Treatise on Geochemistry. Meteorites, Comets and Planets

5155 Pages·2003·137.727 MB·English

by  Davis A.M. (ed.), Holland H.D. (ed.), Turekian K.K. (ed.)

1.01
Origin of the Elements
J. W. Truran, Jr. and A. Heger
University of Chicago, IL, USA
1.01.1 INTRODUCTION 1
1.01.2 ABUNDANCESANDNUCLEOSYNTHESIS 2
1.01.3 INTERMEDIATEMASSSTARS:EVOLUTIONANDNUCLEOSYNTHESIS 3
1.01.3.1 ShellHeliumBurningand12CProduction 4
1.01.3.2 s-ProcessSynthesisinRedGiants 4
1.01.4 MASSIVE-STAREVOLUTIONANDNUCLEOSYNTHESIS 5
1.01.4.1 NucleosynthesisinMassiveStars 7
1.01.4.1.1 Hydrogenburning 7
1.01.4.1.2 Heliumburningandthes-process 7
1.01.4.1.3 Hydrogenandheliumshellburning 7
1.01.4.1.4 Carbonburning 8
1.01.4.1.5 Neonandoxygenburning 8
1.01.4.1.6 Siliconburning 8
1.01.4.1.7 Explosivenucleosynthesis 8
1.01.4.1.8 Thep-process 9
1.01.4.1.9 Ther-process 9
1.01.5 TYPEIaSUPERNOVAE:PROGENITORSANDNUCLEOSYNTHESIS 9
1.01.6 NUCLEOSYNTHESISANDGALACTICCHEMICALEVOLUTION 12
REFERENCES 14
1.01.1 INTRODUCTION Within galaxies, stars and supernovae play the
dominant role both in synthesizing the elements
Nucleosynthesis is the study of the nuclear
from carbon to uranium and in returning heavy-
processes responsible for the formation of the
element-enrichedmattertotheinterstellargasfrom
elements which constitute the baryonic matter of
whichnewstarsareformed.Themassfractionof
theUniverse.TheelementsofwhichtheUniverse oursolarsystem(formed,4.6Gyrago)intheform
is composed indeed have a quite complicated ofheavyelementsis,1.8%,andstarsformedtoday
nucleosynthesis history, which extends from the inourgalaxycanbeafactor2or3moreenriched
firstthreeminutesoftheBigBangthroughtothe (Edvardsson et al., 1993). It is the processes of
present. Contemporary nucleosynthesis theory nucleosynthesisoperatinginstarsandsupernovae
associates the production of certain elements/ thatwewillreviewinthischapter.Wewillconfine
isotopes or groups of elements with a number of ourattentiontothreebroadcategoriesofstellarand
specificastrophysicalsettings,themostsignificant supernovasitewithwhichspecificnucleosynthesis
of which are: (i) the cosmological Big Bang, products are understood to be identified:
(ii) stars, and (iii)supernovae. (i) intermediate mass stars, (ii) massive stars and
Cosmological nucleosynthesis studies predict associated type II supernovae, and (iii) type Ia
thattheconditionscharacterizingtheBigBangare supernovae. The first two of these sites are the
consistent with the synthesis only of the lightest straightforward consequence of the evolution
elements:1H,2H,3He,4He,and7Li(Burlesetal., of single stars, while type Ia supernovae are
2001; Cyburt et al., 2002). These contributions understoodtoresultfrombinarystellarevolution.
