Origin of the Heaviest Elements: the Rapid Neutron-Capture Process
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Origin of the Heaviest Elements: the Rapid Neutron-Capture Process
John J. Cowan∗
HLD Department of Physics & Astronomy,
University of Oklahoma,
440 W. Brooks St.,
Norman, OK 73019,
USA
Christopher Sneden†
Department of Astronomy,
University of Texas, 2515 Speedway,
arXiv:1901.01410v2 [astro-ph.HE] 24 Apr 2020
Austin, TX 78712-1205,
USA
James E. Lawler‡
Physics Department,
University of Wisconsin-Madison,
1150 University Avenue,
Madison, WI 53706-1390,
USA
Ani Aprahamian§ and Michael Wiescher¶
Department of Physics and Joint Institute for Nuclear Astrophysics,
University of Notre Dame,
225 Nieuwland Science Hall,
Notre Dame, IN 46556,
USA
Karlheinz Langanke∗∗ and Gabriel Martı́nez-Pinedo††
GSI Helmholtzzentrum für Schwerionenforschung,
Planckstraße 1, 64291 Darmstadt,
Germany
Institut für Kernphysik (Theoriezentrum),
Department of Physics,
Technische Universität Darmstadt,
Schlossgartenstraße 2,
64298 Darmstadt,
Germany
Friedrich-Karl Thielemann‡‡
Department of Physics,
University of Basel,
Klingelbergstrasse 82, 4056 Basel,
Switzerland
GSI Helmholtzzentrum für Schwerionenforschung,
Planckstraße 1, 64291 Darmstadt,
Germany
(Dated: April 27, 2020)
The production of about half these heavy elements found in nature is assigned to a spe-
cific astrophysical nucleosynthesis process: the rapid neutron capture process (r-process).
Although this idea has been postulated more than six decades ago, the full understand-
ing faces two types of uncertainties/open questions: (a) The nucleosynthesis path in the
nuclear chart runs close to the neutron-drip line, where presently only limited experi-
mental information is available, and one has to rely strongly on theoretical predictions
for nuclear properties. (b) While for many years the occurrence of the r-process has been
associated with supernovae, where the innermost ejecta close to the central neutron star
were supposed to be neutron-rich, more recent studies have cast substantial doubts on
this environment. Possibly only a weak r-process, with no or negligible production of the
third r-process peak, can be accounted for, while much more neutron-rich conditions,
2
including an r-process path with fission-cycling, are likely responsible for the majority
of the heavy r-process elements. Such conditions could result during the ejection of
initially highly neutron-rich matter, as found in neutron stars, or during the fast ejec-
tion of matter which has prior experienced strong electron-captures at high densities.
Possible scenarios are the mergers of neutron stars, neutron-star black hole mergers, but
include also rare classes of supernovae as well as hypernovae/collapsars with polar jet
ejecta and possibly also accretion disk outflows related to the collapse of fast rotating
massive stars. The composition of the ejecta from each event determines the temporal
evolution of the r-process abundances during the “chemical” evolution of the Galaxy.
Stellar r-process abundance observations, have provided insights into, and constraints
on the frequency of and conditions in the responsible stellar production sites. One of
them, neutron star mergers, was just identified thanks to the observation of the r-process
kilonova electromagnetic transient following the Gravitational Wave event GW170817.
These observations, increasingly more precise due to improved experimental atomic data
and high resolution observations, have been particularly important in defining the heavy
element abundance patterns of the old halo stars, and thus determining the extent, and
nature, of the earliest nucleosynthesis in our Galaxy. Combining new results and impor-
tant breakthroughs in the related nuclear, atomic and astronomical fields of science, this
review attempts to provide an answer to the question “How Were the Elements from
Iron to Uranium Made?”
CONTENTS 1. Neutron Capture on neutron-rich nuclei: β-Oslo
method 29
I. Introduction and historical reviews 3 2. Neutron capture by (d, p) surrogate reactions 29
3. Neutron-capture in ring experiments 30
II. Observations 5
A. Stellar Abundances of Neutron-Capture Elements in V. Nuclear modeling of r-process input 30
Metal-Poor Stars 5
A. Nuclear masses 30
B. Atomic Data for the Analysis of neutron-capture
B. Beta-decay half-lives 32
Elements in Metal-Poor Stars 7
C. Abundance Trends in Galactic and Extragalactic C. Neutron captures 34
Stars 9 D. Fission 35
D. The role of long-lived radioactive species 11
E. Kilonovae observations 12 VI. Astrophysical Sites and their ejecta composition 36
A. Possible r-process sites related to massive stars 38
III. Basic Working of the r-process and necessary 1. Neutrino winds from core-collapse supernovae 38
environment conditions 14
2. Electron-capture supernovae 39
A. Modeling Composition Changes in Astrophysical
Plasmas 14 3. Neutrino-induced r-process in the He-shell 39
B. Special features of the r-process and the role of 4. Quark deconfinement supernovae 39
neutron densities and temperatures 16 5. Magneto-rotational supernovae with jets 40
C. How to obtain the required neutron-to-seed ratios 21 6. Collapsars, Hypernovae, long-duration
Gamma-Ray Bursts 41
IV. Experimental developments for r-process studies 22 B. Neutron-star and neutron-star / black hole mergers 43
A. Production of neutron-rich isotopes 23 1. Dynamic ejecta 46
1. Nuclear reactors and fission product sources 24
2. Neutrino Winds and the Effect of Neutrinos 48
2. Spallation sources and ISOL techniques 24
3. Fragmentation sources 24 3. Accretion Disks outflows 49
B. Experimental Achievements in Measuring Nuclear
Properties 25 VII. Electromagnetic signatures of r-process nucleosynthesis 50
1. The experimental study of nuclear masses 26
Mass measurements in storage rings 26 VIII. Abundance evolution in the Galaxy and Origin of the
Mass measurements in traps 26 r-process 54
2. Beta-Decay Studies 27 A. Supernova vs. r-process imprints in early galactic
3. Beta-delayed neutron emission probability evolution 54
measurements 27 B. Galactic Chemical Evolution Modelling 57
C. Experiments towards Neutron Capture Rates 28 1. Homogeneous evolution models 57
2. Inhomogeneous galactic chemical evolution 58
C. Connecting observational constraints on r-process
abundances with different astrophysical sites 59
∗ jjcowan1@ou.edu D. Long-lived Radioactivities: r-process
† chris@astro.as.utexas.edu cosmochronometers and actinide boost stars 61
‡ jelawler@wisc.edu
§ aapraham@nd.edu IX. Final remarks and conclusions 64
¶ Michael.C.Wiescher.1@nd.edu
∗∗ k.langanke@gsi.de Acknowledgments 68
†† g.martinez@gsi.de
‡‡ f-k.thielemann@unibas.ch References 68
3
I. INTRODUCTION AND HISTORICAL REVIEWS mental data are available (see e.g. Käppeler et al., 2011;
Karakas and Lattanzio, 2014; Reifarth et al., 2014). In
At present we know of 118 elements from charge num- order to allow for the production of the heaviest nu-
ber Z = 1 (H) to Z = 118 (Og). Eighty of them have at clei over a timescale of seconds, the r-process operates
least one stable isotope (up to Z = 82, Pb) with Z = 43 far from stability, which requires high neutron densities.
