14.40 o7 d sullivan
DESCRIPTION
Research 5: D SullivanTRANSCRIPT
The Physics of White Dwarf Stars
Denis J SullivanVictoria University of Wellington
October 18, 2011
White dwarf & pulsating WD brief history
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■ 1915 Astronomers: identify white dwarfs (WDs) as unusual.
White dwarf & pulsating WD brief history
2 / 28
■ 1915 Astronomers: identify white dwarfs (WDs) as unusual.
■ ∼1925 Schrodinger, Heisenberg, Pauli, . . . quantum mechanics (QM)developed.
White dwarf & pulsating WD brief history
2 / 28
■ 1915 Astronomers: identify white dwarfs (WDs) as unusual.
■ ∼1925 Schrodinger, Heisenberg, Pauli, . . . quantum mechanics (QM)developed.
■ 1926 Fowler: uses QM to develop a WD theory –electron degeneracy pressure prevents gravitational collapse.
White dwarf & pulsating WD brief history
2 / 28
■ 1915 Astronomers: identify white dwarfs (WDs) as unusual.
■ ∼1925 Schrodinger, Heisenberg, Pauli, . . . quantum mechanics (QM)developed.
■ 1926 Fowler: uses QM to develop a WD theory –electron degeneracy pressure prevents gravitational collapse.
■ 1932 Chandrasekhar: combines special relativity (SR) with QM toobtain a WD theory that predicts a maximum mass (∼ 1.4M⊙ )Eddington not impressed.
White dwarf & pulsating WD brief history
2 / 28
■ 1915 Astronomers: identify white dwarfs (WDs) as unusual.
■ ∼1925 Schrodinger, Heisenberg, Pauli, . . . quantum mechanics (QM)developed.
■ 1926 Fowler: uses QM to develop a WD theory –electron degeneracy pressure prevents gravitational collapse.
■ 1932 Chandrasekhar: combines special relativity (SR) with QM toobtain a WD theory that predicts a maximum mass (∼ 1.4M⊙ )Eddington not impressed.
■ 1964 Landolt accidentally discovers first pulsating WD (DAV,HLTau 76) – periodic variations ∼ 12.5 minutes in a potential WD fluxstandard (Landolt, ApJ, 1968).
White dwarf & pulsating WD brief history
2 / 28
■ 1915 Astronomers: identify white dwarfs (WDs) as unusual.
■ ∼1925 Schrodinger, Heisenberg, Pauli, . . . quantum mechanics (QM)developed.
■ 1926 Fowler: uses QM to develop a WD theory –electron degeneracy pressure prevents gravitational collapse.
■ 1932 Chandrasekhar: combines special relativity (SR) with QM toobtain a WD theory that predicts a maximum mass (∼ 1.4M⊙ )Eddington not impressed.
■ 1964 Landolt accidentally discovers first pulsating WD (DAV,HLTau 76) – periodic variations ∼ 12.5 minutes in a potential WD fluxstandard (Landolt, ApJ, 1968).
■ 1970+ WD pulsations explained by gravity modes driven bymechanism in partial ionization H atmosphere.Note: more common pressure modes have periods: ∼ seconds
t ∼ t ∼1
√ ∼ seconds for WD
WD History (continued)
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■ 1979 First DOV degenerate pulsator discovered (PG1159−035).(McGraw et al.) – explained by driving mechanism in partial ionized Cand O layers.
WD History (continued)
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■ 1979 First DOV degenerate pulsator discovered (PG1159−035).(McGraw et al.) – explained by driving mechanism in partial ionized Cand O layers.
■ 1982 First helium atmosphere WD pulsator discovered (GD358),following theoretical prediction of pulsation driving in He partialionization zone (Winget et al.)
WD History (continued)
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■ 1979 First DOV degenerate pulsator discovered (PG1159−035).(McGraw et al.) – explained by driving mechanism in partial ionized Cand O layers.
■ 1982 First helium atmosphere WD pulsator discovered (GD358),following theoretical prediction of pulsation driving in He partialionization zone (Winget et al.)
■ 1985 Period change due to secular cooling measured from multi-sitephotometry on PG 1159 (Winget et al. 1985).
WD History (continued)
3 / 28
■ 1979 First DOV degenerate pulsator discovered (PG1159−035).(McGraw et al.) – explained by driving mechanism in partial ionized Cand O layers.
■ 1982 First helium atmosphere WD pulsator discovered (GD358),following theoretical prediction of pulsation driving in He partialionization zone (Winget et al.)
■ 1985 Period change due to secular cooling measured from multi-sitephotometry on PG 1159 (Winget et al. 1985).
■ 1990 WET: the Whole Earth Telescope (Nather et al., ApJ 361)
WD History (continued)
3 / 28
■ 1979 First DOV degenerate pulsator discovered (PG1159−035).(McGraw et al.) – explained by driving mechanism in partial ionized Cand O layers.
■ 1982 First helium atmosphere WD pulsator discovered (GD358),following theoretical prediction of pulsation driving in He partialionization zone (Winget et al.)
■ 1985 Period change due to secular cooling measured from multi-sitephotometry on PG 1159 (Winget et al. 1985).