define the primordial compositions both of Stellar nucleosynthesis resulting from the
galaxies and of the first stars formed therein. evolution of single stars is a strong function of
1
2 Origin ofthe Elements
stellar mass (Woosley et al., 2002). Following 1.01.2 ABUNDANCESAND
phases of hydrogen and helium burning, all stars NUCLEOSYNTHESIS
consist of a carbon–oxygen core. In the mass
The ultimate goal of nucleosynthesis theory is,
range of the so-called “intermediate mass” stars
ð1&M=M( &10Þ; the temperatures realized in of course, to explain the composition of the
Universe, as reflected, for example, in the stellar
theirdegeneratecoresneverreachlevelsatwhich
and gas components of galaxies. Significant
carbon ignition can occur. Substantial element
progress has been achieved in this regard as a
production occurs in such stars during the
consequence of a wealth of new information of
asymptotic giant branch (AGB) phase of evo-
cosmic abundances—spectroscopic properties of
lution,accompaniedbysignificantmassloss,and
starsinourgalaxyandofgascloudsandgalaxies
they evolve to white dwarfs of carbon–oxygen
at high redshifts—pouring in from new ground-
(or, less commonly, oxygen–neon) composition.
and space-based observatories. Given that, it
In contrast, the increased pressures that are
remains true that the most significant clues to
experienced in the cores of stars of masses M *
nucleosynthesisarethoseprovidedbyourdetailed
10M( yield higher core temperatures that enable
knowledge of the elemental and isotopic compo-
subsequent phases of carbon, neon, oxygen, and
sitionofsolarsystemmatter.Themassfractionsof
silicon burning to proceed. Collapse of an iron
the stable isotopes in the solar are displayed in
core devoid of further nuclear energy then gives
Figure1.Keyfeaturesthatreflectthenatureofthe
risetoatype IIsupernovaandtheformationofa
nuclearprocessesbywhichtheheavyelementsare
neutron star or black hole remnant (Heger et al.,
formed include: (i) the large abundances of 12C
2003). The ejecta of type IIs contain the ashes of
and 16O, the main products of stellar helium
nuclear burning of the entire life of the star, but
burning;(ii)thedominanceofthea-particlenuclei
are also modified by the explosion itself. They
arethesourceofmostmaterial(bymass)heavier throughcalcium(20Ne,24Mg,28Si,32S,36Ar,and
than helium. 40Ca); (iii) the “nuclear statistical equilibrium”
Observations reveal that binary stellar systems peakatthepositionof56Fe;and(iv)theabundance
comprise roughly half of all stars in our galaxy. peaksintheregionpastironattheneutronclosed
Single star evolution, as noted above, can leave shell positions (zirconium, barium, and lead),
initswakecompactstellarremnants:whitedwarfs, confirming the occurrence of processes of neu-
neutron stars, and black holes. Indeed, we have tron-capture synthesis. The solar system abunda-
evidence for the occurrence of all three types of nce patterns associated specifically with the slow
condensed remnant in binaries. In close binary (s-process) and fast (r-process) processes of
systems, mass transfer can take place from an neutroncapturesynthesisareshowninFigure2.
evolving companion onto a compact object. This Itisimportantheretocallattentiontotherevised
naturally gives rise to a variety of interesting determinations of the oxygen and carbon abun-
phenomena: classical novae (involving hydrogen dances in the Sun. Allende Prieto et al. (2001)
thermonuclear runaways in accreted shells on derivedanaccurateoxygenabundancefortheSun
white dwarfs (Gehrz et al., 1998)), X-ray bursts of log1ðOÞ¼8:69^0:05dex; a value approxi-
(hydrogen/helium thermonuclear runaways on matelyafactorof2belowthatquotedbyAnders
neutron stars (Strohmayer and Bildsten, 2003)), andGrevesse(1989).Subsequently,AllendePrieto
and X-ray binaries (accretion onto black holes). et al. (2002) determined the solar carbon abun-
For some range of conditions, accretion onto dance to be log1ðCÞ¼8:39^0:04dex; and the
carbon–oxygen white dwarfs will permit growth ratioC=O¼0:5^0:07:Thebottomlinehereisa
of the CO core to the Chandrasekhar limit reduction in the abundances of the two most
MCh ¼1:4M(, and a thermonuclear runaway in abundant heavy elements in the Sun, relative to
tocoreleadstoatypeIasupernova. hydrogen and helium, by a factor ,2. The
Inthischapter,wewillreviewthecharacteristics implications of these results for stellar evolution,
ofthermonuclearprocessinginthethreeenviron- nucleosynthesis,theformationofcarbonstars,and
ments we have identified: (i) intermediate-mass galacticchemicalevolutionremaintobeexplored.
