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Structure of the Sun

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Structure of the Sun HELIOSEISMOLOGY (I) STRUCTURE OF THE SUN ² The Sun is the only star for which we can measure internal properties ! test of stellar structure theory ² acoustic mode Composition (heavy elements) from meteorites in the Sun ² (p mode Density, internal rotation from helioseismology n=14, l -20) ² Central conditions from neutrinos HELIOSEISMOLOGY ² The Sun acts as a resonant cavity, oscillating in mil- lions of (acoustic, gravity) modes (like a bell) ! can be used to reconstruct the internal density struc- ture (like earthquakes on Earth) full-disk ² oscillation modes are excited by convective eddies Dopplergram ² periods of typical modes: 1.5 min to 20 min ² velocity amplitudes: » 0:1 m/s ² need to measure Doppler shifts in spectral lines rela- tive to their width to an accuracy of 1:106 . possible with good spectrometers and long integra- tion times (to average out noise) Results ² density structure, sound speed ² depth of outer convective zone: » 0:28 R¯ velocity (km/s) ² rotation in the core is slow (almost like a solid-body) ! the core must have been spun-down with the enve- lope (e±cient core{envelope coupling) SOLAR NEUTRINOS ² Neutrinos, generated in solar core, escape from the Sun and carry away 2 ¡ 6 % of the energy released in H-burning reactions HELIOSEISMOLOGY (II) ² they can be observed in underground experiments ! direct probe of the solar core ² neutrino-emitting reactions (in the pp chains) 1 1 2 + max + ! + + = : H H D e E 0 42 Mev 7 ¡ 7 max + ! + = : Be e Li E 0 86 Mev 8 8 + max ! + + = : B Be e E 14 0 Mev ² The Davis experiment (starting around 1970) has shown that the neutrino flux is about a factor of 3 lower than predicted ! the solar neutrino problem The Homestake experiment (Davis) ² neutrino detector: underground tank ¯lled with 600 tons of Chlorine (C2 Cl4 : dry-cleaning fluid) ² some neutrinos react with Cl 37 37 ¡ e + Cl ! Ar + e ¡ 0:81 Mev ² rate of absorption » 3 £ 10¡35 s¡1 per 37Cl atom ² every 2 months each 37Ar atom is ¯ltered out of the rotation rate (nHz) tank (expected number: 54; observed number: 17) ² caveats convection zone . di±cult experiment, only a tiny number of the neu- r/R trinos can be detected . the experiment is only sensitive to the most en- ergetic neutrinos in the 8B reaction (only minor reaction in the Sun) The Davis Neutrino Experiment Proposed Solutions to the Solar Neutrino Problem ² dozens of solutions have been proposed Model Predictions 1) Astrophysical solutions . require a reduction in central temperature of about 5 % (standard model: 15:6 £ 106 K) . can be achieved if the solar core is mixed (due to convection, rotational mixing, etc.) . if there are no nuclear reactions in the centre (in- ert core: e.g. central black hole, iron core, degen- erate core) . if there are additional energy transport mecha- Homestake Mine nisms (e.g. by WIMPS = weakly interacting par- (with Cl tank) ticles) . most of these astrophysical solutions also change the density structure in the Sun ! can now be ruled out by helioseismology 2) Nuclear physics . errors in nuclear cross sections (cross sections sometimes need to be revised by factors up to » 100) . improved experiments have con¯rmed the nuclear cross sections for the key nuclear reactions Results 3) Particle physics Solar Neutrinos . all neutrinos generated in the Sun are electron neu- 12 10 trinos 1011 Solar Neutrino Spectrum pp Bahcall-Pinsonneault SSM 1010 . if neutrinos have a small mass (actually mass dif- 7Be 109 13 ferences), neutrinos may change type on their path N 108 /s/MeV) 15 between the centre of the Sun and Earth: 2 O 107 pep 17 neutrino oscillations, i.e. change from electron 106 F /s or /cm 2 5 7 ¡ neutrino to or neutrinos, and then cannot be 10 Be 8B 104 detected by the Davis experiment Flux (/cm 103 . vacuum oscillations: occur in vacuum hep 102 1 . 10 matter oscillations (MSW [Mikheyev-Smirnov-- 0.1 1.0 10.0 Neutrino Energy (MeV) Wolfenstein] e®ect): occur only in matter (i.e. as neutrinos pass through the Sun) 100 SNU RECENT EXPERIMENTS << Experiments >> SAGE GALLEX Combined ² resolution of the neutrino puzzle requires more sensi- << SSM >> SSM Bahcall-Pinsonneault SSM tive detectors that can also detect neutrinos from the Turck-Chieze-Lopes SSM << NonSSM >> main pp-reaction S17 = Kam 18 TC = Kam 18 TC = Kam (S17 = 1.3) S34 = Kam 1) The Kamiokande experiment Dar-Shaviv Model Low Opacity (-20%) Large S11 (Kam = 0.39) (also super-Kamiokande) Large S11 (Kam = 0.57) Mixing Model (Kam = 0.44) Mixing Model (Kam = 0.53) MSW (Kam+Cl) I . uses 3000 tons of ultra-pure water (680 tons active 0 20 40 60 80 100 120 140 medium) for Gallium Rate (SNU) ¡ ¡ + e ! + e (inelastic scattering) . about six times more likely for e than and ¡ . observed flux: half the predicted flux (energy de- pendence of neutrino interactions?) The Sudbury Neutrino Observatory 2) The Gallium experiments (GALLEX, SAGE) . uses Gallium to measure low-energy pp neutrinos 1000 tons of heavy water directly 71 71 ¡ e + Ga ! Ge + e ¡ 0:23 Mev . results: about 80 § 10 SNU vs. predicted 132 § 7 SNU (1 SNU: 10¡36 interactions per target atom/s) 3) The Sudbury Neutrino Observatory (SNO) . located in a deep mine (2070 m underground) . 1000 tons of pure, heavy water (D2O) . in acrylic plastic vessel with 9456 light sensors/photo- multiplier tubes . detect Cerenkov radiation of electrons and pho- tons from weak interactions and neutrino-electron scattering . results (June 2001): con¯rmation of neutrino os- cillations (MSW e®ect)?.
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