
1D Boltzmann transport simulation (DD2 EoS)
1050
1051
1052
Lum
inos
ity[e
rg/s
]
678910111213
hEνi[
MeV
] νeνeνx
0 2 4 6 8 10
Time [s]
0.460.480.500.520.540.560.580.60
Ele
ctro
nfra
ctio
n
-
no strong r process allowed (GMP+ 2013)
Introduction r process in mergers Summary
Neutron star mergers: Short gamma-ray bursts and r-process
x [km]
y [
km
]
12.1235 ms
−30 −20 −10 0 10 20 30−30
−20
−10
0
10
20
30
9
9.5
10
10.5
11
11.5
12
12.5
13
13.5
14
14.5
x [km]
y [
km
]
12.6867 ms
−30 −20 −10 0 10 20 30−30
−20
−10
0
10
20
30
9
9.5
10
10.5
11
11.5
12
12.5
13
13.5
14
14.5
x [km]y [
km
]13.4824 ms
−30 −20 −10 0 10 20 30−30
−20
−10
0
10
20
30
9
9.5
10
10.5
11
11.5
12
12.5
13
13.5
14
14.5
x [km]
y [km
]
15.167 ms
−50 0 50−50
−40
−30
−20
−10
0
10
20
30
40
50
9
9.5
10
10.5
11
11.5
12
12.5
13
13.5
14
14.5
Basuswein,Goriely,Janka,ApJ773,78(2013)
Mergers are expected to eject dynamically around 0.001-0.01 M� of neutronrich-material (Ye ∼ 0.01). Impact of weak interactions is not yet fully clear.
Introduction r process in mergers Summary
Neutron star mergers: Short gamma-ray bursts and r-process
Wu,Fernández,GMP,Metzger,arXiv:1607.05290
A similar amount of material less neutron rich Ye & 0.2 is expected to be ejectedfrom the disk. Conditions and ejection mechanism depend on central object(neutron star or black hole).
Both dynamical and disk ejecta may contribute to radioactive electromagnetictransient (kilonova).
Introduction r process in mergers Summary
r process in dynamical ejecta
r-process stars once electron fermi energy drops below ∼ 10 MeVto allow for beta-decays (ρ ∼ 1011 g cm−3).
Important role of nuclear energy production (mainly beta decay).
Energy production increases temperature to values that allow foran (n, γ)� (γ, n) equilibrium for most of the trajectories.
Systematic uncertainties due to variations of astrophysicalconditions and nuclear input
Mendoza-Temis, Wu, Langanke, GMP, Bauswein, Janka, PRC 92, 055805 (2015)
528 trajectories
Introduction r process in mergers Summary
Final abundances different mass models
10-8
10-7
10-6
10-5
10-4
10-3
abundan
ces
at 1
Gyr
FRDM
10-8
10-7
10-6
10-5
10-4
10-3
120 140 160 180 200 220 240
abundan
ces
at 1
Gyr
mass number, A
HFB21
WS3
120 140 160 180 200 220 240
mass number, A
DZ31
Mendoza-Temis, Wu, Langanke, GMP, Bauswein, Janka, PRC 92, 055805 (2015)
Introduction r process in mergers Summary
Temporal evolution (selected phases)
Fission (rates and yields) is fundamental to determine the final r-process abundances.
Introduction r process in mergers Summary
The role of N ∼ 130
120 125 130 135 140
Neutron Number
0
1
2
3
4
5
6
Sn (
MeV
)
DZ31WS3HFB 21FRDM
Yb (Z=70) Isotopes
Both FRDM and HFB models predict a sudden drop in neutron separation energies approachingN ∼ 130 for Z ∼ 70 (shape coexistence region).
Introduction r process in mergers Summary
Global beta-decay calculations
Beta-decay rates determine the speed of matter flow from lightto heavy nuclei.
r-process path determined by neutron separation energies
nuclei with largest impact are those with larger instantaneoushalf-lives.
Despite tremendous progress at RIB facilities (RIBF at RIKEN)most of the half-lives are based on theoretical calculations.
