天体物理概论幻灯片.ppt

天体物理概论第1页，共25页，编辑于2022年，星期五3.1.The Big Bang Part 4.Cosmology Part 4.CosmologyNot an event occurred sometime somewhere.Spacetime came into being with the Big Bang.nRedshifts of galaxiesnThe age of the Universe vs.oldest stars:12.7 13.2 GyrnCMB=Cosmic Microwave BackgroundnAbundances of light elements nucleosynthesis in the first moments2 2第2页，共25页，编辑于2022年，星期五3.2.Dark&Light MatterMost of matter invisible.3 3第3页，共25页，编辑于2022年，星期五Evidence for dark matter Galaxies rotate much faster in their outer regions.extended dark matter surrounding galaxies.Dark matter is needed to bound galaxy clusters.Gravitational lensing Mtotal Mvisible2 independent pieces of evidence for DM in GCs.4 4第4页，共25页，编辑于2022年，星期五3.3.The Abundances Part 4.Cosmology Part 4.CosmologyThe abundance of the elements are quantified by their number fraction or mass fraction:5 5solar(cosmic)abundance第5页，共25页，编辑于2022年，星期五primordial abundance&enrichmentmetals:elements other than H and He Z:metallicity6 6Primordial abundances(in the immediate aftermath of the Big Bang):H,He,D,3He,&7LiStellar evolution&ISM enrichment Part II.Stars P-P chain(M1.5Msun)He:the only product第6页，共25页，编辑于2022年，星期五Supernovae&Explosive NucleosynthesisM8Msun SNe II elements heavier than FeTypical kinetic energy 1051 erg7 7Zhou+06第7页，共25页，编辑于2022年，星期五star formation history&IMF8 8Lu,Zhou,Wang et al.2006IMF=Initial Mass Function:The admixture of stars of different masses when first formed.第8页，共25页，编辑于2022年，星期五3.4.The Gaseous UniverseMetallic-like liquid H at the center of Jupiter9 9H&HeTemperature T Density neState of ionizationH(n=2)How to know T,ne,and H+/H0?Spitzer(1978):for T8x104K,particle encounters are almost always elastic.thermal timescale hours years astrophysical gases are in thermal equilibrium.第9页，共25页，编辑于2022年，星期五Kinetic temperature&Maxwell-Boltzmann velocity distributionGas,energy changed between particles via elastic collisions Statistical mechanics Maxwell-Boltzmann velocity distribution kinetic temperature1010:mean-square particle speed.Root-mean-square(r.m.s)particle speed:most probable speed:mean speed:第10页，共25页，编辑于2022年，星期五the ideal gasAn ideal gas is a gas that obeys the ideal gas law(particle pressure):1111whereis called the mean molecular weight(mi is the mean mass of the ith particle),e.g.,mean atomic metal weight:mi=AimH+electron degeneracy第11页，共25页，编辑于2022年，星期五Statistical equilibrium,(Local)Thermal Equilibrium=(L)TEcollision excitation/de-excitationabsorption excitation/ionizationemission de-excitationThe populations of energy levels are determined by including all processes that both populate&de-populate any given level.In a steady state,the transition rate into any level equals the rate out statistical equilibrium.Equations of statistical equilibrium are set up for each level and involve the density of the particles,the energy density of the radiation field,and coefficients describing collisional,radiative,and spontaneous transition probabilities.The coefficients may themselves be functions of other quantities,such as quantum mechanical parameters or temperature.Extremely complex simplification1212If a gas is in TE,the energy in the radiation field is in equilibrium with the kinetic energy of the particles.LTE:a gas has TE properties,but only locally.第12页，共25页，编辑于2022年，星期五Boltzmann EquationFor LTE,the equations of statistical equilibrium are much simplified and the population of states is given by the Boltzmann Equation:1313Where n is the principle quantum number,Nn is the number of atoms in which electrons are in the nth energy level(e.g.,N1 is the number of atoms with electrons in the ground state),gn is the statistical weight(e.g.,for hydrogen,gn=2n2),and E is the energy difference between state n and the ground state.第13页，共25页，编辑于2022年，星期五General form of Boltzmann EquationRather than 1414Generally,where,is called the partition function.e.g.,at T3500 K,the partition function for hydrogen reverts to the statistical weight of the ground state,i.e.,U=g1exp-0/(kT)=g1=2*12=2.