■ 1990 WET: the Whole Earth Telescope (Nather et al., ApJ 361)
■ 1991 WET observations of PG 1159−035 (Winget et al., ApJ 378)
WD History (continued)
3 / 28
■ 1979 First DOV degenerate pulsator discovered (PG1159−035).(McGraw et al.) – explained by driving mechanism in partial ionized Cand O layers.
■ 1982 First helium atmosphere WD pulsator discovered (GD358),following theoretical prediction of pulsation driving in He partialionization zone (Winget et al.)
■ 1985 Period change due to secular cooling measured from multi-sitephotometry on PG 1159 (Winget et al. 1985).
■ 1990 WET: the Whole Earth Telescope (Nather et al., ApJ 361)
■ 1991 WET observations of PG 1159−035 (Winget et al., ApJ 378)
■ 1994 WET observations of GD 358 (Winget et al., ApJ 430)
WD History (continued)
3 / 28
■ 1979 First DOV degenerate pulsator discovered (PG1159−035).(McGraw et al.) – explained by driving mechanism in partial ionized Cand O layers.
■ 1982 First helium atmosphere WD pulsator discovered (GD358),following theoretical prediction of pulsation driving in He partialionization zone (Winget et al.)
■ 1985 Period change due to secular cooling measured from multi-sitephotometry on PG 1159 (Winget et al. 1985).
■ 1990 WET: the Whole Earth Telescope (Nather et al., ApJ 361)
■ 1991 WET observations of PG 1159−035 (Winget et al., ApJ 378)
■ 1994 WET observations of GD 358 (Winget et al., ApJ 430)
■ WET continues . . . . . .
WD relative size
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Stellar structure equations
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Mechanical structure - P (r), ρ(r),m(r)
dP
dr= −ρ(r)g(r) ; g(r) =
Gm(r)
r2; m(r) =
∫
r
0
ρ(r)4πr2dr
Thermal structure - T (r), L(r), . . .
dT
dr= (· · · ) ;
dL
dr= (· · · )
Common stellar P(r),ρ(r),T(r) profiles
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WD Mechanical Structure
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■ WD support mechanism dominated by electron degeneracy pressure,which is essentially independent of temperature −→ depends on density
WD Mechanical Structure
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■ WD support mechanism dominated by electron degeneracy pressure,which is essentially independent of temperature −→ depends on density
■ Hence in a WD, mechanical structure decoupled from thermal structure
WD Mechanical Structure
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■ WD support mechanism dominated by electron degeneracy pressure,which is essentially independent of temperature −→ depends on density
■ Hence in a WD, mechanical structure decoupled from thermal structure
■ Nonrelativistic (NR) electron gas
n ∝ p3F ; P =1
3vp −→ P ∝ P 5
F −→ P ∝ ρ5
3
■ Extremely relativistic (ER) electron gas
n ∝ p3F ; P =1
3cp −→ P ∝ P 4
F −→ P ∝ ρ4
3
Simple WD mechanical model
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The 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.
d2u
dz2+
2
z
du
dz+
(
u2 −1
x2c + 1
)3
2
= 0
where
u =
(
x2 + 1
x2c + 1
)
1
2
and x =pF(r)
mec
Solve numerically for x(r):
x(r) −→ ρ(r), P (r)
Simple WD mechanical model
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Behaviour of increasingly relativistic particles
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WD density & temperature profiles
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WD Radius vs Mass
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WD Radius vs Mass
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White dwarf spectroscopy (Magellan 6.5m)
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EC20058 (He atm.) and flux standard (H atm.)
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White dwarf time-series photometry
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A WD light curve (MtJohn 1-m)
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WD light curve (Magellan 6.5-m telescope (Chile)
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Two different WD light curves
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A white dwarf with nonsinusoidal pulse shapes
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A white dwarf with nonsinusoidal pulse shapes
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WET (mult-site) time-series light curves
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WET xcov15 DFT, Sullivan et al., MNRAS (2008)
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Nonradial pulsations: spherical harmonics
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Asteroseismology and white dwarf physics
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■ Core chemical composition - stellar nuclear reaction ashes
dominated by 12C and 16O.
■ Core crystallization
■ Convection zone studies
■ Neutrino cooling mechanism
White dwarf cooling models - neutrino cooling
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Plasmon neutrino processes − Feynman diagrams
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Neutrino physics in hot WD plasmas
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■ Basically, neutrinos produced by e− e+ annihilation
■ But where do the positrons come from?
■ Even at WD core temperatures, not enough energy forreal e−,e+ pairs
■ However, plenty of short duration (real) virtual e−,e+ pairs createdcourtesy energy-time uncertainty principle
■ But, these pairs recombine with probability 0.99999 . . .
■ However, this probability is not 1, and there is a ∼ 1 in 10−19 chance offorming neutrino-antineutrino pairs via W±, Z0 exchange/creationprocesses (the electroweak connection).
■ Given the ν mass is ∼ zero, energy conservation permits formation of atwo ν final state from a ∼ KeV photon, but momentum conservationrequires more than a photon in initial state
■ Possible other particles: nuclei, many particles −→ plasmons (this is thedominant mechanism)