14.40 o7 d sullivan

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Research 5: D Sullivan

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14.40 o7 d sullivan

  1. 1. The Physics of White Dwarf Stars Denis J Sullivan Victoria University of Wellington October 18, 2011
  2. 2. White dwarf & pulsating WD brief history s 1915 Astronomers: identify white dwarfs (WDs) as unusual. 2 / 28
  3. 3. White dwarf & pulsating WD brief history s 1915 Astronomers: identify white dwarfs (WDs) as unusual. s ∼1925 Schr¨dinger, Heisenberg, Pauli, . . . quantum mechanics (QM) o developed. 2 / 28
  4. 4. White dwarf & pulsating WD brief history s 1915 Astronomers: identify white dwarfs (WDs) as unusual. s ∼1925 Schr¨dinger, Heisenberg, Pauli, . . . quantum mechanics (QM) o developed. s 1926 Fowler: uses QM to develop a WD theory – electron degeneracy pressure prevents gravitational collapse. 2 / 28
  5. 5. White dwarf & pulsating WD brief history s 1915 Astronomers: identify white dwarfs (WDs) as unusual. s ∼1925 Schr¨dinger, Heisenberg, Pauli, . . . quantum mechanics (QM) o developed. s 1926 Fowler: uses QM to develop a WD theory – electron degeneracy pressure prevents gravitational collapse. s 1932 Chandrasekhar: combines special relativity (SR) with QM to obtain a WD theory that predicts a maximum mass (∼ 1.4M ) Eddington not impressed. 2 / 28
  6. 6. White dwarf & pulsating WD brief history s 1915 Astronomers: identify white dwarfs (WDs) as unusual. s ∼1925 Schr¨dinger, Heisenberg, Pauli, . . . quantum mechanics (QM) o developed. s 1926 Fowler: uses QM to develop a WD theory – electron degeneracy pressure prevents gravitational collapse. s 1932 Chandrasekhar: combines special relativity (SR) with QM to obtain a WD theory that predicts a maximum mass (∼ 1.4M ) Eddington not impressed. s 1964 Landolt accidentally discovers first pulsating WD (DAV, HL Tau 76) – periodic variations ∼ 12.5 minutes in a potential WD flux standard (Landolt, ApJ, 1968). 2 / 28
  7. 7. White dwarf & pulsating WD brief history s 1915 Astronomers: identify white dwarfs (WDs) as unusual. s ∼1925 Schr¨dinger, Heisenberg, Pauli, . . . quantum mechanics (QM) o developed. s 1926 Fowler: uses QM to develop a WD theory – electron degeneracy pressure prevents gravitational collapse. s 1932 Chandrasekhar: combines special relativity (SR) with QM to obtain a WD theory that predicts a maximum mass (∼ 1.4M ) Eddington not impressed. s 1964 Landolt accidentally discovers first pulsating WD (DAV, HL Tau 76) – periodic variations ∼ 12.5 minutes in a potential WD flux standard (Landolt, ApJ, 1968). s 1970+ WD pulsations explained by gravity modes driven by mechanism in partial ionization H atmosphere. Note: more common pressure modes have periods: ∼ seconds 2 / 28
  8. 8. WD History (continued) s 1979 First DOV degenerate pulsator discovered (PG 1159−035). (McGraw et al.) – explained by driving mechanism in partial ionized C and O layers. 3 / 28
  9. 9. WD History (continued) s 1979 First DOV degenerate pulsator discovered (PG 1159−035). (McGraw et al.) – explained by driving mechanism in partial ionized C and O layers. s 1982 First helium atmosphere WD pulsator discovered (GD 358), following theoretical prediction of pulsation driving in He partial ionization zone (Winget et al.) 3 / 28
  10. 10. WD History (continued) s 1979 First DOV degenerate pulsator discovered (PG 1159−035). (McGraw et al.) – explained by driving mechanism in partial ionized C and O layers. s 1982 First helium atmosphere WD pulsator discovered (GD 358), following theoretical prediction of pulsation driving in He partial ionization zone (Winget et al.) s 1985 Period change due to secular cooling measured from multi-site photometry on PG 1159 (Winget et al. 1985). 3 / 28
  11. 11. WD History (continued) s 1979 First DOV degenerate pulsator discovered (PG 1159−035). (McGraw et al.) – explained by driving mechanism in partial ionized C and O layers. s 1982 First helium atmosphere WD pulsator discovered (GD 358), following theoretical prediction of pulsation driving in He partial ionization zone (Winget et al.) s 1985 Period change due to secular cooling measured from multi-site photometry on PG 1159 (Winget et al. 1985). s 1990 WET: the Whole Earth Telescope (Nather et al., ApJ 361) 3 / 28
  12. 12. WD History (continued) s 1979 First DOV degenerate pulsator discovered (PG 1159−035). (McGraw et al.) – explained by driving mechanism in partial ionized C and O layers. s 1982 First helium atmosphere WD pulsator discovered (GD 358), following theoretical prediction of pulsation driving in He partial ionization zone (Winget et al.) s 1985 Period change due to secular cooling measured from multi-site photometry on PG 1159 (Winget et al. 1985). s 1990 WET: the Whole Earth Telescope (Nather et al., ApJ 361) s 1991 WET observations of PG 1159−035 (Winget et al., ApJ 378) 3 / 28