(Tc) and Z = 61 (Pm) being unstable. Another 11 ele- This involves highly unstable nuclei, for which still little
ments up to Z = 94 (Pu) [with the exception of Z = 93, experimental data are available. In addition, the quest
Np] are naturally occurring on earth with sufficiently long for the stellar origin of the required conditions involved
half-lives, while the remaining ones with short half-lives a large number of speculations for many decades (e.g.
have only been either produced in laboratory or possibly Cowan et al., 1991; Arnould et al., 2007). [There are also
also astrophysical environments. The question of how observational indications of intermediate neutron capture
this took place in the Universe is a long-standing one. processes between the s and the r-process, e.g., the i-
Presently we know that of the natural elements/isotopes process (Cowan and Rose, 1977), possibly occurring in
only 1,2 H, 3,4 He and 7 Li originate in the Big Bang, with super-AGB stars (Jones et al., 2016a).] Fig. 1 gives an
problems remaining in understanding the abundance of overview of the major contributions to the solar system
7 abundances. It includes the Big Bang (light elements H,
Li (Cyburt et al., 2016; Pitrou et al., 2018). All other
elements were synthesized in stars, the first ones form- He, Li and their isotopes 1,2 H, 3,4 He and 7 Li, given in
ing a few hundred million years after the Big Bang. The yellow), plus stellar sources, contributing via winds and
majority of stars, which have long evolutionary phases, explosions to the interstellar medium until the formation
are powered by fusion reactions. Major concepts for stel- of the solar system. These stellar burning abundances re-
lar burning were laid out in the 1950s (Burbidge et al., sult from charged-particle reactions up to the Fe-group in
1957; Cameron, 1957), including the then called x-process stellar evolution and explosions (green), and neutron cap-
which today is understood via spallation of nuclei by cos- ture processes. The latter are a superposition of (under-
mic rays (e.g. Prantzos, 2012). During their evolution, stood) slow neutron captures (s-process) in helium burn-
and in explosive end phases, massive stars can synthesize ing of stars (with abundance maxima at closed neutron
elements from C through Ti, the iron-peak elements (e.g., shells for stable nuclei, turquoise), and a rapid neutron
21 ≤ Z ≤ 30 from Sc to Zn) and beyond, (as outlined over capture process (r-process, pink) leading to abundance
many years, e.g. Howard et al., 1972; Woosley and Heger, maxima shifted to lighter nuclei in comparison to the s-
2007; Wanajo et al., 2018; Curtis et al., 2019). The major process. We note, however, that the contributions of the
result is, however, that the production of heavier nuclei i, p, ν, and νp-processes are minor and thus are not read-
up to Pb, Bi, and the actinides requires free neutrons, ily apparent in this figure. The focus of this review will
as charged-particle reactions in stellar evolution and ex- be on the r-process and the understanding of how the
plosions lead typically to full chemical or quasi equilib- corresponding isotopes were synthesized in nature.
ria which favor the abundance of nuclei with the highest Over the years there have been a number of compre-
nuclear binding energies, occurring in the Fe-peak (Hix hensive reviews on this topic (for a selected list see e.g.
et al., 2007). Hillebrandt, 1978; Cowan et al., 1991; Qian and Wasser-
A (very) small number of these heavy isotopes can burg, 2007; Arnould et al., 2007; Sneden et al., 2008;
be produced as a result of charged-particle and photon- Thielemann et al., 2011; Thielemann et al., 2017a,b;
induced reactions in explosive nucleosynthesis, the so Horowitz et al., 2019, and references therein). In or-
called (proton-rich) p-process (e.g. Arnould and Goriely, der to get clues on the r-process origin, a wide range of
2003; Travaglio et al., 2018; Nishimura et al., 2018, and subtopics need to be addressed: (1) nuclear physics input
references therein), and possibly a further contribution to understand the nucleosynthesis path far from stabil-
resulting from interactions with neutrinos in such envi- ity, (2) nucleosynthesis modeling to find out conditions
ronments, including the ν process (Woosley et al., 1990; for neutron densities and temperatures which can repro-
Suzuki and Kajino, 2013; Sieverding et al., 2019) and duce the r-process abundances found in nature, (3) deter-
νp process (Fröhlich et al., 2006b; Pruet et al., 2006; mining whether proposed astrophysical sites can match
Wanajo, 2006). such conditions, (4) observations of stellar abundances
The two main processes involving the capture of free throughout galactic history in order to find out which of
neutrons are the slow (s)-process and the rapid (r)- these sites can contribute during which period of galac-
process (due to low or high densities of neutrons avail- tic evolution, (5) in order to do so with good precision
able and the resulting reaction timescales of neutron cap- a detailed study of the atomic physics is required for
tures). In the s-process, taking place during stellar evo- identifying the strengths of absorption lines needed to
lution and passing through nuclei near stability, there is determine abundances, and (6) detections of long-lived
sufficient time for beta-decay between two neutron cap- radioactive species that can hint towards understanding
tures. The process timescale ranges from hundreds to the frequencies of r-process events in the Galaxy. Thus,
thousands of years. For many of these nuclei experi- a number of connected fields, including atomic physics,4
H Stellar burning 1. the innermost ejecta of regular core-collapse su-
10 10 pernovae CCSNe (e.g. Schramm, 1973; Sato, 1974;
Abundances (Y(Si)= 10 6)
He
s−process peak
s−process peak
s−process peak
r−process peak
r−process peak
Hillebrandt et al., 1976; Hillebrandt, 1978; Woosley
10 8 O et al., 1994; Takahashi et al., 1994; Witti et al.,
C Ne
10 6 Si S Fe 1994; Qian and Woosley, 1996; Hoffman et al.,
D Ca Ni 1997; Thompson et al., 2001; Wanajo et al., 2001;
10 4 Terasawa et al., 2001; Qian and Wasserburg, 2007;
10 2 Ge Sr Farouqi et al., 2010; Roberts et al., 2010, 2012;
Xe Ba Pb Martı́nez-Pinedo et al., 2012; Arcones and Thiele-
10 0 Li B Pt
Be
mann, 2013; Mirizzi, 2015). However, despite all re-
BBN
maining uncertainties in the explosion mechanism,
10 −2
0 20 40 60 80 100 120 140 160 180 200 recent conclusions are that at most a weak r-process
Mass Number, A can occur under these conditions (Wanajo et al.,
FIG. 1 Abundances, Yi , of elements and their isotopes in the 2011; Martı́nez-Pinedo et al., 2012; Roberts et al.,
solar system as a function of mass number Ai = Zi +Ni . Ai Yi 2012; Curtis et al., 2019), because weak interactions
is equal to the mass fraction
P of isotope i, the sum of mass frac- with electron neutrinos and anti-neutrinos from the
tions amounts to 1, i Ai Yi = 1. The present figure utilizes newly formed hot proto-neutron star will either
a scaling, leading (for historical reasons) to an abundance of make initially neutron-rich matter less neutron-rich
106 for the element Si. Element ratios are obtained from so- or even proton-rich or, in case of slightly neutron-
lar spectra, the isotopic ratios from primitive meteorites and
terrestrial values (Asplund et al., 2009; Lodders et al., 2009).
rich matter, sufficiently high entropies are not at-
These values represent a snapshot in time of the abundances tained. Another option for a weak r-process exists
within the gas that formed the solar system. in so-called quark deconfinement supernovae, where
after the collapse of a massive star, leading to a
proto-neutron star, a quark-hadron phase transi-
nuclear physics, stellar spectroscopy, stellar (explosion) tion sets in which causes the subsequent supernova
modeling, and galactic chemical evolution are involved explosion (Fischer et al., 2018, 2020b).
in attempting to answer the long-standing problem of
2. Outer layers of supernova explosions, e.g. the he-
“How Were the Elements from Iron to Uranium Made?”,
lium layer where neutrons are created by (α, n)-
one of the Eleven Science Questions for the New Cen-
reactions, were also suggested (Truran et al., 1978;
tury addressed by the National Academy of Sciences in
Thielemann et al., 1979; Cowan et al., 1980; Hille-
2003 (National Research Council, 2003). Detailed discus-
brandt et al., 1981; Klapdor et al., 1981; Cameron
sions will follow in later sections, here we list a number
et al., 1983; Cowan et al., 1983; Thielemann et al.,
of considered scenarios.
