pierre bergeron - los alamos national laboratory
TRANSCRIPT
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White Dwarf Atmospheres
Pierre BergeronUniversité de Montréal
Sirius B
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What is a stellar atmosphere?
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What is a stellar atmosphere?
Atmosphere
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Input parameters
• Effective temperature (Teff)
• Surface gravity (log g)
• Atmospheric composition (NHe/NH, Z)
• Rotation
• Magnetic field
• Mass loss
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« If the Sun did not have a magnetic field, it would be as boring a star asmost astronomers believe it to be »
(attributed to R.B. Leighton)
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center surface
Basic structure
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center surface
Basic structure
Gilles
Fontaine
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Photosphere
H conv
He conv
P/g 270 km for the Sun
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What are white dwarfs showing us ?
80% are DA stars 20% are «non-DA stars»
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DB
DA
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DB
DA
Gravitational settling
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Magnetic White Dwarfs
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Basic Atmospheric Equations
H : Radiation (Eddington) Flux
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Opacity calculations cross-section + population calculations
bound-bound transition
bound-free transition
free-free transition
scattering
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Population calculations
• Local Thermal Equilibrium (LTE) Saha / Boltzmann
• Non-local Thermal Equilibrium (NLTE) Statistical equilibrium equation
• Equation of state (ideal gas, but not always…)
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H b-f
H f-f
H–
H+2 f-f
e-scat
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+ hydrogen lines
(Stark broadening)
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H–
H+2 f-f
H b-f
H2– f-f
H f-f
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H– b-f / f-f
H2– f-f
H2 collision
induced opacity
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Temperature structure Emergent (Eddington) flux
Boundary conditions for stellar interior models
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Measuring atmospheric parameters using model atmospheres :Spectroscopic technique
NLTE
convective
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Flux calibration is difficult…
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Flux calibration is difficult…
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Balmer line profiles are sensitive to both Teff and log g
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Balmer line profiles are sensitive to both Teff and log g
Stark broadening
atomic level destruction
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i
Hummer-Mihalas occupation probability formalism
j
i
j
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i
Hummer-Mihalas occupation probability formalism
j
i
j
photoexcitation(bound-bound opacity)
i
j
i
photoionisation(bound-free opacity)
κ
e−
wj
wj ››
wj ‹‹
occupationprobability
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i
Hummer-Mihalas occupation probability formalism
j
i
j
photoexcitation(bound-bound opacity)
i
j
i
photoionisation(bound-free opacity)
κ
e−
wj
wj ››
wj ‹‹
occupationprobability
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Spectroscopic technique used for measuring Teff and log gσ(Teff) = 1.2 % Teff σ(log g) = 0.038 dex
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log g converted into mass using the mass-radius relation for WDs(mechanical structure provided by degenerate electrons)
log g ~ 8
Smallerradii !
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log g converted into mass using the mass-radius relation for WDs
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Mass and log g distributions of 770 DA stars
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Chandrasekhar mass ~ 1.36 M⊙
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Chandrasekhar mass ~ 1.36 M⊙
electronsbecome
relativistic
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ZZ Ceti stars + photometrically constant white dwarfs
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Mass distribution as a function of Teff
DA stars from the SDSS / DR4
high-log g problem
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Mass distribution as a function of Teff
DA stars from the SDSS / DR4
high-log g problem
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Application to DB (helium-line) white dwarfs
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Application to DQ (carbon-line) white dwarfs
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Application to DZ (metal-line) white dwarfs
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Application to hot DO (helium-line)
white dwarfs
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Application to DC (featureless!) white dwarfs
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Stellar flux integrated over filter bandpasses (magnitudes)
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http://www.astro.umontreal.ca/~bergeron/CoolingModels/
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http://www.astro.umontreal.ca/~bergeron/CoolingModels/
For Dummies
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Photometric method
fλ = π (R/D)2 4Hλ
Hλ = Hλ(Teff , log g)
D = 1 / π
g = GM / R2
M-R relation for WDs
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Hybrid spectroscopic and photometric approach
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Application to «ultracool» white dwarfs
H2 collision-induced absorption (CIA)
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DB gap
Cooling time →
G. Fontaine & F. Wesemael (1987, 2nd conf. on Faint Blue Stars)
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DB gap
Cooling time →
Evidence for the spectral evolution of white dwarfs
H → HeHe → H
G. Fontaine & F. Wesemael (1987, 2nd conf. on Faint Blue Stars)
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He
H
Photosphere
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He
H
DO DA
Photosphere
Requires only10-16 M
of
hydrogen
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Photosphere
HeII conv
HeI conv
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Photosphere
HeII conv
HeI convH
He
Convective dilution DA DB
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Photosphere
H conv
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Photosphere
H conv
H
He
Convective mixing DA ???
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Photosphere
H conv
H
He
Convective mixing DA ???
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Photosphere
H conv
H
He
Convective mixing DA ???
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Origin of carbon in DQ stars
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Helium convection zone
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Helium convection zone
Carbon core (diffusion tail)
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Helium convection zone
Carbon core (diffusion tail)
Carbon in DQ stars is being dredged up from the core
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The Z in DZ stars !
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