The Radioactive Nuclei 26Al and 60Fe in the Cosmos and in the Solar System - arXiv
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Publications of the Astronomical Society of Australia (PASA)
doi: 10.1017/pas.2021.xxx.
26 60
The Radioactive Nuclei Al and Fe in the Cosmos and
in the Solar System
arXiv:2109.08558v2 [astro-ph.HE] 5 Oct 2021
R. Diehl1 , M. Lugaro2,3,4 , A. Heger4,5,6,7 , A. Sieverding8,9 , X. Tang10 , K. A. Li10 , E. T. Li11 ,
C. L. Doherty2,4 , M. G. H. Krause12 , A. Wallner13,14 , N. Prantzos15 , H. E. Brinkman2,16 ,
J. W. den Hartogh2 , B. Wehmeyer2,12 , A. Yagüe López2 , M. M. M. Pleintinger1 , P. Banerjee17 ,
W. Wang18,19
1 Max Planck Institut für extraterrestrische Physik, D-85748 Garching, Germany
2 Konkoly Observatory, Eötvös Loránd Research Network (ELKH), H-1121 Budapest, Konkoly Thege M. út 15-17, Hungary
3 ELTE Eötvös Loránd University, Institute of Physics, Budapest 1117, Pázmány Péter sétány 1/A, Hungary
4 School of Physics and Astronomy, Monash University, VIC 3800, Australia
5 Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav), Clayton, Vic 3800, Australia
6 Center of Excellence for Astrophysics in Three Dimensions (ASTRO-3D), Australia
7 Joint Institute for Nuclear Astrophysics, 1 Cyclotron Laboratory, NSCL, Michigan State University, East Lansing, MI 48824, USA
8 School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455 , USA
9 Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
10 Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, P.R. China
11 College of Physics and Optoelectronic Engineering, ShenZhen University, P.R. China
12 Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Hertfordshire, AL10 9AB, UK
13 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, 01328 Dresden, Germany
14 Research School of Physics, Australian National University, Canberra, ACT 2601, Australia
15 Institut d’Astrophysique, Paris, France
16 Graduate School of Physics, University of Szeged, Dom ter 9, Szeged, 6720 Hungary
17 Discipline of Physics, Indian Institute of Technology Palakkad, Kerala, India 678557
18 School for Physics and Technology, Wuhan University, Wuhan 430072, P.R. China
19 WHU-NAOC Joint Center for Astronomy, Wuhan University, Wuhan 430072, P.R. China
Abstract
The cosmic evolution of the chemical elements from the Big Bang to the present time is driven by
nuclear fusion reactions inside stars and stellar explosions. A cycle of matter recurrently re-processes
metal-enriched stellar ejecta into the next generation of stars. The study of cosmic nucleosynthesis and
of this matter cycle requires the understanding of the physics of nuclear reactions, of the conditions at
which the nuclear reactions are activated inside the stars and stellar explosions, of the stellar ejection
mechanisms through winds and explosions, and of the transport of the ejecta towards the next cycle,
from hot plasma to cold, star-forming gas. Due to the long timescales of stellar evolution, and because of
the infrequent occurrence of stellar explosions, observational studies are challenging, as they have biases
in time and space as well as different sensitivities related to the various astronomical methods. Here, we
describe in detail the astrophysical and nuclear-physical processes involved in creating two radioactive
isotopes useful in such studies, 26Al and 60 Fe. Due to their radioactive lifetime of the order of a million
years these isotopes are suitable to characterise simultaneously the processes of nuclear fusion reactions
and of interstellar transport. We describe and discuss the nuclear reactions involved in the production
and destruction of 26Al and 60 Fe, the key characteristics of the stellar sites of their nucleosynthesis and
their interstellar journey after ejection from the nucleosynthesis sites. This allows us to connect the
theoretical astrophysical aspects to the variety of astronomical messengers presented here, from stardust
and cosmic-ray composition measurements, through observation of γ rays produced by radioactivity, to
material deposited in deep-sea ocean crusts and to the inferred composition of the first solids that have
formed in the Solar System. We show that considering measurements of the isotopic ratio of 26Al to 60 Fe
eliminate some of the unknowns when interpreting astronomical results, and discuss the lessons learned
from these two isotopes on cosmic chemical evolution. This review paper has emerged from an ISSI-BJ
Team project in 2017–2019, bringing together nuclear physicists, astronomers, and astrophysicists in
this inter-disciplinary discussion.
Keywords: nucleosynthesis – isotope – nucleus:reaction – stars:evolution – interstellar medium –
1
2 Diehl et al.
that such an ensemble of nucleons and isotopes has ex-
perienced along its cosmic trajectory. First, we have
to understand the nucleosynthesis processes themselves,
within stars and stellar explosions, that modify the nu-
clear composition; the nuclear reactions here mostly
occur in low-probability tails at energies of tens of keV,
which in many cases is far from what we can study
by experiments in terrestrial laboratories, so that of-
ten sophisticated extrapolations are required. Beyond
these nuclear reactions and their sites, we have to under-
stand how nuclei are transported in and out of stellar
nucleosynthesis sites and towards the next generation
of stellar nucleosynthesis sites throughout the Galaxy.
A key ingredient is the path through the interstellar
matter towards newly-forming stars, after nuclei have
been ejected from the interior of a star by a stellar wind
or a stellar explosion.
It is possible to measure interstellar isotopes and their
relative abundances directly, by suitably capturing cos-
mic matter and then determining its isotopic compo-
sition, e.g., using mass spectrometry. In fact, cosmic
matter rains down onto Earth continuously in modest
but significant quantity – the discovery of live radioac-
Figure 1. Scientific publications per year, addressing 26 Al (above) tive 60 Fe isotopes in Pacific ocean crusts (Knie et al.,
and 60 Fe (below). A total of >2,000 refereed papers with >25,000
citations and >300,000 reads (for 26 Al) represent the size of the
2004) and in galactic cosmic rays (Binns et al., 2016)
community involved in these topics. (Data and plots from NASA have demonstrated this. It is a major challenge for astro-
ADS). nomical instrumentation, however, to determine abun-
dances of cosmic nuclei for regions that are not accessible
through material transport or spacecraft probes, i.e., in
1 INTRODUCTION different parts of our current and past Universe. For
Understanding the cosmic evolution of the composition example, in starlight spectra only some isotopic signa-
of matter from the Big Bang until the present time re- tures may be recognised, and only when measuring at
quires tracing the ensemble of atomic nuclei through extremely high spectral resolution.