1983; Cowan et al., 1985), later also the collaps-
While there have been many parametric studies in the
ing ONeMg core of massive stars (Wheeler et al.,
early days, assuming a set of neutron densities and tem-
1998). The emergence of realistic pre-explosion
peratures (e.g. Seeger et al., 1965; Kodama and Taka-
stellar models made this site less likely. Further
hashi, 1975; Kratz et al., 1986; Kratz et al., 1988, 1993;
options include — for low abundances of heavy el-
Freiburghaus et al., 1999a; Pfeiffer et al., 2001), the long-
ements in the early Galaxy — sufficient amounts
standing question is, where an r-process with neutron
of neutrons in the He-shell, provided via neutrino
densities of 1026 cm−3 and higher, producing highly un-
interactions (Epstein et al., 1988; Nadyozhin and
stable neutron-rich isotopes of all heavy elements and
Panov, 2007). But this scenario, with low neutron
permitting a fast build-up of the heaviest elements up to
number densities, would not be able to produce the
the actinides, can take place.
solar r-process pattern with its correct peak loca-
As will be discussed later with respect to observations,
tions (Banerjee et al., 2011; Qian, 2014; Banerjee
there are indications that a “weak” and a “strong” r-
et al., 2016).
process seems to occur in nature, and the “strong” com-
ponent is probably the dominant one, accounting essen- 3. Special classes of core-collapse events of massive
tially for solar-system r-process abundances. But some stars with fast rotation and high magnetic fields.
old stars, although displaying abundances of r-process They can either lead to highly magnetized neu-
elements, including Eu, show a strongly declining trend tron stars (magnetars) and neutron-rich jet ejecta
towards heavy elements, and it is not clear whether the (MHD-jet supernovae) along the polar axis (Sym-
third r-process peak with A = 195 or even the actinides balisty et al., 1985; Cameron, 2003; Nishimura
are present. In our review we will focus mostly on the et al., 2006; Winteler et al., 2012; Mösta et al.,
strong r-process, but discuss observations and possible 2014; Nishimura et al., 2015; Mösta et al., 2015;
sites of the weak r-process as well. There have been many Nishimura et al., 2017; Mösta et al., 2018; Halevi
suggestions relating the site of the strong r-process to and Mösta, 2018; Obergaulinger et al., 2018) or to5
black holes, polar jets, and black hole accretion disk of state that ultimately determines the transition from a
outflows (hypernovae/collapsars). The latter have neutron star to a black hole plays an important role in
been attributed to neutron-rich jet ejecta (e.g. Fu- the modeling of these objects. We will not discuss this
jimoto et al., 2008; Ono et al., 2012) and/or the topic further but refer the interested reader to recent re-
creation of r-process elements in black hole accre- views on the nuclear equation of state (Lattimer, 2012;
tion disks (e.g. Pruet et al., 2003, 2004; Siegel et al., Hebeler et al., 2015; Özel and Freire, 2016; Oertel et al.,
2019). The first type of events showed quite some 2017; Tews et al., 2019; Bauswein and Stergioulas, 2019).
promise for producing r-process ejecta, but the ne- Before discussing the r-process astrophysical sources
cessity that very high pre-collapse magnetic fields in detail, a lot of groundwork has to be laid out. Sec-
exist puts constraints on this secenario. The second tion II provides an overview of observations (including
option (collapsars) stands for a high-angular mo- the atomic physics for their correct interpretation), sec-
mentum subset of rotating stars which form black tion III the basic working of an r-process and which con-
holes in combination with long-duration gamma- ditions are needed for its successful operation, sections IV
ray bursts (GRB). A variant of this, based on the and V discuss the impact played by nuclear physics (with
spiraling in of a neutron star via merging with a experimental and theoretical investigations), and sec-
giant in a binary system (leading eventually to ac- tion VI passes through the astrophysical sites which can
cretion, black hole formation, and a black hole ac- fulfill the required conditions. Section VIII combines
cretion disk) has been suggested by Grichener and these astrophysical sites and how their role in galactic
Soker (2019). evolution connects to section II. Finally in the summary
(section IX), after having presented all possible connec-
4. Ejecta from binary neutron star (or BH-neutron
tions, we discuss remaining issues and open questions,
star) mergers have been studied for many years
i.e. whether a single r-process site has been identified by
before the first detection of such an event (e.g.
now, or whether we still might need several sources to
Lattimer and Schramm, 1974; Symbalisty and
explain observations throughout galactic evolution.
Schramm, 1982; Eichler et al., 1989; Freiburghaus
et al., 1999b; Rosswog et al., 2000, 2014; Wanajo
et al., 2014; Goriely et al., 2011; Just et al., 2015a;
Eichler et al., 2015; Goriely et al., 2015; Ramirez- II. OBSERVATIONS
Ruiz et al., 2015; Mendoza-Temis et al., 2015;
A. Stellar Abundances of Neutron-Capture Elements in
Shibagaki et al., 2016; Wu et al., 2016; Lippuner
Metal-Poor Stars
et al., 2017; Thielemann et al., 2017b). After the
gravitational wave detection GW170817 of a neu-
Stellar abundance observations over decades have pro-
tron star merger with a combined total mass of
vided fresh evidence about the nature and extent of heavy
about 2.74 M , (Abbott et al., 2017c, 2019), ac-
element nucleosynthesis. In the case of the s-process
companied by a kilonova observation supporting
there is direct observational evidence of in situ stellar
the production of heavy elements (see e.g. Metzger,
nucleosynthesis with the observation of the radioactive
2017b; Tanaka et al., 2017; Villar et al., 2017), this
element Tc, discovered first by Merrill (1952). Additional
type of event has attracted special attention (see re-
stellar abundance studies have strongly linked this type of
views e.g. by Rosswog et al., 2018; Horowitz et al.,
nucleosynthesis to very evolved He shell-burning asymp-
2019; Shibata and Hotokezaka, 2019, and refer-
totic giant branch stars (e.g. Busso et al., 1999; Käppeler
ences therein). More recent gravitational wave
et al., 2011; Karakas and Lattanzio, 2014). There is no
observations point to further neutron star merg-
similar example for the r-process, related to nucleosyn-
ers (e.g. GW190425 with a combined total mass of
thesis during stellar evolution, as it requires rather exten-
∼ 3.4 M , Abbott et al., 2020), or even neutron
sive neutron fluxes only obtainable in explosive events.
star-black hole merger candidates (e.g. S190426c
Some elements are only formed exclusively or almost so
with a combined total mass in excess of 7 M ,
in the r-process, such as Eu, Os, Ir, Pt, Th and U.
