南京大学天体物理概论课件03物质与辐射

§3.Matter&RadiationEssentialsMatterEssentials:§3.1.TheBigBangPart4.Cosmology§3.2.Dark&LightMatter§3.3.TheAbundancesPart4.Cosmology§3.4.TheGaseousUniverse§3.5.TheDustyUniverse§3.6.TheCosmicRaysRadiationEssentials:§3.7.BlackBodyRadiation§3.8.GreyBodies1§3.1.TheBigBang

Part4.CosmologyNotaneventoccurredsometimesomewhere.SpacetimecameintobeingwiththeBigBang.RedshiftsofgalaxiesTheageoftheUniversevs.oldeststars:12.7–13.2GyrCMB=CosmicMicrowaveBackgroundAbundancesoflightelementsnucleosynthesisinthefirstmoments2§3.2.Dark&LightMatterMostofmatterinvisible.3EvidencefordarkmatterGalaxiesrotatemuchfasterintheirouterregions. extendeddarkmattersurroundinggalaxies.Darkmatterisneededtoboundgalaxyclusters.Gravitationallensing

Mtotal>>Mvisible2independentpiecesofevidenceforDMinGCs.4§3.3.TheAbundances

Part4.CosmologyTheabundanceoftheelementsarequantifiedbytheirnumberfractionormassfraction:5solar(cosmic)abundanceprimordialabundance&enrichmentmetals:elementsotherthanHandHeZ:metallicity6Primordialabundances(intheimmediateaftermathoftheBigBang):H,He,D,3He,&7LiStellarevolution&ISMenrichmentPartII.StarsP-Pchain(M<1.5Msun)He:theonlyproductSupernovae&ExplosiveNucleosynthesisM<8Msun

planetarynebulaWDM>8MsunSNeIIelementsheavier thanFeTypicalkineticenergy~1051erg7Zhou+06starformationhistory&IMF8Lu,Zhou,Wangetal.2006IMF=InitialMassFunction:Theadmixtureofstarsofdifferentmasseswhenfirstformed.§3.4.TheGaseousUniverseMetallic-likeliquidHatthecenterofJupiter9H&HeTemperatureT

DensityneStateofionizationH(n=2)HowtoknowT,ne,andH+/H0?Spitzer(1978):forT<8x104K,particleencountersarealmostalwayselastic.thermaltimescale~hours–yearsastrophysicalgasesareinthermalequilibrium.Kinetictemperature&

Maxwell-BoltzmannvelocitydistributionGas,energychangedbetweenparticlesviaelasticcollisionsStatisticalmechanicsMaxwell-Boltzmannvelocitydistributionkinetictemperature10<v2>:mean-squareparticlespeed.Root-mean-square(r.m.s)particlespeed:mostprobablespeed:meanspeed:theidealgasAnidealgasisagasthatobeystheidealgaslaw(particlepressure):11whereiscalledthemeanmolecularweight(miisthemeanmassoftheithparticle),e.g.,meanatomicmetalweight:mi=AimH+electrondegeneracyStatisticalequilibrium,

(Local)ThermalEquilibrium=(L)TEcollisionexcitation/de-excitationabsorptionexcitation/ionizationemissionde-excitationThepopulationsofenergylevelsaredeterminedbyincludingallprocessesthatbothpopulate&de-populateanygivenlevel.Inasteadystate,thetransitionrateintoanylevelequalstherateout–statisticalequilibrium.Equationsofstatisticalequilibriumaresetupforeachlevelandinvolvethedensityoftheparticles,theenergydensityoftheradiationfield,andcoefficientsdescribingcollisional,radiative,andspontaneoustransitionprobabilities.Thecoefficientsmaythemselvesbefunctionsofotherquantities,suchasquantummechanicalparametersortemperature.Extremelycomplexsimplification12IfagasisinTE,theenergyintheradiationfieldisinequilibriumwiththekineticenergyoftheparticles.LTE:agashasTEproperties,butonlylocally.BoltzmannEquationForLTE,theequationsofstatisticalequilibriumaremuchsimplifiedandthepopulationofstatesisgivenbytheBoltzmannEquation:13Wherenistheprinciplequantumnumber,Nnisthenumberofatomsinwhichelectronsareinthenthenergylevel(e.g.,N1isthenumberofatomswithelectronsinthegroundstate),gnisthestatisticalweight(e.g.,forhydrogen,gn=2n2),and⊿Eistheenergydifferencebetweenstatenandthegroundstate.GeneralformofBoltzmannEquationRatherthan14Generally,where,iscalledthepartitionfunction.e.g.,atT<3500K,thepartitionfunctionforhydrogenrevertstothestatisticalweightofthegroundstate,i.e.,U=g1exp[-0/(kT)]=g1=2*12=2.Ionization&theSahaEq.15TheionizationstateofagasinLTEcanbeexpressedinafashionsimilartotheBoltzmannEq.,whereUK+1&UKarethepartitionfunctionsofthe(K+1)th&Kthionizationstates,respectively,neistheelectrondensity,metheelectronmass,cKistheenergyrequiredtoremoveanelectronfromthegroundstatetotheKthionizationstate.e.g.,hydrogenonlyhasoneelectrontoberemoved,&canonlyexistinthesinglyionizedorneutralstates,theSahaEq.reducesto,§3.5.TheDustyUniverse1.Observationaleffectsofdust16M104extinction:selectiveextinction:e.g.,theratioofselectivetototalextinction:extinctioncurveoftheMilkyWay17structure,composite,&originofdustthesizeofdustygrainsislikelypresentinapowerlawdistribution:18?composite:PAHs=PolycyclicAromaticHydrocarbons;SiO2;CH4;H2O;NH3etc.theoriginofdust:Coolatmospheresofevolvedstars;Metal-richejectaofSNeSmokingquasars(Elvis+00)?§3.6.TheCosmicRaysVictorHess(1912):anelectrometerdischargesmorequicklyathigheraltitudesthesourceofthedischargemustbefromabovetheatmosphere,insteadoftheearth1936,NobelPrizeforphysics19cosmicraycomposite:98%:nucleons:H+(87%);He2+(12%);Zz+(1%)2%:e-/+cosmicrayenergyspectrum:J(E)=KE-G§3.7.BlackBodyRadiationDeviationofthePlanck-Function:theprobabilityoffindingaparticleinanygivenstatedependsonthestatisticalweight&theBoltzmannfactor,exp[-E/(kT)].20ThespecificintensityofablackbodyisdescribedbythePlanckfunction:brightnesstemperature21Rayleigh-JeansLaw(hn

<<kT)WeinLaw(hn

>>kT)(Yuan,Zhou,Komossa+2008,ApJ,685,801)(Zhou,Wang,Wang+2006,ApJ,639,716)Stefan-BoltzmannLawThetotalintensityofablackbodycanbeobtainedbyintegratingthePlanckfunctionovernorl:22Fluxfromastar:Stefan-BoltzmannLaw,scalledStefan-Boltzmannconstant.EstimationofthesurfacetemperatureofthesunT⊙usingS-BLaw:EstimatingT⊙withoutanyknownphysicsmethodofdiscoveringnewphysics!energydensity&pressureinastar23Radiationpressureisastrongfunctionoftemperature.negligibleforlowmasscoolstars;veryimportantforhighmasshotstars,e.g.WR.§3.8.GreyBodiesBlackbodies:absorballincidentradiation.Albedo:thefractionoflightthatisreflected.Objectsthatdonotabsorballincidentradiationarecalledgreybodies.24Itisthereflectedlightthatallowsustoseeplanets,comets,...dust