stars;(ii)massivestarsandtypeIIsupernovae;and Guided by early compilations of the “cosmic
(iii)typeIasupernovae.Thiswillbefollowedbya abundances” as reflected in solar system material
brief discussion of galactic chemical evolution, (e.g.,SuessandUrey,1956),Burbidgeetal.(1957)
whichillustrateshowthecontributionsfromeach and Cameron (1957) identified the nuclear pro-
oftheseenvironmentsarefirstintroducedintothe cessesbywhichelementformationoccursinstellar
interstellarmediaofgalaxies.Reviewsofnucleo- andsupernovaenvironments:(i)hydrogenburning,
synthesisprocessesincludethosebyArnett(1995), which powers stars for ,90% of their lifetimes;
Trimble (1975), Truran (1984), Wallerstein et al. (ii) helium burning, which is responsible for the
(1997), and Woosley et al. (2002). An overview productionof12Cand16O,thetwomostabundant
of galactic chemical evolution is presented by elementsheavierthanhelium;(iii)thea-process,
Tinsley(1980). which we now understand as a combination of
Intermediate Mass Stars:EvolutionandNucleosynthesis 3
Figure1 TheabundancesoftheisotopespresentinsolarsystemmatterareplottedasafunctionofmassnumberA
(thesolarsystemabundancesfortheheavyelementsarethosecompiledbyPalmeandJones(seeChapter1.03).
carbon, neon, and oxygen burning; (iv) the
equilibrium process, by which silicon burning
proceeds to the formation of a nuclear statistical
equilibrium abundance peak centered on mass
A¼56; (v) the slow (s-process) and rapid
(r-process) mechanisms of neutron capture syn-
thesisoftheheaviestelementsðA*60–70Þ;and
(vi)thep-process,acombinationoftheg-process
and the n-process, which we understand to be
responsibleforthesynthesisofanumberofstable
isotopes of nuclei on the proton-rich side of the
valley of beta stability. Our subsequent discus-
sionswillidentifytheastrophysicalenvironments
in which these diverse processes are now under-
stood tooccur.
Figure 2 The s-process and r-process abundances in
solarsystemmatter(basedupontheworkbyKa¨ppeler
etal.,1989).Notethedistinctives-processsignatureat
1.01.3 INTERMEDIATEMASSSTARS:
masses A,88, 138, and 208 and the corresponding
EVOLUTIONAND
r-processsignaturesatA,130and195,allattributable
NUCLEOSYNTHESIS
toclosed-shelleffectsonneutroncapturecross-sections.
It is the r-process pattern thus extracted from solar
Intermediate-mass red giant stars are under-
system abundances that can be compared with the
stoodtobetheprimarysourcebothof12Candof
observed heavy element patterns in extremely metal-
the heavy s-process (slow neutron capture)
deficientstars(thetotalsolarsystemabundancesforthe
heavy elements are those compiled by Anders and elements, as well as a significant source of 14N
Grevesse, 1989), which are very similar to those from and other less abundant CNO isotopes. Their
thecompilationofPalmeandJones(seeChapter1.03). contributions to galactic nucleosynthesis are
4 Origin ofthe Elements
a consequence of the occurrence of nuclear are converted into 12C, followed by the
reactions in helium shell thermal pulses on the 12C(a,g)16O reaction, which forms 16O at the
AGB,thesubsequentdredge-upofmatterintothe expense of 12C. Core helium burning in massive
hydrogen-rich envelope by convection, and mass stars ðM *10M(Þ occurs at high temperatures,
loss.Thisisaverycomplicatedevolution.Current which increases the rate of the 12C(a,g)16O
stellar models, reviewed by Busso et al. (1999), reaction and favors the production of oxygen.
