Presentation on theme: "The Fate of Massive Stars"— Presentation transcript:
1The Fate of Massive Stars Core-Collapse SupernovaeGamma Ray BurstsCosmic Rays
2Midterm 2 Computer problem Choose m=2.0 instead of m=3.0 if you like on problem 6.0
3Core Collapse Supernova Mechanism Start here Extreme conditionsTc~8 x 109 KDensity ~ 1013 kg/m3Electrons are captured by nuclei and protons produced via photo-disintegration…Electron Degeneracy pressure greatly reducedEnergy loss by neutrinos produced in reactions such asNeutrino luminosity ~ 3.1 x W (photon luminosity ~ 4.4 x 1031 W for 20 M)Most of the core’s support in the form of electron degeneracy pressure is suddenly gone….the “floor drops out”!!!Core begins to collapse extremely rapidlyIn the inner portion of the core the collpase is homologous and the velocity of the collapse is proportional to the distance away from the centerAt the radius where the velocity excceeds the local sound speed the inner core decouples from the now supersonic outer coreThe outer core is left behind and nearly in free-fallSpeeds can reach almost 70,000 km/s in the outer coreWithin about one second a volume about the size of the earth has collapsed down to a radius of 50 km!!!!
4Core Collapse Supernova Mechanism Homologous collapse of inner core continues until density exceeds about 8 x kg/m3 (roughly 3 times the density of an tomic nucleus !!!!)The nuclear material that makes up the inner core stiffens because the strong force (normally atractive) suddenly becomes “repulsive” as a a consequence of the Pauli exclusion principle.The inner core rebounds as a result sending pressure waves outward into the infalling material from the outer core. Shock waves move outward when the wave velocity exceeds the sound speedAs the shock wave encounters the outer falling iron core. The high temperatures induce further photo-disintegration robbing the shock of most of its energy (for every 0.1 M of Iron --> shock loses 1.7 x J.Shock stalls becoming nearly stationary (accretion shock)A neutrinosphere develops below the shock. Neutrinos produced from photo-disintegration and electron captureOverlying material is so dense that not even neutrinos can easily escape---> Neutrino heating just behind the shock!!!This allows the shock to resume its march towards the surface….If this does not happen quickly enough the initially outflowing material will fall back to the core …no explosion!!!Complicated modeling business….Assuming this model is correct..The shock will drive the envelope and the remaining nuclear processed material in front of it…The total kinetic energy in the expanding material is ~ 1044 JWhen the material becomes optically thin at a radius of about 100AU …a tremendous optical display results!!!!…1042 J in photonsA peak luminosity of 1036 W (109 L)
8Stellar Remnants of a Core-Collpase Supernova Neutron Star:Initial ZAMS mass < 25 MSupported by neutron degeneracy pressureBlack-holeInitial ZAMS mass > 25 MNeutron degeneracy pressure is not sufficient
9Light Curves and the Radioactive Decay of the Ejecta Light curve is partially explained by radioactive decay of radioactive isotopes produced during supernova explosion!!!!
11The Detection of Neutrinos from SN 1987A Burst of Neutrinos detected at several neutrino detectors on FebTime span of about 12.5 seconds about 3 hours before the arrival of photonsConfirmation of core collapse model of supernovae!!!!Upper limit on neutrino mass of 16eVStill searching for other signs of compact object
12Chemical Abundance Ratio of the Universe Supernovae explosions deposit vast quantities of material that has been “cooked” in the “bowels” of stars…Can check stellar models against observed abundance of elementsCore collapse supernovae are responsible for the significant quantities of Oxygen in the universeProduction of heavier elements occur via r-process nucleosynthesis during supernova events!!!!
13S-process and r-process nucleosynthesis Easier for neutrons to react with high-Z nuclei due to lack of Coulomb repulsion nuclear reactions with neutrons can proceed at low temperatures Producing more massive nuclei that are either stable or unstable against the beta decay reactionIf beta decay half-life is short compared to timescale for neutron captureThis reaction process is called s-process reaction (for slow process)If beta decay half-life is LONG compared to timescale for neutron captureThis results in neutron-rich nuclei through the r-process…need a supernova !!!
14Origin of the elementsNeed Supenovae for the production of elements beyond iron
15Gamma Ray BurstsFirst observed by Vela military satellites in the 1960’s. These satellites were looking for gamma rays produced in terrestrial thermonuclear explosions!!!By 1967 it was clear that bursts of gamma rays were being produced from above!!!About once per day…durations from 10 ms to 1000sWhere are they from?