Two microscopic calculations (GT+FF) have become available:
Covariant density functional theory + QRPA (Marketin+2016)Skyrme finite-amplitude method (Mustonen & Engel2016)
Marketin, Huther, GMP, PRC 93, 025805 (2016)
00
10
20
30
40
50
60
70
80
90
100
110
120
pro
ton
nu
mb
er
00 20 40 60 80 100 120 140 160 180 200 220 240
neutron number
-3 -2 -1 0 +1 +2 +3
log10 TD3C∗1/2 /TFRDM1/2
Mustonen & Engel, PRC 93, 014304 (2016)
Introduction r process in mergers Summary
Impact on r-process abundances
Shorter half-lives for Z & 80 have a strong impact on the position of A ∼ 195 (Eichler+ 2015).
10-7
10-6
10-5
10-4
10-3
10-2
120 140 160 180 200 220 240
abundan
ces
at 1
Gyr
mass number, A
FRDM masses solar r abundanceFRDM+QRPA
D3C*
10-7
10-6
10-5
10-4
10-3
10-2
120 140 160 180 200 220 240
abundan
ces
at 1
Gyr
mass number, A
DZ31 masses solar r abundanceFRDM+QRPA
D3C*
They also affect the robustness of the distribution and the shape of the 2nd peak(Wu+, in preparation)
Introduction r process in mergers Summary
r process on disk ejecta
Black hole disk accretion model from R. Fernández.
Production of all r process nuclides in all diskmodels considered.
0
0.5
1
1.5
2
2.5
3
3.5
4
0.1 0.15 0.2 0.25 0.3 0.35 0.4
ejec
ta m
ass
(10
-4 M
)
Ye,5
M)
10-7
10-6
10-5
10-4
10-3
10-2
0 50 100 150 200 250 300
abundan
ces
at 1
Gyr
mass number, A
solar r abundanceFRDM masses
DZ31 masses
10-7
10-6
10-5
10-4
10-3
10-2
0 50 100 150 200 250 300ab
undan
ces
at 1
Gyr
mass number, A
solar r abundanceFRDM+QRPA
D3C*
Wu, Fernández, GMP, Metzger, MNRAS in press, arXiv:1607.05290
Introduction r process in mergers Summary
Fission barriers
The impact of different fission barriers and yields has not been sufficiently explored.
Goriely & GMP 2015 Giuliali, GMP, Robledo, in preparation
Goriely et al 2007
Möller et al 2009
Giuliani et al 2016
Introduction r process in mergers Summary
Electromagnetic transient (Kilonova)
Radioactive decay ejected material responsible for an electromagnetictransitent: kilonova (likely observed in GRB 130603B)
Kinolova models must address:
Total amount of radioactive energy released(q̇ ∼ t−α , α = 1.1–1.4)Dominating decay channels at different phasesEfficiency of decay products thermalization
Important differences abudances α-decaying nuclei between Pb and U.
10−2
10−1
100
DZ31FRDM
HFB21WS3
Frac
tion
ofen
ergy
β-decayα-decaytotal fission
10−5 10−4 10−3 10−2 10−1 100 101 102
Days
10−2
10−1
100
f tot(
t)
0 5 10 15 20 25 30Days
FRDM
γ-raysfissionfragments
β-particles α-particles
10−2
10−1
100
DZ31ftot,DZ31(t)
ftot,FRDM(t)
f tot(
t)
0 5 10 15 20 25 30Days
Barnes, Kasen, Wu, GMP, ApJ in press, arXiv:1605.07218
Introduction r process in mergers Summary
Kilonova light curve
Light curve contains nuclear physics signatures.
1036
1037
1038
1039
1040
1041
L bol
(erg
ss−
1 )
0 5 10 15 20 25 30Days
FRDM (Beta-decay dominates)DZ31 (Alpha-decay dominates)
Ratio of luminosities at peak value and at late times can be used to constrain the amount of nucleibetween Pb and U produced by the r process.
Introduction r process in mergers Summary
Summary
Neutron star mergers most likely constitute the main r process site. Two type of ejecta:dynamical and disk ejecta.
Dynamical ejecta of neutron star mergers produce a robust r-process abundance patternmainly determined by the fission yields of superheavy nuclei. Role of weak interactions onYe needs to be clarified.
Ejecta from black hole accretion disks produce all r-process nuclides in all modelsconsidered.
Having identified the r process site we can address the role of nuclear physics indetermining the abundances.
Nuclear physics is also fundamental for accurate predictions of kilonova light curves.
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