Lattimer, 2019). The latter two events had no ob-
Their presence in old galactic very metal-poor (VMP)
served electromagnetic counterpart, due to either
halo stars is a clear indication that this process occurred
non-existence or non-detection, related to a larger
in violent astrophysical sites early in the history of the
distance and/or missing precise directions. (Foley
Galaxy (see e.g. Sneden et al., 2008; Thielemann et al.,
et al., 2020; Kyutoku et al., 2020; Barbieri et al.,
2017b, and references therein).
2020; Ackley et al., 2020)
Identification of r-process-rich stars began with the dis-
Most of the astrophysical sites mentioned above involve covery of overabundances of neutron-capture elements in
ejection of material from high densities and involve a neu- the field red giant HD 115444 (Griffin et al., 1982). This
tron star or black hole produced during core-collapse or a was followed by the identification of an r-process pattern
compact binary merger. Hence the high density equation in the well known bright giant HD 122563, even though6
10 2 of these, mostly rare-earth, elements in the stars were
Goriely smaller than in the Sun but with the same relative pro-
Abundances (Y(Si)= 10 6)
10 1 Cowan, Thielemann & Truran
portions, i.e., scaled. This indicated that these stars, that
likely formed early in the history of the Galaxy, experi-
10 0
enced already a pollution by a robust r-process.
10 −1
10 −2
10 −3
10 −4
60 80 100 120 140 160 180 200
Mass Number, A
FIG. 2 Solar r-process abundances as determined by Cowan
et al. (1991) and Goriely (1999). The largest uncertainties
are clearly visible for A . 100 (weak s process region) and
around lead.
its overall neutron-capture element level is depressed rel-
ative to Fe (Sneden and Parthasarathy 1983, see also the
more extensive analysis of Honda et al. 2006). An ini-
tial abundance survey in metal-poor (MP) stars (Gilroy
et al., 1988) considered 20 red giants, finding a common
and easily spotted pattern of increasing overabundances
from Ba (Z = 56) to Eu (Z = 63) among the rare-
earth elements. With better echelle spectrographic data
came discoveries of many more r-process-rich stars, lead-
ing Beers and Christlieb (2005) to sub-classify them as
“r-I” with 0.3 ≤ [Eu/Fe] ≤ +1.0 and [Ba/Eu] < 0, and
as “r-II” with [Eu/Fe] > +1.0 and [Ba/Eu] < 0.
The most detailed deconvolution of abundances into FIG. 3 Top panel: neutron-capture abundances in 13 r-
nucleosynthetic contributions exists for the solar system, II stars (points) and the scaled solar-system r-process-only
as we have accurate abundances down to the isotopic abundances of (Siqueira Mello et al., 2013), mostly adopted
level as a result of meteoritic and solar atmospheric mea- from (Simmerer et al., 2004). The stellar and solar sys-
tem distributions have been normalized to agree for ele-
surements (e.g. Cameron, 1959; Asplund et al., 2009; ment Eu (Z = 63), and than vertical shifts have been ap-
Lodders et al., 2009, see Fig. 1). Identifying the r- plied in each case for plotting clarity. The stellar abundance
process contributions to the solar system neutron-capture sets are: (a) CS 22892-052, (Sneden and Cowan, 2003); (b)
abundances is usually accomplished by first determining HD 115444, (Westin et al., 2000); (c) BD+17 3248, (Cowan
the s-process fractions, (e.g. Käppeler, 1999; Arlandini et al., 2002); (d) CS 31082-001, (Siqueira Mello et al., 2013);
et al., 1999; Burris et al., 2000; Käppeler et al., 2011). (e) HD 221170, (Ivans et al., 2006); (f) HD 1523+0157,
The remaining (residual) amount of the total elemental (Frebel et al., 2007); (g) CS 29491-069, (Hayek et al., 2009);
(h) HD 1219-0312, (Hayek et al., 2009); (i) CS 22953-003,
abundance is assumed to be the solar r-process contri- (François et al., 2007); (j) HD 2252-4225, (Mashonkina et al.,
bution (see Figures 1 and 2). Aside from the so-called 2014); (k) LAMOST J110901.22+075441.8, (Li et al., 2015);
p-process (Arnould and Goriely, 2003; Rauscher et al., (l) RAVE J203843.2-002333, (Placco et al., 2017); (m) 2MASS
2013; Nishimura et al., 2018) that accounts for the minor J09544277+5246414, (Holmbeck et al., 2018). Bottom panel:
heavy element isotopes on the proton-rich side of the val- mean abundance differences for the 13 stars with respect to
ley of instability, as well as the ν-process (Woosley et al., the solar system r-process values.
1990) and the νp-process (Fröhlich et al., 2006b), only
the s and r-processes are needed to explain nearly all of However, the growing literature on abundance analyses
the solar heavy element abundances. of VMP stars has added to our knowledge of the aver-
Early observations of CS 22892-052 (Sneden et al., age r-process pattern, and has served to highlight depar-
1994, 2003) and later CS 31082-001 (Hill et al., 2002; tures from that pattern. Additions to the observational
Siqueira Mello et al., 2013) and references therein), in- results since the review of Sneden et al. (2008) include
dicated a “purely” or “complete” solar system r-process Roederer et al. (2010b, 2014a); Li et al. (2015); Roed-
abundance pattern (see Figure 3). The total abundances erer et al. (2016); Roederer (2017); Aoki et al. (2017);7
Yong et al. (2017); Hansen et al. (2018); Sakari et al. Roederer et al. (2010b).
(2018); Roederer et al. (2018). These additional ob-
servations have shown that there is a complex relation-
ship between light and heavy neutron-capture elements:
Travaglio et al. (2004); Cowan et al. (2005); Hansen and
Primas (2011); Hansen et al. (2012); Aoki et al. (2013);
Ural et al. (2015); Wu et al. (2016). In particular it has
been found in some stars that there is significant observed
star-to-star abundance scatter of lighter neutron-capture
elements (Z ≤ 50), opposite to the heavier ones (Z ≥ 56),
as shown in Fig. 3. For heavy neutron-capture elements,
particularly among the well-studied rare earths, an r-
process origin does not always mean perfect agreement
with the solar r-process pattern. So-called “truncated”
(or incomplete or limited) r-process stars have been iden-
tified with sharp abundance falloffs toward the heavy end
of the rare earths (Honda et al., 2006, 2007; Roederer
et al., 2010a; Boyd et al., 2012). These observed abun-
dance patterns can be described as having a range of
r-process “completeness” with some stars showing only
a partial agreement. The differences in these abundance
patterns have led to a flurry of stellar models and cal-
culations to identify a site or sites for the r-process, and
to determine why stars show differences in these heavy FIG. 4 Differences between stellar and r-process-only solar
element patterns. In addition to the suggestion to the system (s.s.) abundances for four very MP stars with r-
operation of a “weak” r-process, two additional processes process abundance mixes, after Figure 5 of Honda et al. (2007)
and Fig. 11 of Roederer et al. (2010b). The “s.s.,r-only”
have gained currency: the so-called Lighter Element Pri-
abundances are those of Siqueira Mello et al. (2013), mostly
mary Process (LEPP of still unknown origin; Travaglio from Simmerer et al. (2004). The stellar abundance sets
et al., 2004), and the i process (Cowan and Rose 1977; are: CS31082-001, (Siqueira Mello et al., 2013); HD 88609,
see also Denissenkov et al. 2017 and references therein). (Honda et al., 2007); HD 122563, (Honda et al., 2006); and
(While the LEPP and the i process may explain certain HD 221170, (Ivans et al., 2006).