their nuclear transformations on their journey across Astronomical abundance measurements are subject
space and time. These transformations are called nucle- to biases, in particular, because atomic nuclei appear
osynthesis: nuclear reactions that rearrange how protons in different phases, such as plasma, neutral or partially-
and neutrons are grouped into the different isotopes of ionized atoms, or molecules. Therefore, observational
the chemical elements. In nature, nuclear reactions may signals differ from each other. For example, an elemental
occur through collisions or disintegration of nuclei in species may be accelerated as cosmic rays or condensed
hot and energetic environments, such as the Big Bang, into dust, depending on how a meteoric inclusion, such
stellar explosions, the hot interiors of stars, and the inter- as a pre-solar dust grain, had been formed, or how an ion
stellar space where they involve accelerated cosmic-ray mixture may generate an observable spectral line in the
particles. Rearrangements of nucleons through nuclear atmosphere of a star, characteristically absorbing the
reactions therefore drive the change of elemental and starlight originating from the interiors of stars. Observa-
isotopic composition in the Universe from the almost tions of cosmic isotopes are rather direct if radioactive
pure H and He made in the Big Bang to the current rich isotopes can be seen via their radioactive-decay signa-
variety of elements, including C to U, that also enables tures outside stars, i.e., without biases and distortions
biological life. This process is called chemical evolution 1 . from absorption. This is possible when characteristic
In this review, we will disentangle the processes involved γ-ray lines are measured from such radioactive decay.
by picking specific nuclei as examples, and tracing their The detection of characteristic 26Al decay γ rays (Ma-
origins and cosmic journey to us. honey et al., 1982) was the first direct proof that nucle-
The relative abundances of different isotopes in a given osynthesis must be ongoing within the current Galaxy,
material are the result of the nucleosynthetic episodes because this isotope has a characteristic decay half-life
of 0.72 Myr, much shorter than the age of the Galaxy,
1 Although there is no chemistry involved in the evolution of more than 10 Gyr. 26Al, and, similarly, 60 Fe (with a
elemental and isotopic abundances. half-life of 2.62 Myr), both probe recent nucleosynthe-
The Radioactive Universe 3
sis and ejecta transport. They have been measured in
γ rays from interstellar space, have been found in ter-
restrial deposits, and have even been inferred to exist
in specific abundance in the first solids that formed in
the Solar System 4.6 Gyr ago. These two isotopes ex-
emplify a new approach to cosmic chemical evolution
studies, which involves a wide community, from nuclear
physicists through Solar System scientists, astrophysical
theorists, and astronomers working on a broad range
of topics. As a result, there is a significant diversity
of scientific publications addressing these two isotopes,
with discussions increasing in intensity over the past two
decades (Figure 1). This review focuses on discussion of
these two specific isotopes, in relation to the nuclear and
astrophysical processes involved in the cycle of matter
that drives cosmic chemical evolution.
In this paper, we assemble and combine the different
views on this theme from a working group on “Radioac-
tive Nuclei in the Cosmos and in the Solar System” Figure 2. The table of isotopes in the neighbourhood of 26Al.
that met at ISSI-Beijing2 in 2018 and 2019. The team Each isotope is identified by its usual letters and the total number
included astronomers, theorists in various aspects of as- of nucleons, with stable isotopes and black and unstable isotopes in
colored boxes. The second line for unstable isoptopes indicates the
trophysics and nuclear physics, as well as nuclear physics lifetime. The third line lists spin and parity of the nucleus ground
experimentalists. The members of the working group state. The primary decay channel is indicated in the bottom left.
covered a variety of different expertises and interests The stable elements have their abundance fractions on Earth in
and we chose to exploit this diversity to describe the
the last row. (extracted from Karlsruher Nuklidkarte, original by
the JRC of the EU)
journey of cosmic isotopes from a nuclear astrophysics
perspective using the two isotopes 26Al and 60 Fe as exam-
ples. We describe the properties of these nuclei and their ratio of these two isotopes allows to eliminate some of
reactions with other nuclei, the astrophysical processes the unknowns in astrophysical modelling and interpreta-
involved in their production, and how observations of tion. Our conclusions (Section 5) summarise the nuclear
their abundance ratio can be exploited to learn about physics, astrophysics, astronomical, and methodological
which nuclear transformations happen inside stars and issues, and the lessons learned as well as the open ques-
their explosions. tions from the study of 26Al and 60 Fe in the context of
The main goal of this paper is to pose the scientific cosmic chemical evolution.
questions in all their detail, not to provide ultimate
consensus nor answers. We aim to illuminate the approx-
26
imations and biases in our way of arguing and learning, 2 THE COSMIC TRAJECTORY OF AL
as this is important for all theory, observations, and 2.1 Nuclear properties, creation and
experiment. Ideally, we wish to identify critical obser- destruction reactions
vations, experiments, and simulations that can help to
validate or falsify these approximations, towards a bet- 2.1.1 Nuclear properties of 26 Al
ter understanding of the physical processes involved in Figure 2 shows the 26Al isotope within its neighbouring
transforming the initial H and He during cosmic evolu- nuclides, with 27 Al as the only stable isotope of Al. The
tion into the material mix that characterises our current, ground state of 26 Al (26 Alg ) (see Figure 3) has a spin
life-hosting Universe. and parity of 5+ and a β + -decay half-life of 0.717 Myr. It
In Section 2, we focus on the case of 26Al and carry this decays into the first excited state of 26 Mg (1809 keV; 2+ ),
discussion from nuclear properties and reaction physics which then undergoes γ-decay to the ground state of
through cosmic nucleosynthesis sites to interstellar trans- 26
Mg producing the characteristic γ ray at 1808.63 keV.
port and creation of astronomical messengers. Section 3 The first excited state of 26 Al at 228 keV (26 Alm ) is an
discusses the case of 60 Fe and what is different from the isomeric state with a spin and parity of 0+ . It is directly
case of 26Al in relation to each of those processes for connected to the 26 Alg state via the highly-suppressed
60
Fe. Section 4 shows how investigation of the abundance M 5 γ-decay with a half-life of 80,500 yr according to
shell model calculations (Coc et al., 2000; Banerjee et al.,
2018). 26 Alm decays with a half-life of just 6.346 s almost
2 The International Space Science Institute ISSI has its main
home in Bern, Switzerland, and a satellite institute in Beijing.
Scientific workshops and working groups are one main asset of the exclusively to the ground state of 26 Mg via super-allowed
ISSI in support of the scientific community. β + -decay(Audi et al., 2017), with a branching ratio of
4 Diehl et al.
100.0000+0−0.0015 (Finlay et al., 2012). of AGB stars are T = 0.04 - 0.09 GK. In these environ-
In cosmic nucleosynthesis, the correct treatment of ments, 26 Al is produced by 25 Mg(p,γ)26 Alg,m acting on
26
Alm and 26 Alg in reaction network calculations is cru- the initial abundance of 25 Mg within the MgAl cycle
cial (Runkle et al., 2001; Gupta & Meyer, 2001). When shown in Figure 4. 25 Mg can also be produced by the
24
26
Al is produced by a nuclear reaction, it is produced in Mg(p,γ)25 Al(β + )25 Mg reaction chain at the tempera-
an excited state, which rapidly decays to the isomeric ture above 0.08GK. At such low temperatures, there is
and/or ground states by a series of γ-ray cascades. At low no communication between 26 Alg and 26 Alm . 26 Alg may
temperatures (T . 0.15 GK), communication between be destroyed by 26 Alg (p,γ)27 Si and by the β + -decay.
