Research 5: D Sullivan

1. The Physics of White Dwarf Stars
Denis J Sullivan
Victoria University of Wellington
October 18, 2011

2. White dwarf & pulsating WD brief history
s 1915 Astronomers: identify white dwarfs (WDs) as unusual.
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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.
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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.
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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.
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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).
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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
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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.
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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.)
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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).
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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)
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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)
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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)
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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 . . . . . .
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15. WD relative size
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16. Stellar structure equations
Mechanical 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 0
Thermal structure - T (r), L(r), . . .
dT dL
= (· · · ) ; = (· · · )
dr dr
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17. Common stellar P(r),ρ(r),T(r) profiles
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18. WD Mechanical Structure
s WD support mechanism dominated by electron degeneracy pressure,
which is essentially independent of temperature −→ depends on density
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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
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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
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21. Simple WD mechanical model
The following relatively simple differential equation describing x(r)
(which is the electron momentum at the [local] fermi surface) quite accurately
characterises the density and pressure profiles of WDs.
3
d2 u 2 du 1 2
+ + u2 − 2 =0
dz 2 z dz xc + 1
where
1
x2 +1 2 pF (r)
u= and x=
x2 + 1
c me c
Solve numerically for x(r):
x(r) −→ ρ(r), P (r)
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22. Simple WD mechanical model
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23. Behaviour of increasingly relativistic particles
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24. WD density & temperature profiles
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25. WD Radius vs Mass
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26. WD Radius vs Mass
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27. White dwarf spectroscopy (Magellan 6.5m)
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28. EC 20058 (He atm.) and flux standard (H atm.)
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29. White dwarf time-series photometry
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30. A WD light curve (MtJohn 1-m)
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31. WD light curve (Magellan 6.5-m telescope (Chile)
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32. Two different WD light curves
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33. A white dwarf with nonsinusoidal pulse shapes
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34. A white dwarf with nonsinusoidal pulse shapes
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35. WET (mult-site) time-series light curves
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36. WET xcov15 DFT, Sullivan et al., MNRAS (2008)
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37. Nonradial pulsations: spherical harmonics
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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
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39. White dwarf cooling models - neutrino cooling
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40. Plasmon neutrino processes − Feynman diagrams
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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)
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