individual stellar abundances, their contributions to the
total solar system (SS) abundances appear to be very To understand the types and nature of the nucleosyn-
small.) thesis, along with identifying the stellar sites and the
identities of the first stars in our Galaxy, demands highly
An r-process pattern (defined here as [Eu/Ba] > +0.3) precise stellar abundance observations. Those require
can be seen even in MP stars with bulk deficiencies in both high-resolution spectrographic measurements and
neutron-capture elements: In Fig. 4 we show differences accurate atomic data. Thus, the discovery of MP stars re-
in abundances between stellar observations and those of newed efforts to improve atomic data for many heavy (be-
the solar system attributed only to the r-process. Fig. 4 yond the Fe-group) neutron-capture elements (see e.g.,
is similar in structure to those of Honda et al. (2007) Sneden et al., 2009), as discussed below in section II.B.
and Roederer et al. (2010a). As defined in the figure, if
∆ log = 0, then the stellar neutron-capture abundance
set is identical to the solar-system r-process-only distri- B. Atomic Data for the Analysis of neutron-capture
bution. This is clearly the case for elements in the atomic Elements in Metal-Poor Stars
numbers range Z = 57–78, e.g. La–Pt in CS31082-001
(Siqueira Mello et al., 2013). All extremely r-process- Despite a great need for improved transition proba-
rich stars (classified as “r-II”: [Eu/Fe] > +1) have sim- bilities, the identification of lines from neutron-capture
ilar abundance runs in the heavy neutron-capture ele- elements in stellar spectra has been possible for most ele-
ments, as discussed above. However, many MP stars ments using readily available laboratory data from about
with a clear dominance of the r-process, as defined by the middle of the 20th Century. Wavelengths of spectral
[Eu/Ba] > +0.3, have abrupt drop-offs in abundances lines of such elements were measured during the first half
through the rare-earth domain. The most dramatic ex- of the 20th Century using large grating spectrographs
amples are the truncated r-process stars shown in Fig. 4: such as 10 m Rowland circle instruments. These early
HD 122563 (Honda et al., 2006) and HD 88609 (Honda wavelength measurements often achieved 1 part per mil-
et al., 2007). Intermediate cases are abound, as shown in lion (ppm) accuracy and were compiled in the well known8
Atomic Energy Level series by Moore (1971) and for the of recording a million point spectrum in minutes (Brault,
Rare-Earth Elements by Martin et al. (1971). The lat- 1976). Hollow cathode lamps which yield emission spec-
ter of these two works includes more data from Fourier tra for neutral and singly ionized atoms are available for
transform spectrometers (FTSs) and thus achieved ' essentially the entire periodic table.
0.01 ppm or 10 ppb accuracy in many cases. All of these Interest in rare-earth elements is a natural part of stud-
spectroscopic data are now available online1 . Although ies of neutron-capture elements in MP stars. Atoms and
modern optical frequency comb lasers could add many ions with open f-shells have a great many transitions in
additional digits to energy levels, this technology has not the optical. Rare-earths have important applications in
yet been widely applied because of the difficulty in simul- general lighting and in optoelectronics because of their
taneously using it on large numbers of spectral lines. rich visible spectra. Rare-earth elements in MP stars
The situation with respect to transition probabilities are convenient for spectroscopic studies in the optical re-
changed with the development of tunable dye lasers orig- gion accessible to ground based telescopes. Europium
inally by Sorokin and Lankard (1966) in the US and is a nearly pure r-process element and lanthanum is a
Schäfer et al. (1966) in Germany. Although it took some nearly pure s-process element in solar system material.
time to thoroughly control dye laser performance, many Although none of the r-process peaks are in the rare-
research groups had organic dye lasers with broad tun- earth row, the accessibility from the ground is a major
ability, narrow bandwidths (comparable to or less than advantage for rare earths.
Doppler widths), short (few nsec) pulse durations, and
repetition rates in the 10s of Hz. Non-linear techniques,
using crystals and/or gas cells, are needed to access IR
and UV wavelengths, and those were also increasingly
available. The remaining challenge is to make free atoms
and ions of various elements in the periodic table in an
optically thin sample with a low collision rate. There are
several methods, including sputtering metal cathodes,
in a low pressure gas cell (Hannaford and Lowe, 1981),
laser driven plasma sources (e.g. Svanberg et al., 1994)),
and the hollow cathode atom/ion beam source (Duquette
et al., 1981; Salih and Lawler, 1983). The broadly tun-
able organic dye lasers, in combination with a tech-
nique to make low pressure samples of metal atoms and
ions, opened the possibility of using time-resolved laser-
induced-fluorescence (TRLIF) to measure accurate and
precise (about a few %) radiative lifetimes of upper lev-
els on interest in atoms and ions. These lifetimes provide
an accurate and precise total decay rate for transition
probabilities from the selected upper level.
Emission branching fractions (BFs) in rich spectra still
represented a challenge. The same visible and UV capa-
ble FTS instruments (e.g. Brault, 1976)), used to improve
energy levels, became the “work horse” of efforts on BFs
in complex spectra. Reference Ar i and ii lines became in-
ternal standards for many laboratory spectra from hollow
cathode lamps recorded using FTS instruments (Whal- FIG. 5 Comparisons of laboratory data on Sm ii from Univer-
ing et al., 1993, and references therein). The advantages sity of Western Ontario (UWO) and University of Wisconsin
(UW) groups, adapted from Figs. 4 and 5 of Lawler et al.
of interferometric instruments such as the 1 m FTS of (2008). In panel (a) a histogram of differences in lifetimes (τ
the National Solar Observatory on Kitt Peak, AZ, were divided by their uncertainties added in quadrature) is shown,
critical for BF measurements in complex spectra. This along with a dashed line representing a one standard devi-
instrument has a large etendue common to all interfero- ation Gaussian. In panel (b) we show a similar histogram
metric spectrometers, wavenumber accuracy to 1 part in and Gaussian representation for transition probabilities (A-
108 , a limit of resolution as small as 0.01 cm−1 , broad values).