26
Alm and 26 Alg can be ignored due to the negligibly-low Hydrostatic C/Ne shell burning occurs at a temper-
internal transition rates. Therefore, 26 Alm and 26 Alg can ature around 1.2 GK. Here, 26 Al is produced by the
24
be treated as two distinct species with their separate Mg(n,γ)25 Mg(p,γ)26 Alt reaction chain. The detailed
production and destruction reaction rates (Iliadis et al., flow chart is shown in Figure 5. At the temperature
2011). of C/Ne shell burning, 26 Al reaches thermal equilib-
At higher temperatures (T & 0.4 GK), instead, higher rium and can be treated at a single species, 26 Alt (see
excited states of 26 Al can be populated on very short above). Destruction of 26 Al mostly occurs through neu-
timescales by photo-excitation of 26 Alg and 26 Alm re- tron capture reactions. The main neutron sources are
sulting in thermal equilibrium where the abundance the 22 Ne(α,n)25 Mg and 12 C(12 C,n)23 Mg reactions. 26 Alt
ratio of the states are simply given by the Boltzmann is also destroyed by the β + -decay process in C/Ne shell
distribution. In this case, it is sufficient to have just one burning. The explosive C/Ne shell burning may raise the
species of 26 Al in reaction network calculations defined temperature up to 2.3 GK and then quickly cool down
by its thermal equilibrium (26 Alt ), with suitable reaction to 0.1 GK within a time scale of 10 s. The detailed flow
rates that take into account the contributions from all chart in these conditions is shown in Figure 6. 26 Al is pro-
the excited states that are populated according to the duced by the same process as during hydrostatic C/Ne
Boltzmann distribution (Iliadis et al., 2011). shell burning, except that the 23 Na(α,p)26 Mg reaction
The situation becomes complicated at intermediate competes with 23 Na(p,γ)24 Mg and the 25 Mg(α,n)28 Si
temperatures (0.15 GK . T . 0.40 GK). Although, reaction competes with 25 Mg(p,γ)26 Alt . These two α-
26
Alg and 26 Alm can still communicate with each other induced reactions bypass the the production of 26 Alt .
26
via the higher excited states, the timescale required to Alt is primarily destroyed by 26 Alt (n,p)26 Mg instead
achieve thermal equilibrium becomes comparable or even of β + -decay.
longer than the timescale for β + -decay for 26 Alm (as well In an explosive proton-rich environment such as
as β + -decay of higher excited states). Thus, neither the within a nova, the peak temperature may reach about
assumption of thermal equilibrium nor treating 26 Alg 0.3 GK. Here, 26 Al is produced by two sequences
and 26 Alm as two separate species are viable options of reactions: 24 Mg(p,γ)25 Al(β + )25 Mg(p,γ)26 Alg,m , and
24
(Banerjee et al., 2018; Misch et al., 2021). In this case, Mg(p,γ)25 Al(p,γ)26 Si(β + )26 Alg,m , which favours the
it becomes necessary to treat at least the lowest four production of 26 Alm , therefore bypassing the observable
26
excited states as separate species in the reaction network, Alg .
26
along with their mutual internal transition rates, in order Al can also be directly produced in the core-collapse
to calculate the abundance of 26 Al accurately (Iliadis supernova ν process via 26 Mg(νe , e− ) (Woosley et al.,
et al., 2011). However, as will be discussed below, it 1990), when the high-energy (∼ 10 MeV) neutrinos emit-
turns out that the production of 26 Al in stars happen ted during the collapse and cooling of a massive star
mostly either in the low or the high temperature regime, interact with nuclei in the mantle that is processed by
and the problematic intermediate temperature regime the explosion shock at the same time. Neutrino-nucleus
is rarely encountered. reactions that lead to proton emission also increase the
production of 26 Al via the reactions discussed above.
2.1.2 Production and destruction of 26 Al The contribution of the ν process to the total supernova
26
Al is expected to be primarily produced in the hydro- yield is expected to be at the 10% level (Sieverding et al.,
static burning stages of stars through p-capture reactions 2017; Timmes et al., 1995b); we caution that this value
on 25 Mg. These occur in massive stars during core hy- is subject to uncertainties in the neutrino physics and
drogen burning, hydrostatic/explosive carbon/neon shell the details of the supernova explosion mechanism.
burning, and in the hydrogen-burning shell, in some cases
located at the base of convective envelope, of asymptotic 2.1.3 Uncertainties in the relevant reaction rates
giant branch (AGB) stars. Explosive oxygen/neon shell The uncertainties of the rates of main production reac-
burning probably also contributes to the production of tions 25 Mg(p,γ)26 Alg and 25 Mg(p,γ)26 Alm are around
this isotope. All these sites are be described in more 10% at T9 >0.15; at lower temperatures, the uncertain-
detail in Section 2.2. The typical temperatures of the ties are even larger than 30% (Iliadis et al., 2010). Since
H-burning core in massive stars and the H-burning shell there is little communication between 26 Alg and 26 Alm
The Radioactive Universe 5
26Al 26Mg
0.417 MeV
m 0+ 0.228 MeV t = 9.15 s
g 5+ t = 1.04 106 y (T1/2=0.72 106 y)
e- - capture
( 2.7 % )
e- - capture
b+ - decay
( 97.3 % ) 2+
g
1.130 MeV ( 2.4 % )*
b+ - decay
26Al Decay: ( 100 % ) 2+
g
82% b+ - decay ( ~1.17 MeV) 2.938 MeV g
18% e- - capture ( 0.3 % )* 1.809 MeV ( 99.7 % )*
Q=4.0 MeV
Photon yields: (# per decay)
0+
0.511 MeV 1.622
1.130 MeV 0.024
1.809 MeV 0.997 * .= % are relative to one decay of 26Al
2.938 MeV 0.003
Figure 3. The nuclear level and decay scheme of 26 Al (simplified). γ rays are listed as they arise from decay of 26 Al, including
annihilation of the positrons from β + -decay.
27 28 29 30 31 32
Si Si Si Si Si Si
26 27 28 29
Al Al Al Al
23 24 25 26 27 28
26Si 27Si 28Si Mg Mg Mg Mg Mg Mg
β+ (p,γ) 22
Na 23
Na 24
Na 25
Na
(p,γ) (p,γ)
β+
26Alm
Figure 5. Integrated reaction flow for the hydrostatic C/Ne shell
25Al 26Al 27Al
burning calculated with the NUCNET nuclear network code. The
(n,α) thickness of the arrows correspond to the intensities of the flows;
(n,p) red and black arrows show β interactions and nuclear reactions,
(p, γ ) 9.2 s respectively. Here 26 Al is at its thermal equilibrium. Only a frac-
(p,α) β+ (p, γ)
β+(7.2s) 1.04 106 y tion of the flows of Na, Mg, Al and Si are displayed. The neutron
(p,α) (p, γ) source reactions, such as 12 C+12 C and 22 Ne(a,n), are not shown.
23Mg 24Mg 25Mg
26Mg 27 28 29 30 31 32
Si Si Si Si Si Si
β+
23Na
25 26 27 28 29
Al Al Al Al Al
Figure 4. The Na-Mg-Al cycle encompasses production and
destruction reactions, and describes 26 Al in stellar environments.
23 24 25 26 27 28
Mg Mg Mg Mg Mg Mg
22 23 24 25
Na Na Na Na
Figure 6. Same as Figure 5 but for C/Ne explosive burning.
