R‐ProcessNucleosynthesisAndItsSite

MarioA.RiquelmeTheore<calSeminar‐Fall2009

Outline

•  Introduc<on.

•  Nuclearphysicsandther‐process.

•  Possiblesitesforr‐processnucleosynthesis.

•  Abundancesinhalostars.

•  Chemicalevolu<on.

•  Summary

Introduc<on

•  MostoftheHandHeintheUniversewascreatedshortlyaJertheBigBang(alongwithsomelithium).Otherelementslikeberyllium‐9andboron‐11aresynthesizedduetotheinterac<oncosmicrayswiththegasintheISM.

•  Mostoftheotherelementsinnatureareformedthroughnuclearfusioninsidestars.

•  Starsejecttheirmaterialbackintothespace.Thusthereisanintricaterela<onshipbetweenthelifecycleofstarsandthenucleosynthesisofelements.

Introduc<on

•  Bindingenergypernucleusincreasesonlyun<liron‐56.Theproductof

heavierelementsisendothermic.

•  Also,astheprotonnumber(Z)increases,theCoulomb

barrierbecomesan

impedimentfordirect

fusion.

•  Heavierelementsare

synthesizedthrough

n‐captureprocesses.

Introduc<on

•  Thetwomainn‐captureprocesseswerefirstiden<fiedbyBurbidgeetal.1957.

•  Theyaretheslow(s)andrapid(r)processes:

thes‐andr‐processes.

•  Inthes‐processtheneutroncapturehappensina<mescale(n)muchlongerthanthemean<mefor‐decay(),i.e.,n>>.

•  Inthecaseofther‐process:n<<.#

•  While dependsonlyonthenuclearspecies,ndependsstronglyontheenvironment,specificallyonastrongneutronflux.

Introduc<on•  Thefigureshowsthemean fordifferentelementsintermsoftheirproton‐

andneutron‐numbers(Cowanetal.2004).

•  IntheNZplane,theelementsaredivideinto   β‐decaystableand   unstable.

•  Thestableelementsformtheso‐call“valleyofβ‐stability”,whichismarkedbyblackandmagentapoints.

•  Thes‐processpathisclosetothestabilityvalley,whilether‐processhappensfarfromit.

Valleyofβ‐stability

Introduc<on•  Thestrongfluxofneutronisatransientphenomena.AJeritstops,the

nucleiwillβdecayun<ltheyreachthevalleyofstability.Themagentapointsshowstablenucleiproducedbyther‐process.

•  Theblackpointsmark

theelementsmadeby

mainlythes‐processes.•  Thesolarsystem

elementsarean

admixtureofs‐andr‐

processelements

(~50/50).•  Jeweler’selementslike

goldandpla<numare

madealmostexclusively

byther‐process.

Valleyofβ‐stability

Introduc<on•  Thestrongfluxofneutronisatransientphenomena.AJeritstops,the

nucleiwillβdecayun<ltheyreachthevalleyofstability.Themagentapointsshowstablenucleiproducedbyther‐process.

•  Theblackpointsmark

theelementsmadeby

mainlythes‐processes.•  Thesolarsystem

elementsarean

admixtureofs‐andr‐

processelements

(~50/50).•  Jeweler’selementslike

goldandpla<numare

madealmostexclusively

byther‐process.

Valleyofβ‐stability

Introduc<on

•  Thes‐processisrela<velywellunderstood.•  Thenuclearproper<esoftheinvolvedspeciesthatareeasiertomeasure

inthelabthantheonesofther‐process(longer).•  Thesiteisalsomuchbeherconstrained:primarilylow‐andintermediate‐

massstars(lessthan8solarmasses).•  Ther‐processelementforma<onismuchmoreuncertain.•  Thenuclearproper<esofthepar<cipa<ngelementsismuchmoredifficult

tomeasure.•  Andthesiteswherether‐processtakeplaceareamistery.•  R‐processelementforma<onrequireslargeneutronfluxesthatare

associatedtorathercatastrophicevents.ThetwomaincandidatesaretypeII(core‐collapse)supernovaexplosionsandneutronstarmergers.Atpresenttheastrophysicalcondi<onsofthesetwophenomenaarenotwellunderstood(goodreview:Snedenetal.2003).

Nuclearphysicsandther‐process

•  Sinceinther‐processn<<, anelementwithagivenproton‐number(Z)shouldbeabletocapturemanyneutronsbeforeaβ‐decay.

•  Intheenvironmentsofhighfluxesthereistypicallylargetemperaturesandthereforelargequan<<esofgammaphotonsproducingnucleardisintegra<on.

•  Sotheequilibriumabundancesforagivenisotopicchainwillbedeterminedbytheneutrondensityandthetemperature.

•  Thisequilibriumimpliesthatthemaximumabundanceofpar<cipa<ngelementswillbecharacterizedbysimilarneutronenergysepara<ons(Sn),whichistheenergyreleasedbythecaptureofaneutron.

Nuclearphysicsandther‐process

•  Tofirstorder,theneutronenergysepara<on,Sn,determinesthemaximumneutronabundanceofeachisotope,anddefinesa“path”intheNZplane.

•  Inthefigure,themagentalineshowsanexampleofar‐processpath(Sn=2‐3MeV).

•  Whenthefluxofneutronsstops,

thenucleimigratetowardsthevalleyof‐stability,formingthe

r‐processstablenucleishownin

magentacolor.

•  Butknowingthepathisnot

enoughtomakepredic<onaboutabundancesofr‐process

elements.

r‐processpath

Nuclearphysicsandther‐process•  Indeed,theabundanceswilldependonthe<methatpar<clesspendineach

partofthepath.Thisdependson.#

•  Infact,whenthenumberofneutronscorrespondtotheso‐called“magicnumbers”(closedneutronshells)

(2,8,20,28,50,…)theelementstendtobesignificantlymorestable.