spectral coverage from the UV to IR, and the capability
Rare-earth elements tend to be singly ionized in the
photospheres of F, G, and K stars of interest for many
elemental abundance studies. The spectrum of singly ion-
1 http://physics.nist.gov/asd ized samarium (Sm ii) received special attention (Lawler9 et al., 2006; Rehse et al., 2006), Lawler et al. (2008) com- first r-process peak such as a As and Se have their va- pleted comparisons from the two sets of measurements. lence electrons in nearly closed p-shells. The huge gap Fig. 5 shows a histogram of lifetime measurement dif- between the ground and first resonance levels exists in ferences between the two studies with a one standard both the neutral and ion energy level structure, although deviation Gaussian superposed, and a similar histogram the neutral atom population is dominant in most stars comparison for Einstein A coefficients which include BFs. of interest for both of these elements. A similar prob- It is clear from these histograms that radiative lifetime lem arises for Te at the second r-process peak with only uncertainties are overly conservative and BFs uncertain- deep UV lines. Fortunately HST time was allocated for a ties are satisfactory but perhaps slightly too optimistic study of Te i lines in multiple MP stars (Roederer et al., in at least one of two sets of measurements. 2012). The success of the Te study inspired a careful Uncertainties in radiative lifetimes from TRLIF exper- search through the HST archives for one or more stars iments have proven to be easier to minimize than uncer- with sufficiently deep UV spectral coverage for observa- tainties in emission BFs. Various techniques can con- tions on all three r-process peaks (Roederer and Lawler, veniently be used to check for optical depth (vary the 2012). Unfortunately the star HD 160617 is likely the atom/ion beam intensity), to check for collisional effects only such star with sufficient deep UV spectral coverage. (throttle a vacuum pump), and to eliminate errors from Laboratory data sets for many of the lighter r-process Zeeman quantum beats (zero the magnetic field in the elements are included. Laboratory data sets for many of experimental region for short lifetimes and introduce a the lighter r-process elements could be improved, but a high, 30 Gauss, magnetic field for long lifetimes). Most successor telescope to HST with a high resolution spec- importantly benchmark lifetimes in simple spectra such trograph and UV capability will be needed to exploit as He i, Be i, Be ii, Mg ii, etc., which are well known improvements in the laboratory data. from accurate theory, can be periodically re-measured as The discovery of a single line of U II in an MP star an end-to-end test of the TRLIF experiment (Den Hartog (complicated by being located on the shoulder of a much et al., 2002). There are multiple challenges in BF mea- stronger Fe i line) by (Cayrel et al., 2001) was a milestone surements. It is essential to have a reliable relative ra- in stellar spectroscopy. Despite this complication, there diometric calibration, and a source that is optically thin is some confidence in its identification. Thorium is also for strong lines of interest. One must resolve lines of an element of choice for stellar chronometry (e.g. Sneden interest from nearby blending partners and line identifi- et al., 2003). cations must be correct. These latter two constraints are most easily achieved using FTS instruments due to their exceptional resolving power and absolute wave number C. Abundance Trends in Galactic and Extragalactic Stars accuracy and precision. Weak lines from an upper level of interest are clearly most vulnerable to blending, poor As already discussed in section II.A, the galactic MP signal-to-noise ratios (S/N), and other problems. Uncer- stars show indications of neutron-capture abundances, in tainty migrates to weak lines because BFs from an upper fact, it appears as if ALL such stars (to an observational level of interest sum to unity by definition. limit) exhibit some level of neutron-capture abundances. Elements with wide hyperfine structure and/or a wide In addition, observations have indicated the presence of range of isotopes require some additional effort, but in elements such as Ba in nearby dwarf spheroidal galax- most cases the needed hyperfine splitting (hfs) data can ies (e.g. Shetrone et al., 1998, 2003; Venn et al., 2003; be extracted from FTS spectra. The existence of even Skúladóttir et al., 2019). Recently there has been evi- a few hfs data from single frequency laser measurements dence of these elements in ultra-faint dwarf (UFD) galax- is helpful since such data can serve to constrain nonlin- ies, structures of only about 104 M and possibly being ear least square fitting of partially resolved hfs patterns also the building blocks and substructures of the early in FTS data. Laboratory transition probability measure- Galaxy (Brauer et al., 2019). By now more than 10 UFDs ments on rare-earth ions were summarized during a study are discovered around our Galaxy, being very metal-poor of Ce ii by Lawler et al. (2009) and were applied to five with metallicities of [Fe/H] ≈ −3 (Kirby et al., 2013; r-process rich very MP stars in a companion paper by Frebel and Norris, 2015; Ji et al., 2019b; Simon, 2019), Sneden et al. (2009). The most striking conclusion from and most of them show very low r-process enhancements. the decade long rare-earth study is that the relative r- However, one of them (Reticulum II) shows highly r- process abundance pattern is stable over time and space. process enhanced stars comparable to galactic r-process Third r-process peak elements, including Os, Ir, and Pt rich stars such as CS 22892-052 (Roederer, 2013; Ji et al., were observed in MP stars by Cowan et al. (2005). Some 2016; Roederer, 2017; Ji and Frebel, 2018) which seems useful lines of Os i and Ir i are accessible to ground based to go back to one very early r-process event. In addition studies. Unfortunately lines suitable for abundance stud- to Reticulum II, a further dwarf galaxy, Tucana III, has ies of many lighter neutron-capture elements are not ac- recently been observed and also shows r-process features cessible to ground based observations. Elements near the (Hansen et al., 2017; Marshall et al., 2019a,b).
10
FIG. 7 Abundances as a function of metallicity for [Mg/Fe]
(panel a) and [Eu/Fe] (panel b). This is an update of Fig. 14
in (Sneden et al., 2008). Red straight lines are approximate
FIG. 6 Abundances of [Sr/Fe] vs. [Ba/Fe] in a large number
fits to the averages of halo, thick disk, and thin disk stars.
of galactic and extragalactic stars from Roederer (2013) and
Black dashed lines in panel (b) highlight the growing star-to-
references therein.
star scatter in [Eu/Fe] with decreasing metallicity. Individual
data points are taken from Fulbright (2000); Hill et al. (2002);
Reddy et al. (2003); Cayrel et al. (2004); Simmerer et al.
We show in Fig. 6, (taken from Roederer, 2013, and (2004); Cohen et al. (2004); Barklem et al. (2005); Reddy et al.
references therein), a compilation of abundances in both (2006); François et al. (2007); Bensby et al. (2014); Roederer
galactic and extragalactic stars. In these observations et al. (2014a); Battistini and Bensby (2016).