6 Diehl et al. at T9
The Radioactive Universe 7
103 2.2.1 Low- and Intermediate-Mass Stars
26 Mg + e ction Low and intermediate mass stars (of initial masses
s se
≈ 0.8−8 M ) become asymptotic giant branch (AGB)
cm2)
lcros stars after undergoing core H and He burning. An AGB
102
tota
star consists of a CO core, H and He burning shells
g.s.)
42
2 6 Al ( surrounded by a large and extended H-rich convective
g to
cross section (10
in envelope. These two shells undergo alternate phases
ch
bran of stable H burning and repeated He flashes (thermal
101 pulses) with associated convective regions. Mixing events
(called third dredge ups) can occur after thermal pulses,
whereby the base of the convective envelope penetrates
inwards, dredging up material processed by nuclear re-
exp + RPA
100 RPA only actions from these deeper shell burning regions into the
envelope. Mass is lost through a stellar wind and progres-
0 20 40 60 sively strips the envelope releasing the nucleosynthetic
Neutrino energy (MeV) products into the interstellar environment (see Karakas
& Lattanzio (2014) for a recent review of AGB stars.).
Figure 7. Cross section for the reaction 26 Mg(νe , e− ) compar- The production of 26 Al3 within AGB stars has been
ing results based entirely on theoretical calculations (red lines) the focus of considerable study (e.g., Norgaard 1980;
and results based on the experimentally measured Gamow-Teller
strength distribution (blue lines). The experimentally determined Forestini et al. 1991; Mowlavi & Meynet 2000; Karakas &
distribution increases the strength at low energies and gives a Lattanzio 2003; Siess & Arnould 2008; Lugaro & Karakas
larger cross section for the transitions to the 26 Al ground state. 2008; Ventura et al. 2011). Here we do not attempt a
review of the extensive literature, but briefly summarize
the relevant nucleosythesis, model uncertainties, stellar
yields, and the overall galactic contribution.
achieve accurate rates for astrophysical applications.
The main site of 26 Al production in low-mass AGB
Finally, the cross section for 26 Mg(νe , e− )26 Al is dom- stars is within the H-burning shell. Even in the lowest
inated by the transition to the isobaric analog state of mass AGB stars, temperatures are such (≥ 40 MK), that
the 26 Mg ground state at 228.3 keV and further contribu- the MgAl chain can occur and the 26 Al is produced via
tions from a number of Gamow-Teller (GT) transitions the 25 Mg(p,γ)26 Al reaction. The H burning ashes are
at low energies. Zegers et al. (2006) have used charge subsequently engulfed in the thermal pulse convective
exchange reactions to determine the GT strength distri- zone, with some 26 Al surviving and later enriching the
bution of 26 Mg. Sieverding et al. (2018b) have calculated surface via the third dredge up. In AGB stars of masses
the cross section based on these experimental results ≥ 2-3 M (depending on metallicity) the temperature
with forbidden transitions at higher energies. Figure 7 within the thermal pulse is high enough (> 300 MK)
shows a comparison between the theoretical cross sec- to activate the 22 Ne(α,n)25 Mg reaction. The neutrons
tion based on the Random Phase Approximation and produced from this reaction efficiently destroy the 26 Al
the values using the experimentally determined strength (via the 26 Al(n,p)26 Mg and 26 Al(n,α)23 Na channels),
at low energies. The particle emission branching has leaving small amounts to be later dredged to the surface.
been calculated with a statistical model code (Loens, In more massive AGB stars another process is able to
2010; Rauscher et al., 2000). While the theoretical model produce 26 Al: the hot bottom burning. This hot bottom
captures the total cross section quite well, the values burning takes place when the base of the convective
for transition to the 26 Mg ground state are substantially envelope reaches high enough temperatures for nuclear
underestimated in the calculations. burning (∼ 50-140 MK). Due to the lower density at the
base of the convective envelope than in the H burning
shell, higher temperatures are required here to activate
2.2 Cosmic Nucleosynthesis Environments the Mg-Al chain of nuclear reactions. The occurrence of
hot bottom burning is a function of initial stellar mass
Here we address stellar nucleosynthesis, as we know it and metallicity, with higher mass and/or lower metallic-
from models and theoretical considerations, in greater ity models reaching higher temperatures. The lower mass
detail first for stars that are not massive enough to end limits for hot bottom burning (as well as its peak tem-
in a core collapse, then for the different nucleosynthe- peratures) also depend on stellar models, in particular
sis regions within massive stars and their core-collapse on the treatment of convection (e.g.Ventura & D’Antona
supernovae; and finally, we comment on other explo-
sive sites such as novae and high-energy reactions in 3 Note that in this and the following sections, for sake of sim-
interstellar matter. plicity, the notation 26 Al represents 26 Alg , unless noted otherwise.
8 Diehl et al.
of the initial envelope 24 Mg has been transmuted to
25
Mg, and the intershell 25 Mg is efficiently dredged-up
via the third dredge-ups. The decreasing trend in 26 Al
yield for the most massive metal-poor models is due
to their shorter AGB phase, less third dredge-up and
higher hot-bottom burning temperatures, which activate
the destruction channel 26 Al(p,γ)27 Si.
The contribution from AGB stars to the galactic inven-
tory of 26 Al has been estimated at between 0.1-0.4 M
(e.g., Mowlavi & Meynet 2000). More recently Siess &
Arnould (2008) also included super-AGB stars4 yields
in this contribution, and also their impact seems to be
Figure 8. AGB star yields of 26 Al for the range of metallicities rather modest. Even when factoring in the consider-
(Z = 0.02 - 0.0001) as a function of initial mass. Results taken able uncertainties impacting the yields, AGB stars are
from Karakas (2010) and Doherty et al. (2014a,b)
expected to be of only minor importance to the Galac-
tic 26 Al budget at solar metallicity. However, Siess &
Arnould (2008) noted that at lower metallicity, around
2005). Values from representative models of the Monash
that of the Magellanic clouds (Z=0.004-0.008), the con-
group (Karakas, 2010) are ∼ 5 M at metallicity Z=0.02,
tribution of AGB and super-AGB stars may have been
decreasing to ∼ 3.5 M at Z=0.0001. Typically, there is
far more significant.
larger production of 26 Al by hot bottom burning when
temperatures at the base of the envelope are higher and 2.2.2 Massive Stars and their core-collapse
the AGB phase is longer. The duration of the AGB phase supernovae
is set by the mass loss rate, which is a major uncertainty
Massive stars are defined as stars with main-sequence
in the predicted 26 Al yields (Mowlavi & Meynet, 2000;
masses of more than 8 − 10 M . They are characterized
Siess & Arnould, 2008; Höfner & Olofsson, 2018).