第14页，共25页，编辑于2022年，星期五Ionization&the Saha Eq.1515The ionization state of a gas in LTE can be expressed in a fashion similar to the Boltzmann Eq.,where UK+1&UK are the partition functions of the(K+1)th&Kth ionization states,respectively,ne is the electron density,me the electron mass,c cK is the energy required to remove an electron from the ground state to the Kth ionization state.e.g.,hydrogen only has one electron to be removed,&can only exist in the singly ionized or neutral states,the Saha Eq.reduces to,第15页，共25页，编辑于2022年，星期五3.5.The Dusty Universe1.Observational effects of dust1616M104extinction:selective extinction:e.g.,the ratio of selective to total extinction:第16页，共25页，编辑于2022年，星期五extinction curve of the Milky Way1717第17页，共25页，编辑于2022年，星期五structure,composite,&origin of dustthe size of dusty grains is likely present in a power law distribution:1818?composite:PAHs=Polycyclic Aromatic Hydrocarbons;SiO2;CH4;H2O;NH3 etc.the origin of dust:nCool atmospheres of evolved stars;nMetal-rich ejecta of SNenSmoking quasars(Elvis+00)?第18页，共25页，编辑于2022年，星期五3.6.The Cosmic RaysVictor Hess(1912):an electrometer discharges more quickly at higher altitudes the source of the discharge must be from above the atmosphere,instead of the earth 1936,Nobel Prize for physics1919cosmic ray composite:98%:nucleons:H+(87%);He2+(12%);Zz+(1%)2%:e-/+cosmic ray energy spectrum:J(E)=KE-G第19页，共25页，编辑于2022年，星期五3.7.Black Body RadiationDeviation of the Planck-Function:the probability of finding a particle in any given state depends on the statistical weight&the Boltzmann factor,exp-E/(kT).2020The specific intensity of a black body is described by the Planck function:第20页，共25页，编辑于2022年，星期五brightness temperaturebrightness temperature2121Rayleigh-Jeans Law(hn n n n kT)kT)(Yuan,Zhou,Komossa+2008,ApJ,685,801)(Zhou,Wang,Wang+2006,ApJ,639,716)第21页，共25页，编辑于2022年，星期五Stefan-Boltzmann LawStefan-Boltzmann LawThe total intensity of a black body can be obtained by integrating the Planck function over n or l:2222Flux from a star:Stefan-Boltzmann Law,s s called Stefan-Boltzmann constant.Estimation of the surface temperature of the sun T using S-B Law:Estimating T without any known physics method of discovering new physics!第22页，共25页，编辑于2022年，星期五energy density&pressure in a star2323Radiation pressure is Radiation pressure is a strong function of temperaturea strong function of temperature.nnegligible for low mass cool stars;negligible for low mass cool stars;n nvery important for high mass hot stars,e.g.WR.very important for high mass hot stars,e.g.WR.第23页，共25页，编辑于2022年，星期五3.8.Grey BodiesnBlack bodies:absorb all incident radiation.nAlbedo:the fraction of light that is reflected.nObjects that do not absorb all incident radiation are called grey bodies.2424It is the reflected light that allows us to see planets,comets,.dust grains.absorbed light that heats.第24页，共25页，编辑于2022年，星期五homework25251.1.An apparent magnitude of a star is V=1An apparent magnitude of a star is V=1mm.95.Determine its flux density.95.Determine its flux density in units of erg sin units of erg s-1-1 cm cm-2-2 A A-1-1 and erg s and erg s-1-1 cm cm-2-2 Hz Hz-1-1.Do the same conversion.Do the same conversion for B=1for B=1mm.95.95.*The effective wavelengths of the V and B bands are*The effective wavelengths of the V and B bands are l l l leffeff(V)=(V)=5450A and 5450A and l l l leffeff(B)=4380A.The zero points are ZP(B)=4380A.The zero points are ZPn n n n(V)=ZP(V)=ZPl l l l(V)=0,(V)=0,ZPZPn n n n(B)=-0(B)=-0mm.120,and ZP.120,and ZPl l l l(B)=-0(B)=-0mm.601.Also useful are the relations.601.Also useful are the relations between flux density and magnitude:mbetween flux density and magnitude:mn n n n=-2.5logf=-2.5logfn n n n-48.598-ZP-48.598-ZPn n n n.and and mml l l l=-2.5logf=-2.5logfl l l l-21.100-ZP-21.100-ZPl l l l.2.2.An MOIII star is measured to have B=8An MOIII star is measured to have B=8mm.76 and V=7.76 and V=7mm.09.Please.09.Please estimate the distance to the star.estimate the distance to the star.*The intrinsic color of MOIII stars is(B-V)*The intrinsic color of MOIII stars is(B-V)0 0=1=1mm.57,and th absolute magnitude is M.57,and th absolute magnitude is MV V=-0=-0mm.2.2.第25页，共25页，编辑于2022年，星期五