the Sr abundance acts as a surrogate for the overall
metallicity of these stars and Ba indicates the enrichment
of neutron-capture elements. The figure illustrates that tensive scatter. These observations, combined with those
stars down to the lowest metallicities contain Sr and/or from ultrafaint dwarf galaxies, indicate that the heavy
Ba. In a solar mix these are predominantly s-process ele- r-process elements are made in rare events which con-
ments, i.e. their s-process isotopes dominate in present tribute significant amounts of material, when they occur
solar abundances. If massive stars with fast rotation (see Fig. 7). Such abundance comparisons can be used
rates contributed already some s-process in early galac- to put constraints on the site (or sites) for the r-process
tic evolution (Frischknecht et al., 2016), this could be due in terms of (a) ejecta compositions, (b) amounts of r-
to such s-process sources. However, global trends, where process ejecta, and (c) their mixing with the extended
observed elemental or isotopic ratios can be deconvolved interstellar medium in order to understand the history of
into s- and r-process contributions, show an s-process ap- element formation in the Galaxy (i.e., Galactic Chemical
pearance only in later periods of galactic evolution. Thus, Evolution, GCE). With respect to (a) Ji et al. (2019a)
this compilation strongly suggests that all of these stars have analyzed extended sets of low metallicity observa-
have been enriched in r-process material, which also has tions with [Fe/H] < −2.5 (attempting to select stars be-
implications for early nucleosynthesis in galaxies. ing polluted only by single events) and [Eu/Ba] > 0.4 (to
Clues about early galactic nucleosynthesis are also ensure a pure r-process origin, avoiding s-process con-
found in comparison of elements with different nucleosyn- tributions) with the aim to find the typical lanthanide
thesic origin. We show one such comparison in Figure 7, (plus actinide) fraction XLa among the global r-process
observed in halo stars, i.e. containing elements synthe- element distribution. This permits on the one hand to
sized prior to the formation of these stars. It is evident look for variations among old stars, indicating apparently
that alpha elements (such as Mg) appear early in galactic a different result for the bulk of low metallicity stars
evolution at low metallicities, originating from fast evolv- with log XLa ≈ −1.8, while the most r-process enriched
ing massive stars and core-collapse supernovae as their stars with [Eu/Fe] > 0.7 have log XLa > −1.5. This
final endpoints. Such events occur with a high frequency measure will also permit comparisons to future kilonova
during galactic evolution and show little scatter. Com- events, if observations allow to determine this quantity
mon r-process elements, like Eu, display, however, an ex- (see section VII). With respect to (b) and (c) of the list11
above we will return to galactic evolution issues later in
section VIII, after having presented the nucleosynthesis 0.9 Fe I
yields of different astrophysical sites. Fe I
The eventual demise of the Hubble Space Telescope,
0.8
able to obtain high-quality UV observations, will ham-
Normalized flux
per future progress in the observation of heavy elements Nd II
in low-metallicity stars. The James Webb Space Tele- 0.7
scope (JWST), the scientific “successor” of HST, will U II
have no UV capability but an IR capability. Identifi- 0.6
cation of neutron-capture element lines in the IR region CN
could provide new avenues for understanding the opera- 0.5
tion and nature of the r-process (see subsection II.B). [U/Fe] = 1.4 ± 0.2
0.4 [U/Fe] = −∞
D. The role of long-lived radioactive species
Residual (%)
4
Identification and detailed spectroscopic analysis of a 0
handful of r-II stars, e.g., CS 22892-052 (Sneden et al.,
1994, 2003) CS 31082-001 (Hill et al., 2002; Siqueira −4
Mello et al., 2013, and references therein), and HE 1523- 3859.2 3859.4 3859.6 3859.8 3860.0
0901 (Frebel et al., 2007) brought forth detections of Wavelength (Å)
the long-lived very heavy neutron-capture radioactive el-
ements Th (t1/2 = 13.0 Gyr) and U (t1/2 = 4.6 Gyr), FIG. 8 Synthesis and derived abundance for U in the star
which can only be made in the r-process, and in ad- 2MASS J09544277+5246414 from Figure 2 of (Holmbeck
dition an neutron-capture element abundances ranging et al., 2018).
from Z ≈ 30 to 92, indicating also an r-process pattern.
This makes detailed comparisons possible between obser-
vations and r-process theory. More Th detections have r-process contribution, while the [U/Th] did not show
been made since then, and more recently U has also been these anomalies (Mashonkina et al., 2014). Such an ac-
detected in some halo stars. Due to its shorter half-life, tinide boost is found in a few stars with metallicities
its abundance is inherently smaller and detections are [Fe/H] ≈ −3. This indicates that (a) an r-process was
difficult. Shown in Fig. 8 from (Holmbeck et al., 2018) is already contributing in very early galactic evolution, but
a uranium detection in 2MASS J09544277+5246414, the also (b) with possibly varying conditions for producing
most actinide-enhanced r-II star known. the heaviest elements, dependent on the r-process site.
These Th and U discoveries led to cosmochronology Unfortunately it has proved difficult to obtain U detec-
estimates, independent of a cosmological model, based tions in many stars, but it is surprising that an actinide
solely on decay half-lives of involved isotopes. This boost has not been seen at higher metallicities (see Fig. 9
method requires, however, Th/U ratios from theoreti- from Holmbeck et al., 2018).
cal r-process predictions (geared to fit a solar r-process In addition to observations of long-lived radioactive
pattern) plus the observed abundance ratios. This en- species seen via the spectra of stars throughout galac-
abled estimates on the decay-time since the birth of a tic evolution, there have also been detections in deep-sea
star (when the addition of new material from nucle- sediments, indicating more recent additions of these ele-
osynthesis sites stopped) and promising results were ob- ments to the earth. While the discussion in II.C points to
tained (Cowan et al., 1991; Cowan et al., 1999; Kratz rare strong r-process events in the early Galaxy, the latter
et al., 2000; Schatz et al., 2002; Hill et al., 2017). The detections, suggest the same in recent history. Long-lived
same can in principle also be done utilizing the Th/Eu radioactive species can act as witness of recent additions
ratio for some stars, yielding values in concordance with to the solar system, dependent on their half-lives. For a
cosmological age estimates (see above). The fact that review on the signature of radioactive isotopes alive in the
some stars seem to have experienced an “actinide boost”, early solar system see e.g. Davis and McKeegan (2014).
i.e. an enhanced amount of Th and U in comparison Two specific isotopes have been utilized in recent years
to lighter r-process elements, could point back to a to measure such activities in deep sea sediments. One of
non-universal r-process production pattern and possi- them, 60 Fe, has a half-life of 2.6 × 106 yr and can indicate
bly varying r-process compositions from different pro- recent additions from events occurring up to several mil-
ductions sites. This made the [Th/Eu] chronology un- lion years ago. 60 Fe is produced during the evolution and
certain or non-reliable for such stars (e.g. Cayrel et al., explosion of massive stars, leading to supernovae (Thiele-
2001; Honda et al., 2004), having experienced a non-solar mann et al., 2011; Wanajo et al., 2013; Limongi and Chi-12
Halo r -I
tal findings (Wallner et al., 2015) are in agreement with
0.0 −15.4
J0954+5246 Halo r -II an r-process origin from a rare event. This can explain
−0.2 −6.1 the 244 Pu existing initially in the very early solar system
as well as the low level of more recent additions witnessed
log (Th/Eu)
Age (Gyr)
−0.4 3.3 in deep-sea sediments over the past few hundred million
−0.6 12.6
years.
−0.8 21.9
DES J033523−540407
E. Kilonovae observations
−1.0 31.3
−3.5 −3.0 −2.5 −2.0 −1.5 For many years a connection between observations of
[Fe/H] short-duration gamma-ray bursts (sGRBs), supernova-
like electromagnetic transients (macronovae/kilonovae),
FIG. 9 Th/Eu ratios for stars with detected thorium abun- and compact binary mergers has been postulated (see
dances from Holmbeck et al. (2018). One can see that at low e.g. Piran, 2004). The first observational evidence of an
metallicities around [Fe/H]≈-3 quite a number of so-called ac- excess of near infrared emission over the standard GRB
tinide boost stars can be found. If utilizing initial r-process
afterglow came in 2013 with the observation of GRB
production ratios which fit solar r-abundances (Schatz et al.,
2002), unreasonable, and even negative, ages of these stars 130603B by Tanvir et al. (2013)2 and suggested a ther-
are obtained, not at all consistent with their metallicity, which mal component consistent with kilonova emissions. Fur-
points to the formation of these stars in the very early Galaxy. ther, evidence has been obtained from a reanalysis of the
GRB 060614 (Yang et al., 2015), GRB 050709 (Jin et al.,
2016), and GRB 070809 (Jin et al., 2020) afterglow data
including a first estimate of the kilonova emission tem-
perature (Jin et al., 2015). See Gompertz et al. (2018)
effi, 2018; Thielemann et al., 2018). It is found in deep-
for a review of kilonova candidates associated to short
sea sediments which incorporated stellar debris from a
GRB observations.