by relatively high ratios of temperature over density
As the temperature at the base of the convective en- (T /ρ) throughout their evolution. Due to this, such stars
velope increases two other reactions become important. tend to be more luminous. Unlike lower-mass stars, they
First, at ∼ 80 MK, 24 Mg is efficiently destroyed via avoid electron degeneracy in the core during most of
24
Mg(p, γ)25 Al(β + )25 Mg leading to more seed 25 Mg for their evolution. Therefore, core contraction leads to a
26
Al production, Second, at above 100 MK, the 26 Al smooth increase of the temperature. This causes the
itself is destroyed via 26 Al(p,γ)27 Si(β + )27 Al. This last ignition of all stable nuclear burning phases, from H, He,
reaction has the largest nuclear reaction rates uncer- C, Ne, and O burning up to the burning of Si both in
tainty within the Mg-Al chain, variations of this rate the core and in shells surrounding it. The final Fe core is
within current uncertainties greatly modify the AGB bound to collapse, while Si burning continues in a shell
stellar 26 Al yield (Izzard et al., 2007; van Raai et al., and keeps on increasing the mass of the core. During
2008). this complex sequence of core and shell burning phases,
Figure 8 shows the 26 Al yields for a range of metal- many of the elements in the Universe are made. A sub-
licites (Z = 0.02 - 0.0001) as a function of initial mass stantial fraction of those newly-made nuclei are removed
from the Monash set of models of Karakas (2010) and from the star and injected into the interstellar medium
Doherty et al. (2014a,b). The relative efficiency of the by the core-collapse supernova explosion, leaving be-
two different modes of production are evident: in the hind a neutron star or a black hole. The collapse of the
lower mass models, where 26 Al is enhanced only by the core is accompanied by the emission of a large number
third dredge-ups of the H-shell ashes, show a low yield of neutrinos. The energy spectrum of these neutrinos
of ≈ 10−8 − 3 × 10−7 M . The more massive AGB reflects the high temperature environment from which
stars that undergo hot bottom burning, instead, have they originate, with mean energies of 10 − 20 MeV. The
substantially higher yield of ≈ 10−6 − 10−4 M . fact that these neutrinos could be observed in Supernova
Metallicity also has an impact to the AGB 26 Al yield, 1987A is a splendid confirmation of our understanding
in particular for intermediate-mass AGB stars. The of the the lives and deaths of massive stars (Burrows &
larger yields at Z=0.004 and 0.008, when compared Lattimer, 1987; Arnett, 1987).
to Z=0.02, are primarily due to their higher tempera- The mechanism that ultimately turns the collapse of a
tures and longer AGB phases. At the lowest metallicity stellar core into a supernova explosion is an active field of
(Z=0.0001) the seed 25 Mg nuclei are not present in suf-
ficient amounts to further increase the 26 Al yield even
4 Super-AGB stars are the most massive AGB stars
(≈ 7−10 M ) which have undergone central C burning prior
with a higher temperature and similar duration of the to the super-AGB phase - for a recent review, see Doherty et al.
AGB phase. This is the case even thought the majority (2017).
The Radioactive Universe 9
C/Ne burning and 26 Al is efficiently produced in the
region of suitable peak temperature around 2.3 GK.
As we will show, this is the dominant contribution
C-burning for stars in the mass range 10 − 30 M .
4. Neutrino interactions during the explosion can also
explosive- affect the abundance of 26 Al.
burning
H-burning Figure 9 shows the profile of the 26Al mass fraction
for a 15 M stellar model, calculated with the KEPLER
hydrodynamics code in spherical symmetry. The pre-
supernova as well as the post-explosion abundance pro-
files are shown, and the production mechanisms indi-
cated. We now discuss in detail each of the four main
Figure 9. Mass fraction profiles of 26 Al indicating regions of
mechanisms listed above.
different production mechanisms. H core and shell burning: The production in the
convective core H burning during the main sequence
mostly depends on the size of the convective core and
research. In our current understanding, a combination of the initial amount of 25 Mg. 26 Al in the region that
neutrino heating and turbulent fluid motion are crucial undergoes core He burning is destroyed due to neutron-
components for successful explosions (see Janka, 2012; capture reactions, but some of it may survive in the
Burrows & Vartanyan, 2021, for reviews of the status of layers outside of the burning region. The 26 Al produced
core-collapse modeling). Due to the multi-dimensional during H core burning is also threatened by the lifetime
nature and multi-physics complexity of this problem, of the star. Since the post-main-sequence, i.e., post-H-
simulations of such explosions from first principles are burning, evolution of a star can take more than 0.1
still in their infancy (Müller, 2016). Parametric models, Myr, due to the exponential radioactive decay most of
however, have proven to be able to explain many proper- this early made 26 Al decays before it can be ejected
ties of supernovae, although they need to be fine-tuned by a supernova explosion. In H-shell burning, 26 Al is
accordingly (Burrows & Vartanyan, 2021). also produced and it is more likely to survive until it is
The supernova explosion expels most of the stellar ejected. In cases in which the H-burning contribution
material that had been enriched in metals by the hy- is important for the final 26 Al yield, this component is
drostatic burning and the explosion shock itself. Before sensitive to H burning conditions and in particular to
the explosion, strong winds already take away some of the treatment of convection.
the outer envelopes of these massive stars, especially Another way for 26 Al from H burning to contribute
in the luminous blue variable and Wolf-Rayet phases to the ejecta is mass loss. For single stars, mass is lost
of evolution (as will be discussed in detail see below). via stellar winds driven by radiation pressure (Cassinelli,
This ejected material also contains a range of radioactive 1979; Vink, 2011). Thus, it is stronger for more luminous,
isotopes, including some with lifetimes long enough to more massive stars. Stellar mass loss has been a subject
be observable long after the explosion has faded, such of study for a long time (Lamers et al., 1999; Vink, 2011),
as 26 Al and 60 Fe. In this section we describe the various but the details of the implementation in models still gives
ways in which 26 Al is made in massive stars and the rise to significant uncertainties (Farrell et al., 2020). The
ensuing supernova explosion. H-burning contribution to 26 Al is most-important for
The production of 26 Al always operates through the massive stars with initial mass > 30 M for which stellar
25
Mg(p,γ)26 Al reaction, which is active during differ- winds are strong enough to remove material from the
ent epochs of the stellar evolution. We can distinguish H burning regions below the H envelope (Limongi &
four main phases that contribute to the production of Chieffi, 2006b).