23Gamma Ray Bursts (long duration) Peculiar supernova explosions of massive stars.Generated by a central engine that is likely to be a newborn black hole at the heart of the dying starSupernovae are all of spectral type Ibccore-collapse supernovae
25Cosmic RaysCosmic rays of energies up to about 1015 eV are believed to be produced in Supernova explosionsFermi-theory of shock accelerationMagnetic confinement of charged particles …Larmor Radius
26The Degenerate Remnants of Massive Stars I The Discovery of Sirius BWhite DwarfsThe Physics of Degenerate Matter
27The Discovery of Sirius B Bessel noted that the path of Sirius did not follow a straight line through space…He concluded that Sirius must be in a Binary star system with an orbital period of 50 yearsAlvin Clark later “discovered” Sirius B using his father’s new 18 inch refractor in 1862PropertiesLA=23.5 L LB=0.03 LTA=9910K TB=27,000KMA=2.3 L MB=1.05 LTemperature and Luminosity of BRB=0.008 RSirius B is smaller than Earth with the mass of the Sun!!!!Average density ~ 3.0 x 109 kg/m3Surface gravity ~ 4.6 x 106 m/s2
28White DwarfsClass of Star that has approximately the mass of the Sun in a size of the EarthActually come in all colors Temperatures range from less than 5000K to more than 80,000KComplete sample difficult to obtainOccupy a region of the H-R diagram that is roughly parallel to and below the main-sequence
30Classes of White Dwarfs DA White Dwarfs:Pressure broadened Hydrogen absorption lines in spectrumLargest group. About 2/3.DB White Dwarfs:Hydrogen lines absentHelium absorption linesAbout 8% of sampleDC White Dwarfs:No lines. Only continuum devoid of featuresAbout 14%DQ White Dwarfs:Carbon features in spectraDZ White Dwarfs:Evidence of metal lines
31Central Conditions in White Dwarfs The birth of White Dwarfs Central PressureAbout 1 million x > SunCentral TemperatureWhite Dwarfs are “manufactured” in the cores of low-intermediate mass (< 8-9 M) near the ends of their lives on the Asymptotic Giant Branch.Most white dwarfs consist completely of ionized carbon and oxygen nuclei, because any star with a helium core mass M>0.5 M will undergo fusion.As the aging giant star expels its surface layers, the core is exposed as a white dwarf progenitorHydrogen not present in appreciable amounts below the surface layers of white dwarf.Material at center must be incapable of fusion at these densities and temperatures
32Spectra and Surface Composition Exceptionally strong pull of the white Dwarf’s gravityPull heavier nuclei below the surfaceVertical Stratification of Nuclei (in about 100 years!!)A thin layer of hydrogen rests on a layer of helium on top of a carbon-oxygen core.Explains hydrogen line spectrum of DA white dwarfsNon-DA white dwarfs possibly explained by eitherNo hydrogen left to form a surface layerConvective mixing between helium and hydrogen layers
33Pulsating White Dwarfs White Dwarfs of T~12,000K lie within the instability strip of the H-R diagram.Pulsate with periods between 100 and 1000 s.ZZ Ceti variable stars. Also known as DAV or variable DA white dwarfsThe pulsation periods correspond to non-radial g-modes that resonate within the white dwarf’s surface layers of hydrogen and helium.Horizontal displacements --> little change in radius.Brightness variations (few tenths of magnitude) due to temperature variationSuccessful modeling of pulsating white dwarfs . Predicted DBV stars….Hydrogen partial ionization zone DAVHelium partial ionization zone DB
35Electron Degeneracy Pressure The Physics of Degenerate Matter The Pauli Exclusion Principle and Electron DegeneracyWhat can support a white dwarf against the relentless pull of its own gravity???Electron Degeneracy PressurePauli Exclusion Principle allows at most one fermion in each quantum state.In a gas at STP only one of every 107 quantum states is occupied. Pauli Exclusion Principle is insignificant in this case.What happens when T--> 0K???Energy Removed from systemParticles are forced into lower energy statesOnly one Fermion allowed in each quantum stateAll of the Particles can not crowd into ground stateFermions will fill up the lowest available unoccupied state
36The Fermi Energy Even at T=0K there are fermions in excited states. The motion of these fermionsproduces a Pressure.At T=0K all of the lower energy states and none of the higher energy states are occupiedSuch a Fermion gas is said to be completely degenerateThe maximum energy of any electron in a completely degenerate gas at T=0K is known as the Fermi Energy.
43Electron Degeneracy Pressure Estimate The Electron Degeneracy Pressure by using two key ideasThe Pauli Exclusion PrincipleHeisenberg’s Uncertainty PrincipleMaking the unrealistic assumption that all of the electrons have the same momentumIn a completely degenerate electron gas, the electrons are packed as tightly as possibleFor a uniform number density the separation between neighboring electrons is ne-1/3Pauli exclusion principleElectrons must maintain their identity as separate particles. That means the uncertainty in their positions can not be larger than their separation, I.e