nearby explosion about two million years ago (Knie et al.,
Following the seminal work of Li and Paczyński (1998),
2004; Ludwig et al., 2016; Wallner et al., 2016; Sørensen
first predictions of light curves powered by radioactive
et al., 2017). Such a contribution is consistent with a
decay were done by Metzger et al. (2010b); Roberts et al.
supernova origin and related occurrence frequencies, wit-
(2011); Goriely et al. (2011). These initial studies used
nessing the last nearby event. Another isotope utilized,
244 grey opacities appropriate to the Fe-rich ejecta in type
Pu, has a half-life of 8.1 × 107 yr and would con-
Ia SNe and predicted peak luminosities at timescales of
tain a collection from quite a number of contributing
a day in the blue. However, the opacity of heavy r-process
events. If the strong r-process would take place in every
elements is substantially higher due to the high density
core-collapse supernova from massive stars, about 10−4 –
of line transitions associated with the complex atomic
10−5 M of r-process matter would need to be ejected
structure of lanthanides and actinides. This lead to a
per event in order to explain the present day solar abun-
light curve peak at timescales of a week in the red/near-
dances (see Fig. 39). The recent 244 Pu detection (Wall-
infrared (Kasen et al., 2013; Barnes and Kasen, 2013;
ner et al., 2015) is lower than expected from such predic-
Tanaka and Hotokezaka, 2013). Metzger et al. (2015)
tions by two orders of magnitude, suggesting that con-
speculated on the possibility that in fast expanding ejecta
siderable actinide nucleosynthesis is very rare (permitting
unburned neutrons are left and lead via their decay to a
substantial decay since the last nearby event). This in-
ultraviolet/blue precursor event. Early blue emission has
dicates that (regular) core-collapse supernovae did not
also been suggested to originate from the hot cocoon that
contribute significantly to the strong r-process in the so-
surrounds the GRB jet as it crosses the ejecta (Gottlieb
lar neighborhood for the past few hundred million years,
et al., 2018).
but does not exclude a weak r-process contribution with
On August 17, 2017 the gravitational wave event
very minor Eu production (Fields et al., 2019; Wallner
GW170817 was observed (Abbott et al., 2017c) and iden-
et al., 2019). Thus, in addition to the inherent problems
tified as merger of two neutron stars. With the com-
of (regular) core-collapse supernova models (to be dis-
bination of gravitational wave signals and electromag-
cussed in later sections) to provide conditions required
netic observations, its location was identified (Abbott
for a strong r-process — also producing the actinides in
et al., 2017d), a (weak) sGRB detected (Abbott et al.,
solar r-process proportions — these observational con-
2017b) (weak probably due to an off-axis observation, Wu
straints from nearby events also challenge them as source
of main r-process contributions. A recent careful study of
the origin of the strong r-process with continuous accre-
2
tion of interstellar dust grains into the inner solar system see https://kilonova.space for an up to date catalog of kilonova
(Hotokezaka et al., 2015) concluded that the experimen- observations13
2.5
Bolometric luminosity(erg/s)
1042
1041 2.0 0818
0819
1040 1.5 0820
0821
Flux
0822
1039 1.0 0823
Smartt et. al. (2017)
0824
Villar et. al. (2018)
1038 Tanvir et. al. (2017) 0.5 0825
Kasliwal et. al. (2018) 0826
1037 0827
1 10 100 0.0 5000 10000 15000 20000 25000
Wavelength (Å)
time (days)
FIG. 10 (left panel) Bolometric light curve of AT 2017gfo, the kilonova associatted with GW170817. The filled black triangles
are from Smartt et al. (2017). Uncertainties derived from the range of values given in the literature (Waxman et al., 2018;
Cowperthwaite et al., 2017; Smartt et al., 2017) are shown as a grey band. Also shown are lower limits (empty triangles) on the
late-time luminosity as inferred from the Ks band with VLT/HAWK-I (Tanvir et al., 2017) (black) and the 4.5 µm detections
by the Spitzer Space Telescope from Villar et al. (2018) (green) and Kasliwal et al. (2019) (blue) (adapted from Wu et al., 2019).
(right panel) Evolution of the kilonova flux spectrum during the first 10 days. Each spectrum is labelled by the observation
epoch. The shaded areas mark the wavelength ranges with very low atmospheric transmission (Reprinted by permission from
Springer Nature, Pian et al., 2017).
and MacFadyen, 2018; Mooley et al., 2018), accompanied presence of luminous visual wavelength (“blue”) emission
by secondary X-ray and radio signals. at early times was interpreted by most groups as arising
Within eleven hours of the merger the electromagnetic from the fastest outer layers of the ejecta, which con-
transient, named AT 2017gfo, was observed in the ultra- tained exclusively light r-process nuclei with a relatively
violet, optical and near infrared wavelength bands in the low visual wavelength opacity (Metzger and Fernández,
galaxy NGC 4993 (Arcavi et al., 2017; Chornock et al., 2014; Nicholl et al., 2017b; Drout et al., 2017), see how-
2017; Coulter et al., 2017; Cowperthwaite et al., 2017; ever (Waxman et al., 2018; Kawaguchi et al., 2018). The
Drout et al., 2017; Evans et al., 2017; Kasliwal et al., observed transition of the emission colors to the near-
2017; Nicholl et al., 2017b; Pian et al., 2017; Smartt et al., infrared confirmed predictions for the inner ejecta layers
2017; Soares-Santos et al., 2017; Tanvir et al., 2017). The containing lanthanide elements, with atomic mass num-
left panel of Fig 10 shows the bolometric light curve for ber A & 140 (Kasen et al., 2013; Barnes and Kasen, 2013;
the two-week-long epoch of detailed observations adapted Tanaka and Hotokezaka, 2013).
from Wu et al. (2019). The figure also includes late-time In order to explain the color evolution of the emission,
observations from the Ks band with the VLT/HAWK- models with at least two-components are necessary. Sim-
I (Tanvir et al., 2017) and the 4.5 µm detections by ulations suggest that at least three components are neces-
the Spitzer Space Telescope (Villar et al., 2018; Kasli- sary to account for the ejecta of neutron star mergers: dy-
wal et al., 2019). The right panel shows the evolution of namic, winds, and secular outflows from the disk (Perego
the kilonova flux spectra from the X-shooter VLT spec- et al., 2017a). Combining all observations Villar et al.
trograph during the first 10 days from Pian et al. (2017). (2017) find a best-fit kilonova model consisting of three-
Further analysis of these spectra even led to the first components: a “blue” lanthanide-poor component (opac-
identification of an element, Sr (Watson et al., 2019). ity κ = 0.5 cm2 g−1 ) with Mej ≈ 0.020 M , moving
The luminosity and its evolution agreed with predic- with a velocity of approximately 0.27 c, an intermedi-
tions for the light powered by the radioactive decay of ate opacity “purple” component (κ = 3 cm2 g−1 ) with
heavy nuclei synthesized via the r-process in the neutron- Mej ≈ 0.047 M at 0.15 c, and a “red” lanthanide-rich
rich merger ejecta (Li and Paczyński, 1998; Metzger component (κ = 10 cm2 g−1 ) with Mej ≈ 0.011 M at
et al., 2010b; Roberts et al., 2011; Barnes and Kasen, 0.14 c. The three-component model is compatible with
2013; Rosswog et al., 2018) (see section VII). Additional a two-component model containing only blue and red
evidence is provided by the spectral/color evolution. The components. The blue component is expected to con-You can also read