allow the formation of low mass ð,1:5M(Þ Typically, the 16O/12C ratio in the oxygen-rich
carbon stars, which represent the main source of mantlesofmassivestarspriortocollapseisafactor
s-process nuclei. A detailed review of the ,2–3 higher than the solar values. Massive stars
nucleosynthesis products (chemical yields) for are thus the major source of oxygen, while low-
low- and intermediate-mass stars is provided by and intermediate-mass stars dominate the pro-
Marigo (2001). ductionofcarbon.
Redgiantstarshaveplayedasignificantrolein Theadvantageoftheheliumshellsoflow-and
the historical development of nucleosynthesis intermediate-mass stars for 12C production arises
theory. While the pivotal role played by nuclear from the fact that, for conditions of incomplete
reactions in stars in providing an energy source helium burning, the 12C/16O ratio is high.
sufficienttopowerstarsliketheSunoverbillions Following a thermal pulse in the helium shell,
of years was established in the late 1930s, it convective dredge-up of matter from the helium
remained to be demonstrated that nuclear pro- shell brings helium, s-process elements, and a
cesses in stellar interiors might play a role in the significant mass of 12C to the surface. It is the
synthesis of heavy nuclei. The recognition that surface enrichment associated with this source of
heavy-element synthesis is an ongoing processin 12C that leads to the condition that the envelope
stellarinteriorsfollowedthediscoverybyMerrill 12C/16Oratioexceeds1,suchthat“carbonstar”is
(1952)ofthepresenceoftheelementtechnetiumin born. Calculations of galactic chemical evolution
red giant stars. Since technetium has no stable indicate that this source of carbon is sufficient to
isotopes,andthelongest-livedisotopehasahalf- accountforthelevelof12Cingalacticmatter.The
lifet1=2,4:6Myr;itspresenceinred-giantatmos- levels of production of 14N, 13C, and other CNO
pheres indicates its formation in these stars. isotopesinthisenvironmentaresignificantlymore
This confirmed that the products of nuclear difficult to estimate, and thus the corresponding
reactions operating at high temperatures and contributions of AGB stars to the galactic
densities in the deep interior can be transported abundances ofthese isotopesremain uncertain.
by convection to the outermost regions of the
stellarenvelope.
The role of such convective “dredge-up” of 1.01.3.2 s-ProcessSynthesisinRedGiants
matter in the red-giant phase of evolution of
The formation of most of the heavy elements
1–10M( stars is now understood to be an occursinoneoftwoprocessesofneutroncapture:
extremely complex process (Busso et al., 1999).
the s-process or the r-process. These two broad
On the first ascent of the giant branch (prior to
divisions are distinguished on the basis of the
helium ignition), convection can bring the pro-
relative lifetimes for neutron captures (t) and
ducts of CNO cycle burning (e.g., 13C, 17O, and electron decays ðt Þ: The condition thattn.t ;
14N) to the surface. A second dredge-up phase b n b
wheret isacharacteristiclifetimeforb-unstable
occurs following the termination of core helium b
nuclei near the valley of b-stability, ensures that
burning.Thecriticalthirddredgeup,occurringin
as captures proceed the neutron-capture path will
the aftermath of thermal pulses in the helium
itselfremainclosetothevalleyofb-stability.This
shells of these AGB stars, is responsible for the defines the s-process. In contrast, when t ,t ;
transport of both 12C and s-process nuclei n b
it follows that successive neutron captures
(e.g., technetium) to the surface. The subsequent
will proceed into the neutron-rich regions off
lossofthisenrichedenvelopematterbywindsand
the b-stable valley. Following the exhaustion of
planetary nebula formation serves to enrich the
theneutronflux,thecaptureproductsapproachthe
interstellar media of galaxies, from which new
position of the valley of b-stability by b-decay,
stars are born. A brief review of the mechanisms
forming the r-process nuclei. The s-process and
ofproductionof12Candthe“main”componentof
r-processpatternsinsolarsystemmatterarethose
thes-processofneutroncapturenucleosynthesisis
shown inFigure 2.
presentedin the followingsections.
