Nuclear Reactions and Solar Neutrinos ASTR 2110 Sarazin. Davis Solar Neutrino Experiment

Transcription

1 Nuclear Reactions and Solar Neutrinos ASTR 2110 Sarazin Davis Solar Neutrino Experiment

2 Hydrogen Burning Want 4H = 4p è 4 He = (2p,2n) p è n, only by weak interaction Much slower than pure fusion protons Deuterium (pn) lifetime pep alternative Only low Z elements, works at low T ε ~ T 4

3 Hydrogen Burning C only catalyst net result 4 H è He Time scales for late A star

4 Helium Burning Triple Alpha

5 Helium Burning Triple Alpha First step endothermic è reversed unless another He hits first Requires high T ~ 10 8 K, high density

6 Carbon Burning, Etc. Explains why elements beyond Fe are very rare, can t be made by exothermic fusion reactions Explains why most common elements beyond H have Z = N = even = multiples of He

7 Fe

8 Carbon Burning, Etc. Explains why elements beyond Fe are very rare, can t be made by exothermic fusion reactions Explains why most common elements beyond H have Z = N = even = multiples of He

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10 Fusion Reactions log ε (erg/s/gm) Sun CNO pp 3α C burning log T (K) 8

11 Thermal Stability of Stars Fusion: Makes lots of energy Very temperature sensitive } explosives hotter BANG more energy

12 Thermal Stability of Stars Why don t stars explode? Stars have negative heat capacity!

13 Energies is Stars PE = 2 TE E = TE TE TE 0 E E PE PE Stars have negative heat capacity If you add energy (heat), they get cooler!!

14 Thermal Stability of Stars Why don t stars explode? Stars have negative heat capacity! Add energy, get cooler Add energy, pressure increases, gas expands, gas gets cooler Cooler è less fusion è less energy Fusion in stars è perfect thermostat! Stars are incredibly stable!

15 Exceptions: Thermal Stability of Stars 1. Virial theorem applies to whole of star Parts of star exchange energy è pulsations 2. Thermal gas pressure only KE = E KE = (3/2) PV = (3/2) NkT V only if P = P gas Degeneracy pressure, KE = (3/2) P deg V, but P deg = func. of density only, not temperature Fusion in degenerate regions è explosion

16 Fusion The Key to the Stars

17 Fusion The Key to the Stars Make stellar energy, provide source of light Make all heavy elements (C, N, O, Fe, ) Make stars stable (or unstable) Change composition of stars they evolve Set lifetime of stars, t = E nucl / L

18 Tests of Stellar Structure Theory ASTR 2120 Sarazin Davis Solar Neutrino Experiment

19 Tests of Stellar Structure Theory At some level, all of astronomy (rest of this course) Critical tests? Sun is good since it is close

20 Solar Oscillations Helio-seismology P ~ (G ρ ) -1/2 But, many normal modes of oscillation Can also be done for other stars

21 Solar Oscillations One normal made of Sun (red and blue shifts)

22 Solar Oscillations Some modes probe surface, some deep interior

23 Solar Oscillations Spectrum of frequencies of oscillation

24 Solar Neutrino Experiments Test idea: Fusion powers stars Cannot see into Solar core with light, but neutrinos should escape Unfortunately, main neutrinos made by pp reaction are very low energy, hard to detect Detect side reactions

25 Solar Neutrino Experiments 14 Mev!

26 Solar Neutrino Experiments

27 Solar Neutrino Experiments

28 Davis Experiment Mainly detect 8 B ν s ν + 37 Cl 37 Ar + e - Normal chlorine - use 100,000 gal of C 2 Cl 4 (dry-cleaning fluid) Argon inert, bubble out of tank 37 Ar decays (beta decay), detect every single atom! Homestead Gold Mine, South Dakota

29 Davis Experiment

30 Davis Experiment

31 Davis Experiment

32 Davis Experiment Predict ~ 5.6 SNU (solar neutrino units) Observe only 1.8 ± 0.3 SNUs ~ 1/3 expected What is wrong? 1. Experiment wrong - No 2. Nuclear reaction rates wrong - No 3. Astronomy wrong (solar model incorrect) - No 4. Physics wrong - Yes! Neutrinos are NOT mass-less and stable

33 Neutrino Oscillations Three flavors of neutrinos ν e ν µ ν τ

34 Neutrino Oscillations

35 Sudbury Experiment

36 Super-Kamiokande Experiment

37 Neutrino Oscillations Three flavors of neutrinos ν e Yes! ν µ ν τ

38 Nobel Prize 2002

39 Stellar Models ASTR 2110 Sarazin

40 Five Equations dp dr = GM(r)ρ r 2 Hydrostatic equilibrium dm dr = 4πr2 ρ Mass dl dr = 4πr2 ερ Thermal equilibirum dt dr = min " $ # $ % $ 3κρ 1 64πσ r 2 T L(r) T P dp dr radiation convection P = ρkt µm p + P rad + P deg Equation of state Energy transport

41 Boundary Conditions Take radius r as independent variable P(r), T(r), ρ(r), L(r), M(r) 0 r R * Boundary Conditions: Center (r = 0): M(0) = 0, L(0) = 0 Surface (r = R * ): M(R * ) = M *, P(R * ) = 0, ρ(r * ) = 0 Two Problems: 4 differential equations, 5 boundary conditions What is R *?

42 Boundary Conditions Change independent variable to mass M r(m ), P(M ), T(M ), ρ(m ), L(M ), 0 M M * dm dr = dr 4πr2 ρ dm = 1 4πr 2 ρ dx dm = dx dr dr dm

43 Five (New) Equations dr dm = 1 4πr 2 ρ radius (formerly mass) dp dm = GM(r) Hydrostatic equilibrium 4πr 4 dl dm = ε Thermal equilibirum dt dm = min " $ # $ % $ 3κ 1 256π 2 σ r 4 T L(r) T P dp dm P = ρkt µm p + P rad + P deg Equation of state radiation convection Energy transport

44 Boundary Conditions Center (M = 0): r(0) = 0, L(0) = 0 Surface (M = M * ): P(M * ) = 0, ρ(m * ) = 0 Better! 4 differential equations 4 boundary conditions R * is derived

45 Vogt-Russell Theorem Necessary inputs: Mass of star M * Mean particle mass μ, opacity κ, energy production ε Depend on ρ, T, composition Vogt-Russell Theorem: Star is uniquely determined by mass M * & composition

46 Solar Model ρ c = 147 gm/cm 3 T c = 15 million K Burning hydrogen in core ~ 20% of radius Radiative zone ~ inner 85% of radius Convective zone ~ outer 15% of radius ρ (gm/cm 3 ) T (10 6 K)

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48 Main Sequence Choice of composition: Try mainly hydrogen and helium Surface of Sun and most other stars Interstellar gas è what stars form from Vary mass M * Reproduce main sequence (normal stars) All burning H in their cores Main Sequence = stars made of hydrogen burning H in their cores

49 Main Sequence high mass mass low mass

50 Hydrogen: Most abundant element Best nuclear fuel Burned first Main Sequence Stars spend most of life on main sequence, only leave it when they start to die