26
Al during massive star evolution and the supernova Stellar rotation may significantly increase mass loss
explosion (Limongi & Chieffi, 2006b). and the mixing efficiency (Groh et al., 2019; Ekström
et al., 2012), which has significant impact on the 26 Al
1. In H core and shell burning 26 Al is produced from yields. Stellar-evolution models that include a descrip-
the 25 Mg that is present due to the initial metallic- tion of rotation have been developed for decades (see
ity. Maeder & Meynet, 2000; Heger et al., 2000, for exten-
2. During convective C/Ne shell burning 26 Al is pro- sive reviews). However, the effects are still not well-
duced from the 25 Mg that results from the MgAl understood. A major challenge is to model the transport
cycle with protons provided by the C fusion reac- of angular momentum within stars (Aerts et al., 2019).
tions. This determines how fast the internal regions of the
3. The supernova explosion shock initiates explosive star rotate at different radii and different latitudes. Fric-
10 Diehl et al.
tion from laminar and turbulent flows between layers
of different velocity transports angular momentum, and 3
Coriolis forces add complexity. It is, therefore, far from
4
straightforward to determine how much rotation-induced
mixing happens in different regions of a star. This affects 5
transport of heat and of material, and thus where and
6
how nuclear burning may occur.
log(26Al/M )
Brinkman et al. 2021 (submitted to ApJ), 0 km/s
A wealth of information has become available on in- 7 Brinkman et al. 2021 (submitted to ApJ), 150 km/s
Brinkman et al. 2021 (submitted to ApJ), 300 km/s
ternal rotation rates of low-mass stars (Aerts et al., 8
Ekstrom et al. 2012, 0 km/s
Ekstrom et al. 2012, 0.4 crit
2019), thanks to asteroseismology studies, e.g., with data Limongi & Chieffi 2018, 0 km/s
from the Kepler and TESS spacecrafts (Borucki et al., 9 Limongi & Chieffi 2018, 150 km/s
Limongi & Chieffi 2018, 300 km/s
2010; Ricker et al., 2015). These internal rotation rates 10 Limongi & Chieffi 2018, 0 km/s, SN
Limongi & Chieffi 2018, 150 km/s, SN
can help us to investigate the stellar interiors directly. Limongi & Chieffi 2018, 300 km/s, SN
1110
This led, for example, to the insight that rotation has 20 30 40 50 60 70 80
Mass (M )
a negligible effect on the slow neutron-capture process
nucleosynthesis in low-mass AGB stars (den Hartogh
Figure 10. 26 Al yields from three stellar evolution codes with
et al., 2019). Information on the internal rotation rates of different implementations of stellar rotation. Shown are contribu-
massive stars is more sparse, while there is information tions from winds of solar metallicity stars (Ekström et al., 2012;
available on the rotation rates of black holes and neutron Limongi & Chieffi, 2018; Brinkman et al., 2021), and supernova
stars, which are the final phases of massive star evolu- yields (Limongi & Chieffi, 2018). Initial rotation rates of 0 (non-
rotating), 150, and 300 km s−1 are considered, as indicated in the
tion.Recently, Belczynski et al. (2020) investigated how legend. Yields are in units of M . Based on Figure 4b of Brinkman
to match the LIGO/Virgo-derived compact-star merger et al. (2021).
rates, and their black hole masses and spins.They con-
cluded that massive stars transport angular momentum
more efficiently than predicted by current stellar evolu- to collapse completely into black holes, and therefore do
tion models, and thus slow down their rotation rate. This not eject 26 Al in their supernova. In Section 4.2, we will
was attributed by these authors to the effect of magnetic also consider this comparisons in the light of population
fields via the Tayler-Spruit dynamo (Spruit, 2002) or synthesis for both 26 Al and 60 Fe (Figures 40 and 39).
similar processes (e.g. Fuller et al., 2019). None of the The presence of a binary companion may also have a
current published massive star yields include this effect significant impact on the 26 Al yields from massive stars,
so far, which means that the currently available yields because binary interactions can affect the mass loss. As
from rotating massive stars may likely overestimate the shown by Sana et al. (2012), massive stars are rarely
effects of rotation. single stars: most, if not all, are found in binary or even
Recent nucleosynthesis models including stellar rota- multiple systems. If close enough, the stars within such
tion allow us to get estimates of the impact of rotational a system can interact and the gravitational pull between
mixing on the stellar yields. Figure 10 illustrates several the stars affects the mass loss, known as Roche lobe
characteristic cases. Rotation generally is found to in- overflow. In turn the mass loss affects the internal struc-
crease 26 Al yields, due to the fact that the H-burning ture and thus further evolution of the star. Figure 11
convective core is more extended and therefore more shows how binarity may affect 26 Al yields across the
25
Mg is burnt into 26 Al, which is also mixed up more stellar-mass range (Brinkman et al., 2019).When the
efficiently due to rotation, and due to the fact that these binary interaction takes place during the main sequence
stars experience more mass loss than their non-rotating or shortly after, but before helium is ignited in the core,
counterparts. For the lowest-mass models, 13 and 15 M , the impact on the amount of the ejected 26 Al can be
Limongi & Chieffi (2018) find a large increase in the significant, and mostly prominent at the lower mass-end
yields of rotating models, which is due to a significant in- of massive stars (10-35 M ). Single stars in this mass
crease in the mass-loss. This large increase is however not range lose only a small fraction of their whole H enve-
found in the other two studies. For the higher mass-end, lope, leaving a significant amount of 26 Al locked inside.
30 M and up, the mass-loss is less affected by stellar However, when part of a binary system, much more of
rotation, and the yields only increase slightly, compared the envelope can be stripped off because of mass transfer,
to the non-rotating models. This is the same for all which exposes the deeper layers of the star, those that
three studies. The supernova yields for the lowest-mass were once part of the H-burning core, and now are the
stars, shown in Figure 10, are another factor of 10-100 regions where most of the 26 Al is located (Brinkman
higher than the rotating single-star yields from the same et al., 2019). For more massive stars (M∗ ≥35 M ), in-
set. Supernova yields for higher masses are zero in the stead, mass loss through the stellar winds is strong even
scenario discussed by Limongi & Chieffi (2018), because for single stars: these are the Wolf-Rayet stars observed
stars with an initial mass higher than 25M are assumed to expose their He, C, N, or O-rich regions to the sur-The Radioactive Universe 11
26
Al yields. In principle, this depends on the explosion
dynamics and in particular on the explosion energy. The
peak temperature, however, only scales very weakly with
4
the explosion energy, and therefore, even very weak ex-
5 plosions with energies of the order of 1050 ergs produce
enough 26 Al to dominate over the contribution from
C/Ne shell burning to the total yield.
log(26Al/M )
6
7 Explosive contributions: For the explosive contribu-
tion of core-collapse supernovae two main quantities
8 affect the 26 Al production. First, as a pre-requisite, the
Ekstrom et al. 2012 amount of produced 26 Al scales with the 24 Mg mass
9 Limongi & Chieffi, 2018
Brinkman et al. 2019, single star fraction in the C/Ne layer, because the production pro-
10 Brinkman et al. 2019, effective binary yields ceeds through 24 Mg(n, γ)25 Mg(p, γ)26 Al. This depends
10 20 30 40 50 60 70 80 on the conditions of C core and shell burning during
Mass (M )
the hydrostatic evolution of the star and the C-burning
Figure 11. Yields of 26 Al for various single star studies, as well
reaction rates, as discussed above (Section 2.1). Second,
as the effective binary yields defined as the average increase of the the optimal peak temperature for 26 Al production is
yield from a single star to the primary star of a binary system, in a narrow range between 2.1 GK and 2.5 GK. This
when considering a range of periods (see Brinkman et al., 2019, depends on the reaction rates of 25 Mg(p,γ)26 Al, on the
neutron capture reaction on 26 Al, and on the neutron
for details).