Theenvironmentprovidedbythermalpulsesin
theheliumshellsofintermediate-massstarsonthe
AGB provides conditions consistent with the
1.01.3.1 ShellHeliumBurning
and12CProduction synthesis of the bulk of the heavy s-process
isotopes through bismuth. Neutron captures in
Stellarheliumburningproceedsbymeansofthe AGBstarsaredrivenbyacombinationofneutron
“triple-alpha” reaction in which three 4He nuclei sources: the 13C(a,n)16O reaction provides
Massive-star Evolution and Nucleosynthesis 5
the bulk of the neutron budget at low-neutron
densities, while the 22Ne(a,n) 25Mg operating at
high temperatures helps to set the timescale for
criticalreaction branches.Thiss-processsite(the
main s-process component) is understood to
operate in low-mass AGB stars ðM ,1–3M()
and to be responsible for the synthesis of the
s-process nuclei in the mass range A*90:
Calculations reviewed by Busso et al. (1999)
indicate a great sensitivity both to the character-
istics of the 13C “pocket” in which neutron
production occurs and to the initial metallicity of
the star. In their view, this implies that the solar
system abundances are not the result of a unique
s-processbutrathertheconsequenceofacompli- Figure 3 Evolution of the central temperature and
catedgalacticchemicalevolutionaryhistorywhich density in stars of 15M( and 25M( from birth as
witnessedmixingoftheproductsofs-processingin hydrogenburningstarsuntilironcorecollapse(Table1).
stars of different metallicity and a range of 13C Ingeneral,thetrajectoriesfollowalineofr/T3,but
with some deviation downward (towards higher r
pockets. We can hope that observations of the
at a given T) due to the decreasing entropy of the
s-processabundancepatternsinstarsasafunction
core. Nonmonotonic behavior is observed when
of metallicity will ultimately be better able to nuclear fuels are ignited and this is exacerbated in the
guideandtoconstrainsuchtheoreticalmodels. 15M( model by partial degeneracy of the gas (source
Woosleyetal.,2002).
1.01.4 MASSIVE-STAREVOLUTION
ANDNUCLEOSYNTHESIS the bottom of burning region (central or shell
burning) and depleting the fuel elsewhere in
Generallyspeaking, theevolution ofamassive
that region at the same time. As a result, shell
starfollowsawell-understoodpathofcontraction
burning of this fuel then commences outside that
toincreasingcentraldensityandtemperature.The
region.