  13. 13. WD History (continued) s 1979 First DOV degenerate pulsator discovered (PG 1159−035). (McGraw et al.) – explained by driving mechanism in partial ionized C and O layers. s 1982 First helium atmosphere WD pulsator discovered (GD 358), following theoretical prediction of pulsation driving in He partial ionization zone (Winget et al.) s 1985 Period change due to secular cooling measured from multi-site photometry on PG 1159 (Winget et al. 1985). s 1990 WET: the Whole Earth Telescope (Nather et al., ApJ 361) s 1991 WET observations of PG 1159−035 (Winget et al., ApJ 378) s 1994 WET observations of GD 358 (Winget et al., ApJ 430) 3 / 28
  14. 14. WD History (continued) s 1979 First DOV degenerate pulsator discovered (PG 1159−035). (McGraw et al.) – explained by driving mechanism in partial ionized C and O layers. s 1982 First helium atmosphere WD pulsator discovered (GD 358), following theoretical prediction of pulsation driving in He partial ionization zone (Winget et al.) s 1985 Period change due to secular cooling measured from multi-site photometry on PG 1159 (Winget et al. 1985). s 1990 WET: the Whole Earth Telescope (Nather et al., ApJ 361) s 1991 WET observations of PG 1159−035 (Winget et al., ApJ 378) s 1994 WET observations of GD 358 (Winget et al., ApJ 430) s WET continues . . . . . . 3 / 28
  15. 15. WD relative size 4 / 28
  16. 16. Stellar structure equationsMechanical structure - P (r), ρ(r), m(r) r dP Gm(r) = −ρ(r)g(r) ; g(r) = ; m(r) = ρ(r)4πr2 dr dr r2 0Thermal structure - T (r), L(r), . . . dT dL = (· · · ) ; = (· · · ) dr dr 5 / 28
  17. 17. Common stellar P(r),ρ(r),T(r) profiles 6 / 28
  18. 18. WD Mechanical Structure s WD support mechanism dominated by electron degeneracy pressure, which is essentially independent of temperature −→ depends on density 7 / 28
  19. 19. WD Mechanical Structure s WD support mechanism dominated by electron degeneracy pressure, which is essentially independent of temperature −→ depends on density s Hence in a WD, mechanical structure decoupled from thermal structure 7 / 28
  20. 20. WD Mechanical Structure s WD support mechanism dominated by electron degeneracy pressure, which is essentially independent of temperature −→ depends on density s Hence in a WD, mechanical structure decoupled from thermal structure s Nonrelativistic (NR) electron gas 3 1 5 5 n∝ pF ; P = vp −→ P ∝ PF −→ P ∝ ρ 3 3 s Extremely relativistic (ER) electron gas 1 4 n∝ p3 F ; 4 P = cp −→ P ∝ PF −→ P ∝ ρ 3 3 7 / 28
  21. 21. Simple WD mechanical modelThe following relatively simple differential equation describing x(r)(which is the electron momentum at the [local] fermi surface) quite accuratelycharacterises the density and pressure profiles of WDs. 3 d2 u 2 du 1 2 + + u2 − 2 =0 dz 2 z dz xc + 1where 1 x2 +1 2 pF (r) u= and x= x2 + 1 c me cSolve numerically for x(r): x(r) −→ ρ(r), P (r) 8 / 28
  22. 22. Simple WD mechanical model 9 / 28
  23. 23. Behaviour of increasingly relativistic particles 10 / 28
  24. 24. WD density & temperature profiles 11 / 28
  25. 25. WD Radius vs Mass 12 / 28
  26. 26. WD Radius vs Mass 13 / 28
  27. 27. White dwarf spectroscopy (Magellan 6.5m) 14 / 28
  28. 28. EC 20058 (He atm.) and flux standard (H atm.) 15 / 28
  29. 29. White dwarf time-series photometry 16 / 28
  30. 30. A WD light curve (MtJohn 1-m) 17 / 28
  31. 31. WD light curve (Magellan 6.5-m telescope (Chile) 18 / 28
  32. 32. Two different WD light curves 19 / 28
  33. 33. A white dwarf with nonsinusoidal pulse shapes 20 / 28
  34. 34. A white dwarf with nonsinusoidal pulse shapes 21 / 28
  35. 35. WET (mult-site) time-series light curves 22 / 28
  36. 36. WET xcov15 DFT, Sullivan et al., MNRAS (2008) 23 / 28
  37. 37. Nonradial pulsations: spherical harmonics 24 / 28
  38. 38. Asteroseismology and white dwarf physics s Core chemical composition - stellar nuclear reaction ashes dominated by 12 C and 16 O. s Core crystallization s Convection zone studies s Neutrino cooling mechanism 25 / 28
  39. 39. White dwarf cooling models - neutrino cooling 26 / 28
  40. 40. Plasmon neutrino processes − Feynman diagrams 27 / 28
  41. 41. Neutrino physics in hot WD plasmas s Basically, neutrinos produced by e− e+ annihilation s But where do the positrons come from? s Even at WD core temperatures, not enough energy for real e− ,e+ pairs s However, plenty of short duration (real) virtual e− ,e+ pairs created courtesy energy-time uncertainty principle s But, these pairs recombine with probability 0.99999 . . . s However, this probability is not 1, and there is a ∼ 1 in 10−19 chance of forming neutrino-antineutrino pairs via W± , Z0 exchange/creation processes (the electroweak connection). s Given the ν mass is ∼ zero, energy conservation permits formation of a two ν final state from a ∼ KeV photon, but momentum conservation requires more than a photon in initial state s Possible other particles: nuclei, many particles −→ plasmons (this is the dominant mechanism) 28 / 28

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