sources. If the temperature is above the optimal value,
i.e., at smaller radii, charged particle reactions efficiently
face. These stars lose their H envelope and reveal deeper destroy the produced 26 Al. If the temperature is too low,
layers during their main-sequence evolution or shortly overcoming the Coulomb barrier in 25 Mg(p, γ)26 Al is
after even without having a companion, and the impact harder, reducing the production. Figure 12 shows the
of binarity is found to be insignificant, especially for 26
Al mass fraction profile as a function of the peak tem-
initial masses of 50M and higher. perature reached at a given radius. Across the mass
However, there are still many uncertainties concerning range of progenitor models, between 13 M and 30 M ,
mass loss in general, and even more so in the combina- the highest 26 Al mass fraction is reached for the same
tion of binary evolution and mass transfer, as orbital peak temperature. The maximum mass fraction, i.e.,
separations change in response to stellar evolution, and the height of the peak in Figure 12, depends mostly on
different phases of mass transfer may occur (Podsiad- the local mass fraction of 24 Mg (which is required to
lowski et al., 2004; Sana et al., 2012). For wide binaries, produce 25 Mg by neutron captures during the explosion).
little may change with respect to single-star evolution; Since the peak temperature at a given radius depends
but for close binaries, the impact on stellar evolution on the explosion energy, different explosion energies
may be large (Sana et al., 2012). Moreover, the coupling move the peak to different densities. This also changes
of binary evolution and mass transfer with rotation and the value of the maximum 26 Al mass fraction. Since
its effect on the core-collapse explosive yields have not charged-particle induced nuclear reactions are highly
been explored yet. temperature-dependent, the peak temperature is the
Convective C/Ne shell burning: The production of most important quantity. Density and seed abundance
26
Al in this region in the pre-supernova stage can be sub- enter linearly, the range of peak 26 Al mass fraction
stantial, as shown in Figure 9 where the mass fraction is relatively narrow, within a factor two. The site of
can reach values up to above 10−4 . The production of the explosive production of 26 Al is relatively far away
26
Al in C/Ne burning requires the existence of a convec- from the stellar core, and therefore not very sensitive
tive shell that burns C at a sufficiently high temperature. to the dynamics of the explosion mechanism itself. The
Convection is necessary for the supply of fresh 25 Mg, explosion energy, however, depends on the supernova
to which the C-burning reactions provide the protons. engine, and thus affects the position of the peak mass
At the same time, convection moves 26 Al out of the fraction. The amount of matter exposed to the critical
hottest burning regions, where it is destroyed quickly. optimal conditions can also change by asymmetries of
However, the 26 Al produced in this process does not the explosion. While Figure 12 shows that the mech-
contribute to the final yield because it is later destroyed anism is always qualitatively similar, the actual yield
by the high temperatures induced by the explosion shock significantly depends on the mass of material that is
(Figure 9). Different models for the same mass range, contained in the region that reaches this temperature.
however, may obtain almost no 26 Al produced during This varies much more between models, and gives rise
convective C-burning, while the production later during to the non-monotonic dependence of the 26 Al yields on
explosive burning still may lead to very similar overall the progenitor mass shown in Figure 12.The optimal12 Diehl et al.
s13 s22 without
10 4 s14 s23 12 low energies
s15 s24 high energies
s16 s25
s17
s18
s26
s27
10
10 5
yield (10 5M )
s19 s28
mass fraction
s20
s21
s29
s30
8
10 6 6
4
10 7
2
1 2 3 4 5 15 20 25 30
peak temperature (GK) progenitor mass
Figure 12. Mass fraction of 26 Al for mass shells from a range Figure 13. 26 Al supernova yields from massive star progenitors
of core-collapse supernova models. Results are shown for a whole in the range of 13-30 M showing the also the contribution of the
series of models with initial masses between 13 M and 30 M ν process and its dependence on the neutrino spectra (Sieverding
(as indicated in the legend). The temperature that leads to the et al., 2018b). The progenitor models and explosion trajectories
largest 26 Al mass fractions is very similar in all the models. have been calculated with the KEPLER hydrodynamics code.
temperature is largely determined by the nuclear reac- however, had assumed relatively large energies for the
tion rates and thus independent of the progenitor model supernova neutrinos that are not supported by current
(Figure 12). simulation results. The contribution of the ν process to
26
Al is significantly reduced when such lower neutrino
Neutrino interactions: 26 Al yields are also coupled
energies are adopted. For neutrino spectra with tempera-
directly to the neutrino emission from core collapse
ture Tνe = 2.8 MeV for νe and Tνx = 4 MeV for all other
by the ν process, as mentioned in Section 2.1.2. The
flavors, the increase of the 26 Al yield due to neutrinos
neutrinos that are copiously emitted from the cooling
is reduced to at most 10 % (Figure 13).
proto-neutron star during a supernova explosions are
sufficiently energetic and numerous to induce nuclear There are still large uncertainties in the prediction
reactions in the outer layers of the star. Such reactions of the neutrino emission from a supernova explosion.
on the most abundant species have been found to be During the early phases of vigorous accretion, the neu-
responsible for, or at least contribute, to the solar abun- trino spectra can be much more energetic, increasing the
dances of a handful of rare isotopes, including, 7 Li, 11 B, contribution of the ν process (Sieverding et al., 2018a).
19
F, 138 La, and 180 Ta. The ν process also leaves traces Neutrino flavor oscillations add additional uncertainty.
in the supernova yields of long-lived radioactive isotopes, The production of 26 Al occurs mostly at densities below
such as 10 Be, 92 Nb, and 98 Tc and contributes to the the critical density for neutrino flavor transformations
explosive yield of 26 Al. This occurs in a direct and an due to the MSW-resonance5 . Due to collective non-linear
indirect way. A direct production channel exists through effects, however, neutrino flavor transformations may
26
Mg(νe , e− ). Indirectly, neutral-current inelastic neu- occur below the region relevant for the production of
26
trino scattering can lead to nuclear excitations that decay Al. This could lead to a significant increase of the
by proton emission, i.e., reactions such as 20 Ne(νx , νx0 p), νe spectral temperature. Tentative calculations indicate
where νx includes all neutrino flavors. This provides that the 26 Al yield may be increased by up to a fac-
another source of protons for 25 Mg(p, γ)26 Al to occur, tor 2, if a complete spectral swap between νe and the
and enhances production of 26 Al outside of the optimal heavy flavor neutrinos takes place below the O/Ne layer
temperature region. With re-evaluated neutrino-nucleus (Sieverding et al., 2020).