contractioniscausedbytheenergylossofthestar,
Table 1 summarizes the burning stages and
duetolightradiatedfromthesurfaceandneutrino
theirdurationsfora20M( star.Thetimescalefor
losses (see below). The released potential energy
helium burning is ,10 times shorter than that of
isinpartconvertedintointernalenergyofthegas
hydrogen burning, mostly because of the lower
(Virial theorem). This path of contraction is
energyreleaseperunitmass.Thetimescaleofthe
interrupted by nuclear fusion—first hydrogen is
burning stages and contraction beyond central
burned to helium, then helium to carbon and
helium burning is greatly reduced by thermal
oxygen. This is followed by stages of carbon,
neutrino losses that carry away energy in situ,
neon, oxygen, and silicon burning, until finally a
instead of requiring that it be transported to the
core of iron is produced, from which no more
stellar surface by diffusion or convection. These
energy can be extracted by nuclear burning. The
lossesincreasewithtemperature(as /T9).When
onsetsoftheseburningphasesasthestarevolves
the star has built up a large-enough iron core,
throughthetemperature–densityplaneareshown
inFigure3,forstarsofmasses15M( and25M(: exceeding its effective Chandrasekhar mass (the
maximum mass for which such a core can be
Eachfuelburnsfirstinthecenterofthestar,then
stable),thecorecollapsestoformaneutronstaror
in one or more shells (Figure 4). Most burning
ablackhole(seeWoosleyetal.(2002)foramore
stages proceed convectively: i.e., the energy
production rate by the burning is so large and extended review). A supernova explosion may
centrally concentrated that the energy cannot be result (e.g., Colgate and White, 1966) that ejects
transported by radiation (heat diffusion) alone, mostofthelayersoutsidetheironcore,including
and convective motions dominate the heat trans- many of the ashes from the preceding burning
port. The reason for this is the high-temperature phase. However, when the supernova shock front
sensitivity of nuclear reaction rates: for travelsoutward,forabrieftimepeaktemperatures
hydrogenburninginmassivestars,nuclearenergy are reached, that exceed the maximum tempera-
generation has a /T18 dependence, and the turesthathavebeenreachedineachregioninthe
dependence is even stronger for later burning preceding hydrostatic burning stages (Table 2).
stages. The important consequence is that, due to This defines the transient stage of “explosive
theefficientmixingcausedbytheconvection,the nucleosynthesis”thatiscriticaltotheformationof
entire unstable region evolves essentially chemi- an equilibrium peak dominated by 56Ni (Truran
cally homogeneously—replenishing the fuel at et al.,1967).
6 Origin ofthe Elements
Figure4 Interiorstructureofa22M( starofsolarcompositionasafunctionoftime(logarithmoftimetillcore
collapse)andenclosedmass.Greenhatchingandredcrosshatchingindicateconvectiveandsemiconvectiveregions.
Convectiveregionsaretypicallywellmixedandevolvechemicallyhomogeneously.Blueshadingindicatesenergy
generationandpinkshadingenergyloss.Bothtakeintoaccountthesumofnuclearandneutrinolosscontributions.
Thethickblacklineatthetopindicatesthetotalmassofthestar,beingreducedbymasslossduetostellarwinds.
Note that the mass loss rate actually increases at late times of the stellar evolution. The decreasing slope
ofthetotalmassofthestarinthefigureisduetothelogarithmicscalechosenforthetimeaxis.
Table 1 Hydrostatic nuclear burning stages in massive stars. The table gives burning stages, main and secon-
daryproducts(ashes),typicaltemperaturesandburningtimescalesfora20M(star,andthemainnuclearreactions.
An ellipsis (···) indicates more than one product of the double carbon and double oxygen reactions, and a chain
ofreactionsleadingtothebuildupofirongroupelementsforsiliconburning.
Fuel Mainproducts Secondaryproducts T Duration Mainreaction
(109K) (yr)
H He 14N 0.037 8.1£106 4H!4He(CNOcycle)
He O,C 18O,22Ne 0.19 1.2£106 34He!12C
s-Process 12Cþ4He!16O
C Ne,Mg Na 0.87 9.8£102 12Cþ12C!···
Ne O,Mg Al,P 1.6 0.60 20Ne!16Oþ4He
20Neþ4He!24Mg
O Si,S Cl,Ar, 2.0 1.3 16Oþ16O!···
K,Ca
Si Fe Ti,V,Cr, 3.3 0.031 28Si!24Mgþ4He···
Mn,Co,Ni 28Siþ4He!24Mg···
Massive stars build up most of the heavy heavyelementsuptoatomicmassnumbers80–90
elements from oxygen through the iron group frominitialiron,convertinginitialcarbon,oxygen,
fromtheinitialhydrogenandheliumofwhichthey and nitrogen into 22Ne, thus providing a neutron
areformed.Theyalsomakemostofthes-process sourceforthes-process.Massivestarsareprobably