The β‐decaylifecanbeoneortwoordersofmagnitudelarger.Then‐capturecross‐sec<onisalsosmaller.Sothenucleitendtoconcentratemorearoundtheseregions,andproducepeaksofabundance.

Neutronmagicnumbers

Nuclearphysicsandther‐process

•  Thisunderscorestheimportanceofknowingnuclearproper<esof

theelementslikeSn, τβ,etctobeabletocomparemodelswith

abundanceobserva<ons.

•  Fissionalsoplaysanessen<alrole

inthesynthesisofr‐process

elementssince,itdeterminesthe

heaviestnucleiproducedinan

r‐process(Cowanetal.1991).

•  Fissionalsocontributestothe

distribu<onoflighterelements.

Nuclearphysicsandther‐process

Peakinthes‐processabundanceshouldhappenathighermassnumber(A)

Nuclearphysicsandther‐process

•  Peaksins‐process

abundancesare

indeedshiJedtowardslarger

massnumber(A).

Examplecorresponds

tosolarsystemabundances.

Possiblesites

•  Acri<calparameteristhenumberofneutronperseednucleus.InordertogettonucleiwithA~200,itisnecessarytohave~150neutronsperseednucleus(Febeinggenerallythelightestone).

•  Butwhereinnaturedoesonefindtheappropriatecondi<ons?

•  TypeIISNe.

Asthecorecollapsesitwouldejectarichfluxofneutronsthatwouldaccountfortheforma<onofr‐processelements.

Manyuncertain<esremain:theexplosionmechanism,theroleofneutrinointerac<on,roleofmul<dimensionalhydrodynamicinstabili<es,theequa<onofstateofultradensemaher,etc.

Possiblesites

•  Neutronstarmergers.

Theenormousdensityofneutrons(~1033cm‐3)availableinthemergerofneutronstarswouldbuildupheavyelementsandleadtofissioninveryshort<mescales.

Itcouldproduce"fissionrecycling”viasubsequentn‐capture(seecalcula<onofFreiburghausetal.1999).Itwouldbeconsistentwith

r‐processelementsinthesolarsystemforA>130.

Abundancesinhalostars.

•  Muchofthenewknowledgeabouttheforma<onoftheheaviestelementshasbeengainedthroughhighresolu<onspectroscopyofstarsintheGalaxy,specificallytheso‐calledhalostars.

•  Thesurfaceabundanceofstarsreflectthecomposi<onoftheforma<onmaterial.

•  HalostarsareamongsttheveryoldeststarsintheGalaxy,thereforetheyprovideinvaluableinforma<onaboutthechemicalcomposi<onoftheISMatearly<mes.

•  AverywellstudiedhalostarisCS22892‐052,whoseFeabundanceisaboutonethousandthofsolar(veryold;Snedenetat.2003b).

Abundancesinhalostars.•  AnstrikingfeatureistheagreementbetweenCS22892‐052abundanceand

ther‐processabundanceofthesolarsystemforZ>56(Ba).

•  Thisistellingusmainlytwothings:•  First,ther‐processseemstobeaveryrobustprocess.Whereverandhowever

ishappeningitisunderwellconstrainedastrophysicalcondi<ons.•  Ithappensveryfast(fewmillionyears),beforetheforma<onofs‐processelements.•  ThedisagreementforZ=40‐50ispuzzlingandmightbesugges<ngdifferentkindofr‐processforlightandheavyelements(Wassenburgetal.2000).

Chemicalevolu<on•  Thepossibilitythatlightandheavyr‐processelementsareproducedby

differentindifferentenvironmentsissupportedbythechemicalevolu<onofdifferenthalostars.

•  Figureshowstheabundanceasafunc<onof“<me”(Feabundance)ofdifferentr‐processelements.Ge(Z=36;light)showsadifferentevolu<onandmuchlessscaheringatearly

<mescomparedtoEu(Z=63;heavy).•  ThelargescaheringofEuatearly<messuggestssuggeststhatatfirstther‐processelementswerespreadinhomogeneously.Thiseffectislessimportantiftheelement,likeGe,isproducedinalargerrangeofsites/condi<ons.

Chemicalevolu<on

•  Theshort<meinwhichther‐processelementsshouldhavebeenspread(fewmillionyears)pointstotypeIISNeasthefavoredcandidates(neutronstarmergerswouldevolvetooslowly).

•  Inthecontextofforma<onincore‐collapseSNe,observa<onspointtomorethanonesite/condi<onfortheforma<onofr‐process.

Summary•  ElementsheavierthanFerequireendothermicnuclearreac<ons.•  Theyareformedbyneutron‐capture,ins‐andr‐processes(~50/50inthesolar

system).•  Theastrophysicalsiteforther‐processisanopenques<on.•  Possibili<es:TypeIIcore‐collapseSNeandneutronstarmergers.•  Observa<onofabundancesinhalosstarsshowanstrikingagreementwithsolar

systemr‐processelementsforZ>56.•  Chemicalevolu<onshowsthatheavierelementswereformedmoreslowly.•  Thisistellingus:R‐processisveryrobust.Itssiteevolvesquiterapidly(fewmillionyears).Therecouldbedifferentsites/condi<onsforheavyandlightr‐processelements.•  ThisfavorsthetypeIIcore‐collapseSNeinsteadofneutronstarmergersasthe

site.Buttheques<onofthewhereinthesupernovaortheeffectoftheprogenitormassinthekindofr‐processelementiss<llopen.