cross sections, a study of 1D explosions for progenitors 5 Neutrino mass eigenstates propagate differently in matter
in the mass range 13-30 M has confirmed an early with a density gradient, so that the neutrino flavor, the sum of
finding (Timmes et al., 1995b) that the ν process in- the mass eigenstates, may change upon propagation. This MSW
creases the 26 Al yields by up to 40 %. Previous studies, effect was discovered in 1985 by Mikheev, Smirnov, and Wolfstein.The Radioactive Universe 13
R.C. Reedy / Nuclear Instruments and Methods in Physics Researc
2.2.3 Other explosive events: Thermonuclear The excita
supernovae, Novae, X-ray bursts, and (p,an) cross
sections wer
kilonovae
of calculated
Other cosmic environments are also plausible candidate for Fe and C
to host the nuclear reactions that produce 26 Al. Ex- was assumed
tremely hot plasma temperatures are likely when matter sections. The
Ca(p,x)36Cl r
falls onto compact objects: the gravitational energy re- duced reactio
leased by a proton that falls onto a neutron star is 2 ton-induced
GeV. Therefore, nuclear reactions are expected on the Early calc
surfaces of neutron stars and in the accretion disks that core were co
accompany newly-forming black holes. However, signifi- rates for mak
restrial meas
cant cosmic contributions to 26 Al from these objects are
unlikely for two reasons: (i) these events are so energetic 4. Summary
that nuclei are decomposed into nucleons and α particles,
and the 26 Al abundance in such conditions will be low; Cross sec
being routine
and (ii) there is hardly significant material ejected from 10
Be, 14C, 21N
such compact regions. a decade or m
One example of an exception, i.e., where significant also yielded p
material is ejected, is the recent observational confirma- on in situ ter
lated produc
tion of a kilonova (Abbott et al., 2017). Here, the forma- particle fluxe
tion of a compact object after collision of two neutron While the
stars has evidently led to brightening of the object from surements ar
freshly-produced radioactivity, and the spectra of the sections for e
kilonova light can be interpreted as a hint at overabun- Fig. 3. The elemental cross sections (in millibarns) as a function of neutron energy
Figure 14. Cross sections for spallation reactions of cosmic rays
tor irradiatio
samples with
(in MeV) for 26Al (0.7-Myr) from Al and Si and for 36Cl from K and Ca. The cross
dance in nuclei heavier than Fe (Smartt et al., 2017). If (adapted from Reedy,
sections between 2013).
points are Reactions
a linear areonindicated
interpolation in the legend,
a log–log plot. ses of meteo
consolidated (in view of the significant uncertainties due and include 26 Al production from neutron reactions. For proton
to atomic-line unknowns and explosion asymmetries), 3.3.4. Cross sections for making 14C
ments for na
this represents a potential signature of rapid neutron For oxygen and neutron energies below 35 MeV, the measured
tally-measur
capture (r-process) nucleosynthesis. An ejected amount near
crossthe maximum
sections for makingof 14nuclear binding
C [30] were used. Theenergy, which
higher energies
for neutron
56 26 ously-irradia
of such material in the range 10−4 up to 10−2 M h̃as is were
reached for the
1.1 times Ni.
versionAl will also
adopted by [3]be
forproduced herein.
fitting 14C measured and stable p
been inferred (Abbott et al., 2019). However, being pre- But the reaction
in lunar paths
samples. The are driven
excitation to tighter-bound
functions for Si were basednu- on mono-energe
clei under thesesections
circumstances.
[26] shiftedThe results from3by models
14
Si(p,x) C cross to lower energies 5 MeV
dominantly a result of neutron reactions, this ejected because of the different reaction threshold energies (for He emit-
spallation ne
material is not expected to hold any significant amounts show relatively low 26Al yields of ∼ 10−8 M (Iwamoto
ted with incident neutrons). Calculated rates for making 14C in
irradiations s
etKnyahinya
al., 1999;wereNomoto & Leung, 2018). Therefore, weexcita-
con-
More test
of 26 Al. Similarly, nuclear reactions that occur on the consistent with measurements [9,11]. The samples usin
surfaces of neutron stars in binary systems are unlikely sider supernovae
tion functions of Type
for making 14
Ia toO be
C from andrather unimportant
Si are shown in Fig. 2. use the lates
to contribute any significant cosmic 26 Al. Such reactions contributors to cosmic 26Al . and transpor
have been observed in the form of Type-I X-ray bursts Novae
3.3.5. are
Cross also for
sections potential
making 26contributors
Al of 26 Al in the help to show
Galaxy, as mentioned in Section 2.1.2. Herein, hot ground-
hydro-
26
(Bildsten, 2000; Galloway et al., 2008). These are ther- The excitation functions for making Al (the 0.7-Myr
Acknowledg
26
monuclear runaway explosions after accretion of critical gen burning reactions can lead to significant
state isomer) are not easy to calculate because the short-lived
Al produc-iso-
mer 26mAl beta decays directly to 26Mg and 26 not to 26Al. Some pro-
amounts of H and He on neutron star surfaces. Even tion
ton (Jose & Hernanz,
cross sections for 26Al 1998), with using
were measured Al mass fractions
good gamma-ray
Members
He ashes may ignite and create super-bursts. The rapid around
detection10−3 for the
systems more-massive
prior to AMS. The early O-Ne white
proton dwarfs.
excitation A
func-
tion of this p
comments th
proton capture (rp) process during an explosive hydro- major
tions uncertainty
were adopted in fornova modelling
neutron reactionsismaking
how the 26
Alobserva-
from Al
ject of the Na
gen burst will process surface material up the isotope tionally
26
inferred large ejected masses would be generated;
and Si [34] and are shown in Fig. 3. Calculated rates for making
preparation,
Al in terrestrial SiO2 were possibly low compared to early work
sequence out to Sm, and hence also include 26 Al produc- this appears to ask forandsome
[5]. Rates in Knyahinya Apollocurrently
15 deep drillunknown
core are source
in good
Program sup
tion. Characteristic afterglows have been observed, that ofagreement
energy towith make nova explosions
measurements [4,9,10]. more violent. A total
contribution from novae to Galactic 26Al of 0.1–0.4M
References
are powered from the various radioactive by-products,
likely including 26 Al (Woosley et al., 2004). have
3.3.6.been
Crossestimated from self-consistent
sections for making 36
Cl models (Jose & [1] R.C. Reedy,
Hernanz, 1998). forHigher values may possibly occur39 under
[2] J.C. Gosse,
Another hot and dense nuclear-reaction site is the Cross sections measured a particles emitted from K irradi- [3] R.C. Reedy,
thermonuclear runaway in a white dwarf star after ig- favourable circumstances,
ated by neutrons with energies with
up toup to ∼
about 15 10 −6
M for an
MeV determine the [4] K.J. Kim, J.
[5] J. Masarik,
individual nova part
(Starrfield et al., Cl 1993).
36
most important of the K(n,x) excitation function. The K
nition of carbon fusion. This is believed to produce a curve in Fig. 3 shows that the cross sections near 10 MeV are
[6] D.C. Argent
supernova of type Ia. Herein, temperatures of several
review.
much higher than at higher energies, and those energies are the [7] R.C. Reedy,
GK and high densities of order 108−10 g cm−3 allow for 2.2.4 Interstellar
ones where neutron spallation reactions
fluxes are very high. The cross sections above [8]
[9]
I. Leya, J. M
R.C. Reedy,
the full range of nuclear reactions reaching nuclear sta- Cosmic-ray
30 MeV were nuclei
basedare
on characterised as relativistic
estimates for similar reactions, andpar-
are Southon, A
tistical equilibrium (Seitenzahl & Townsley, 2017). From ticles by definition; therefore, when they collide with
about 0.6–0.7 of recently measured proton cross sections. [10] R.C. Reedy,
such an equilibrium, one expects that the main products interstellar matter, the energies in the colliding-system
will be iron-group isotopes and elements, i.e., products coordinates exceed the threshold for nuclear reactions.You can also read
